62 results on '"Schofield AJ"'
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2. Evolution of the low-temperature Fermi surface of superconducting FeSe1−xSx across a nematic phase transition
- Author
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Coldea, AI, Blake, SF, Kasahara, S, Haghighirad, AA, Watson, MD, Knafo, W, Choi, ES, McCollam, A, Reiss, P, Yamashita, T, Bruma, M, Speller, SC, Matsuda, Y, Wolf, T, Shibauchi, T, Schofield, AJ, Department of Physics, Kyoto University, Kyoto University [Kyoto], Florida State University [Tallahassee] (FSU), Laboratoire de Physique et d'Etude des Matériaux (UMR 8213) (LPEM), Ecole Superieure de Physique et de Chimie Industrielles de la Ville de Paris (ESPCI Paris), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), School of Physics (SUPA), University of Edinburgh, University of St Andrews. Centre for Designer Quantum Materials, and University of St Andrews. School of Physics and Astronomy
- Subjects
[PHYS.COND.CM-S]Physics [physics]/Condensed Matter [cond-mat]/Superconductivity [cond-mat.supr-con] ,QC Physics ,DAS ,Correlated Electron Systems ,QC ,ComputingMilieux_MISCELLANEOUS - Abstract
This work was mainly supported by EPSRC (EP/L001772/1, EP/I004475/1, EP/I017836/1). A.A.H. acknowledges the financial support of the Oxford Quantum Materials Platform Grant (EP/M020517/1). A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement No. DMR-1157490 and the State of Florida. This research was supported in part by the National Science Foundation under Grant No. NSF PHY17-48958. Part of this work was supported by HFML-RU/FOM and LNCMI-CNRS, members of the European Magnetic Field Laboratory (EMFL) and by EPSRC (UK) via its membership to the EMFL (grant no. EP/N01085X/1). Part of this work was supported by Programme Investissements d’ Avenir under the programme ANR-11-IDEX-0002-02, reference ANR-10-LABX-0037-NEXT. The authors would like to acknowledge the use of the University of Oxford Advanced Research Computing (ARC) facility in carrying out part of this work. A.I.C. thanks the hospitality of KITP supported by the National Science Foundation under Grant No. NSF PHY- 1125915. A.I.C. acknowledges an EPSRC Career Acceleration Fellowship (EP/I004475/1). The existence of a nematic phase transition in iron-chalcogenide superconductors poses an intriguing question about its impact on superconductivity. To understand the nature of this unique quantum phase transition, it is essential to study how the electronic structure changes across this transition at low temperatures. Here, we investigate the evolution of the Fermi surfaces and electronic interactions across the nematic phase transition of FeSe1−xSx using Shubnikov-de Haas oscillations in high magnetic fields up to 45 T in the low temperature regime down to 0.4 K. Most of the Fermi surfaces of FeSe1−xSx monotonically increase in size except for a prominent low frequency oscillation associated with a small, but highly mobile band, which disappears at the nematic phase boundary near x ~ 0.17, indicative of a topological Lifshitz transition. The quasiparticle masses are larger inside the nematic phase, indicative of a strongly correlated state, but they become suppressed outside it. The experimentally observed changes in the Fermi surface topology, together with the varying degree of electronic correlations, will change the balance of electronic interactions in the multi-band system FeSe1−xSx and promote different kz-dependent superconducting pairing channels inside and outside the nematic phase. Publisher PDF
- Published
- 2019
3. Participation in Free and Open Source Communities: An Empirical Study of Community Members’ Perceptions
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Schofield, AJ and Cooper, GS
- Abstract
Although the defining factors of Free and Open Source Software (FOSS) are generally seen as the availability and accessibility of the source code, it is what these facilitate that is perhaps of more significance. Source code availability allows the sharing of code, skills, knowledge, and effort, focused on a particular piece of software under development. The result of this is the FOSS community, which although often perceived as a single group, is actually many small groups, each bound by a common interest in a particular piece of software and using the Internet as a communication medium. Although there have been studies focusing on the motivation of FOSS developers to contribute to software, there has been little investigation into the motives, attitudes, and the culture within the communities as a whole. There is much more to most of these communities than software development. Many also have extensive support networks for the use of software, portals for research, and social facilities. This paper describes the results of an investigation into how FOSS community members perceive the communities that they belong to, their reasons for being in the community, and the manner in which they participate.
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- 2006
4. Free and Open Source Software Communities as a Support Mechanism
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Schofield, AJ and Mitra, A
- Published
- 2005
5. Evaluation of Open Source Software: Comparative Perspectives Between Proprietary and Open Code Development
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Schofield, AJ and Mitra, A
- Published
- 2004
6. Knowledge and resource sharing in free and open source software community environments
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Schofield, AJ and Cooper, GS
- Abstract
This thesis describes a research study which investigated on-line communities based \ud around Free and Open Source Software (FOSS) support and development. \ud Specifically, the study examined how knowledge and resources are shared within \ud these communities. Emphasis was placed on the issues of organisation and \ud governance of these communities, collaboration, and the motivation of the \ud participants involved. The research was carried out from an interpretive philosophical \ud perspective and was therefore exploratory in nature, focusing on the perspectives of \ud FOSS community participants. Following an in-depth literature review on the topics \ud of FOSS and Knowledge Management, a multi-method approach was employed using \ud three data collection techniques; an on-line questionnaire targeting participants in \ud FOSS interest groups and communities, interviews with community participants and \ud observation of two FOSS communities.\ud The results of the research have revealed interesting findings relating to the social, \ud managerial and technological mechanisms which facilitate knowledge transfer and \ud creation in FOSS communities, and the important role that leadership styles, \ud participant motivations, organisational structures and cultures, degrees of formality, \ud relationships between participants and the use of information technology have to play \ud in the success or failure of FOSS projects and communities. The research has shown \ud that extremely strict or relaxed forms of leadership are likely to cause problems in a \ud project and community, that a wide array of motivational factors drive participants, \ud that many and varied forms of organisational structure and cultures exist, often \ud influenced by leadership styles, that levels of formality in a community can have \ud significant effects on knowledge sharing and collaboration, that social interaction acts \ud as a foundation for FOSS activities but has little significance beyond that, and that to \ud a certain extent, disagreements and arguments between participants and even project \ud forking can be beneficial to a FOSS project and community. A generic finding was \ud that FOSS communities vary considerably, based on the factors described above. This \ud research therefore goes some way towards creating a generic model of FOSS \ud communities and projects and specifies a number of 'recipes' for project and/or \ud community success.
7. Classification images for aerial images capture visual expertise for binocular disparity and a prior for lighting from above.
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Skog E, Meese TS, Sargent IMJ, Ormerod A, and Schofield AJ
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- Humans, Cues, Individuality, Learning, Vision Disparity, Lighting
- Abstract
Using a novel approach to classification images (CIs), we investigated the visual expertise of surveyors for luminance and binocular disparity cues simultaneously after screening for stereoacuity. Stereoscopic aerial images of hedges and ditches were classified in 10,000 trials by six trained remote sensing surveyors and six novices. Images were heavily masked with luminance and disparity noise simultaneously. Hedge and ditch images had reversed disparity on around half the trials meaning hedges became ditch-like and vice versa. The hedge and ditch images were also flipped vertically on around half the trials, changing the direction of the light source and completing a 2 × 2 × 2 stimulus design. CIs were generated by accumulating the noise textures associated with "hedge" and "ditch" classifications, respectively, and subtracting one from the other. Typical CIs had a central peak with one or two negative side-lobes. We found clear differences in the amplitudes and shapes of perceptual templates across groups and noise-type, with experts prioritizing binocular disparity and using this more effectively. Contrariwise, novices used luminance cues more than experts meaning that task motivation alone could not explain group differences. Asymmetries in the luminance CIs revealed individual differences for lighting interpretation, with experts less prone to assume lighting from above, consistent with their training on aerial images of UK scenes lit by a southerly sun. Our results show that (i) dual noise in images can be used to produce simultaneous CI pairs, (ii) expertise for disparity cues does not depend on stereoacuity, (iii) CIs reveal the visual strategies developed by experts, (iv) top-down perceptual biases can be overcome with long-term learning effects, and (v) CIs have practical potential for directing visual training.
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- 2024
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8. What surprises the Mona Lisa? The relative importance of the eyes and eyebrows for detecting surprise in briefly presented face stimuli.
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Skog E, Qian CS, Parmar A, and Schofield AJ
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- Humans, Female, Follow-Up Studies, Emotions physiology, Happiness, Eyebrows, Facial Expression
- Abstract
The classification image (CI) technique has been used to derive templates for judgements of facial emotion and reveal which facial features inform specific emotional judgements. For example, this method has been used to show that detecting an up- or down-turned mouth is a primary strategy for discriminating happy versus sad expressions. We explored the detection of surprise using CIs, expecting widened eyes, raised eyebrows, and open mouths to be dominant features. We briefly presented a photograph of a female face with a neutral expression embedded in random visual noise, which modulated the appearance of the face on a trial-by-trial basis. In separate sessions, we showed this face with or without eyebrows to test the importance of the raised eyebrow element of surprise. Noise samples were aggregated into CIs based on participant responses. Results show that the eye-region was most informative for detecting surprise. Unless attention was specifically directed to the mouth, we found no effects in the mouth region. The eye effect was stronger when the eyebrows were absent, but the eyebrow region was not itself informative and people did not infer eyebrows when they were missing. A follow-up study was conducted in which participants rated the emotional valence of the neutral images combined with their associated CIs. This verified that CIs for 'surprise' convey surprised expressions, while also showing that CIs for 'not surprise' convey disgust. We conclude that the eye-region is important for the detection of surprise., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Author(s). Published by Elsevier Ltd.. All rights reserved.)
- Published
- 2023
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9. Accuracy and Precision of a Novel Photogate System to Measure Toe Clearance on Stairs.
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Skervin TK, Thomas NM, Schofield AJ, Hollands MA, Maganaris CN, O'Brien TD, Baltzopoulos V, and Foster RJ
- Abstract
Background: Toe clearance on stairs is typically measured using optoelectronic systems, though these are often constrained to the laboratory, due to their complex setups. Here we measured stair toe clearance through a novel prototype photogate setup and compared this to optoelectronic measurements. Methods: Twelve participants (age 22 ± 3 years) completed 25 stair ascent trials, each on a seven-step staircase. Toe clearance over the fifth step edge was measured using Vicon and the photogates. Twenty-two photogates were created in rows through laser diodes and phototransistors. The height of the lowest photogate broken at step-edge crossing was used to determine photogate toe clearance. A limits of agreement analysis and Pearson's correlation coefficient compared the accuracy, precision and relationship between systems. Results: We found a mean difference of -1.5 mm (accuracy) between the two measurement systems, with upper and lower limits (precision) of 10.7 mm and -13.8 mm, respectively. A strong positive correlation was also found ( r = 70, n = 12, p = 0.009) between the systems. Discussion: The results suggest that photogates could be an option for measuring real-world stair toe clearances, where optoelectronic systems are not routinely used. Improvements to the design and measurement factors may help to improve the precision of the photogates.
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- 2023
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10. Second-order texture gratings produce overestimation of height in depictions of rectangles and steps.
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Schofield AJ
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- Humans, Optical Illusions, Visual Perception
- Abstract
The horizontal-vertical illusion (HVI) has been proposed as a method to increase the perceived height of steps, increase toe clearance and prevent falls. High contrast vertical stripes are placed on the step riser abutting a horizontal edge-highlighter creating 'T' junctions which are thought to promote the illusion. Various configurations of the HVI were tested including luminance gratings (L) and second-order modulations of contrast (CM), spatial frequency (FM) and orientation (OM). Observers were asked to compare the apparent height of gratings with that of either filled, unmodulated rectangles or unfilled rectangles. Rectangles were presented alone or as part of a step with a highlighter. In some conditions highlighters matched the properties of the grating; in others or not. In one critical experiment, the HVI was compared for steps with highlighters that were separated from the riser by a thin line and those where the risers and highlighters were continuous. All gratings except FM appeared taller when presented in the step configuration with a continuous, matching highlighter. This effect was greatly reduced when a thin line separated the grating from the highlighter and abolished for mis-matched highlighters and risers. In the rectangle conditions, all cues appeared taller than blank rectangles and L and CM appeared taller than filled-unmodulated rectangles. In conclusion, second-order cues may be useful for inducing the HVI onto steps. However, the ability of vertical stripes and edge-highlighters to accentuate perceived step height may be due to aggregation of the highlighter into the grating rather than the normal horizontal-vertical illusion., Competing Interests: Declaration of Competing Interest The author declares that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Author. Published by Elsevier Ltd.. All rights reserved.)
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- 2022
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11. Observing separate spin and charge Fermi seas in a strongly correlated one-dimensional conductor.
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Vianez PMT, Jin Y, Moreno M, Anirban AS, Anthore A, Tan WK, Griffiths JP, Farrer I, Ritchie DA, Schofield AJ, Tsyplyatyev O, and Ford CJB
- Abstract
An electron is usually considered to have only one form of kinetic energy, but could it have more, for its spin and charge, by exciting other electrons? In one dimension (1D), the physics of interacting electrons is captured well at low energies by the Tomonaga-Luttinger model, yet little has been observed experimentally beyond this linear regime. Here, we report on measurements of many-body modes in 1D gated wires using tunneling spectroscopy. We observe two parabolic dispersions, indicative of separate Fermi seas at high energies, associated with spin and charge excitations, together with the emergence of two additional 1D "replica" modes that strengthen with decreasing wire length. The interaction strength is varied by changing the amount of 1D intersubband screening by more than 45%. Our findings not only demonstrate the existence of spin-charge separation in the whole energy band outside the low-energy limit of the Tomonaga-Luttinger model but also set a constraint on the validity of the newer nonlinear Tomonaga-Luttinger theory.
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- 2022
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12. Using a stair horizontal-vertical illusion to increase foot clearance over an inconsistently taller stair-riser.
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Skervin TK, Thomas NM, Schofield AJ, Hollands MA, Maganaris CN, O'Brien TD, Baltzopoulos V, and Foster RJ
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- Female, Humans, Male, Movement physiology, Outcome Assessment, Health Care, Young Adult, Foot physiology, Illusions
- Abstract
Introduction: Stair falls can be caused by inconsistent stair dimensions. During ascent, inconsistently taller stair risers lead to reduced foot clearances as the inconsistency goes unnoticed. A stair horizontal-vertical illusion increases perceived riser heights and foot clearance and could offset reduced foot clearances over inconsistently taller risers, though this might impact other stair safety measures., Method: Twelve participants (age: 22 (3) years) ascended a seven-step staircase under three conditions: i) all steps consistent in riser height (consistent), ii) a 1cm increase in step 5 riser height (inconsistent) and iii) a 1cm increase in step 5 riser height, superimposed with a stair horizontal-vertical illusion (illusion). Vertical foot clearance, foot overhang, and margins of stability were assessed over step 4, 5 and 6. Perceived riser height due to the illusion was determined through a computer perception test. A One-Way Repeated Measures ANOVA compared biomechanical variables between conditions. A One Sample t test compared perceived riser height to the true height., Results: Over the inconsistent step 5, foot clearance reduced by 0.8cm compared to consistent. Illusion increased foot clearance by 1.1cm and decreased foot overhang by 4% compared to inconsistent. On step 4 the illusion led to more anterior instability compared to inconsistent. Illusion and inconsistent led to more mediolateral stability compared to consistent. The illusion increased perceived riser height by 12%., Discussion: Foot clearance reductions over inconsistently taller risers can be offset by a stair horizontal-vertical illusion. Additional benefits included a safer foot overhang and unaffected stability over the inconsistent riser. Changes to step 4 stability might have resulted from leaning forward to look at the step 5 illusion. The stair horizontal-vertical illusion could be a practical solution for inconsistently taller stair risers, where a rebuild is usually the only solution., Competing Interests: The authors have declared that no competing interests exist.
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- 2021
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13. The missing N1 or jittered P2: Electrophysiological correlates of pattern glare in the time and frequency domain.
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Tempesta AJ, Miller CE, Litvak V, Bowman H, and Schofield AJ
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- Electroencephalography, Electrophysiological Phenomena, Humans, Glare, Migraine Disorders
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Excessive sensitivity to certain visual stimuli (cortical hyperexcitability) is associated with a number of neurological disorders including migraine, epilepsy, multiple sclerosis, autism and possibly dyslexia. Others show disruptive sensitivity to visual stimuli with no other obvious pathology or symptom profile (visual stress) which can extend to discomfort and nausea. We used event-related potentials (ERPs) to explore the neural correlates of visual stress and headache proneness. We analysed ERPs in response to thick (0.37 cycles per degree [c/deg]), medium (3 c/deg) and thin (12 c/deg) gratings, using mass univariate analysis, considering three factors in the general population: headache proneness, visual stress and discomfort. We found relationships between ERP features and the headache and discomfort factors. Stimulus main effects were driven by the medium stimulus regardless of participant characteristics. Participants with high discomfort ratings had larger P1 components for the initial presentation of medium stimuli, suggesting initial cortical hyperexcitability that is later suppressed. The participants with high headache ratings showed atypical N1-P2 components for medium stripes relative to the other stimuli. This effect was present only after repeated stimulus presentation. These effects were also explored in the frequency domain, suggesting variations in intertrial theta band phase coherence. Our results suggest that discomfort and headache in response to striped stimuli are related to different neural processes; however, more exploration is needed to determine whether the results translate to a clinical migraine population., (© 2021 The Authors. European Journal of Neuroscience published by Federation of European Neuroscience Societies and John Wiley & Sons Ltd.)
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- 2021
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14. The next step in optimising the stair horizontal-vertical illusion: Does a perception-action link exist in older adults?
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Skervin TK, Thomas NM, Schofield AJ, Hollands MA, Maganaris CN, and Foster RJ
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- Accidental Falls, Aged, Biomechanical Phenomena, Foot, Gait, Humans, Illusions
- Abstract
Introduction: Tripping on stairs results from insufficient foot to step edge clearance and can often lead to a fall in older adults. A stair horizontal-vertical illusion is suggested to increase the perceived riser height of a step and increase foot clearance when stepping up. However, this perception-action link has not been empirically determined in older adults. Previous findings suggesting a perception-action effect have also been limited to a single step or a three-step staircase. On larger staircases, somatosensory learning of step heights may be greater which could override the illusory effect on the top step. Furthermore, the striped nature of the existing stair horizontal-vertical illusion is associated with visual stress and may not be aesthetically suitable for use on public stairs. These issues need resolving before potential future implementation on public stairs., Methods: Experiment 1. A series of four computer-based perception tests were conducted in older (N = 14: 70 ± 6 years) and young adults (N = 42: 24 ± 3 years) to test the influence of different illusion designs on stair riser height estimation. Participants compared images of stairs, with horizontal-vertical illusions or arbitrary designs on the bottom step, to a plain stair with different bottom step riser heights and selected the stair they perceived to have the tallest bottom riser. Horizontal-vertical illusions included a previously developed design and versions with modified spatial frequencies and mark space ratios. Perceived riser height differences were assessed between designs and between age groups. Experiment 2. To assess the perception-action link, sixteen older (70 ± 7 years) and fifteen young (24 ± 3 years) adults ascended a seven-step staircase with and without horizontal-vertical illusions tested in experiment 1 placed onto steps one and seven. Foot clearances were measured over each step. To determine whether changes in perception were linked to changes in foot clearance, perceived riser heights for each horizontal-vertical illusion were assessed using the perception test from experiment 1 before and after stair ascent. Additional measures to characterise stair safety included vertical foot clearance, margins of stability, foot overhang, stair speed, and gaze duration, which were assessed over all seven steps., Results: Experiment 1. All horizontal-vertical illusion designs led to significant increases in the perceived riser height in both young and older adults (12-19% increase) with no differences between age groups. Experiment 2. On step 7, each horizontal-vertical illusion led to an increase in vertical foot clearance for young (up to 0.8 cm) and older adults (up to 2.1 cm). On step 1 significant increases in vertical foot clearance were found for a single horizontal-vertical illusion when compared to plain (1.19 cm increase). The horizontal-vertical illusions caused significant increases in the perceived riser height (young; 13% increase, older; 11% increase) with no differences between illusion design, group or before and after stair ascent. No further differences were found for the remaining variables and steps., Conclusion: Results indicate a perception-action link between perceived riser height and vertical foot clearance in response to modified versions of the horizontal-vertical illusion in both young and older adults. This was shown with no detriment to additional stair safety measures. Further evaluating these illusions on private/public stairs, especially those with inconsistently taller steps, may be beneficial to help improve stair safety for older adults., (Copyright © 2021 Elsevier Inc. All rights reserved.)
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- 2021
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15. Momentum-dependent power law measured in an interacting quantum wire beyond the Luttinger limit.
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Jin Y, Tsyplyatyev O, Moreno M, Anthore A, Tan WK, Griffiths JP, Farrer I, Ritchie DA, Glazman LI, Schofield AJ, and Ford CJB
- Abstract
Power laws in physics have until now always been associated with a scale invariance originating from the absence of a length scale. Recently, an emergent invariance even in the presence of a length scale has been predicted by the newly-developed nonlinear-Luttinger-liquid theory for a one-dimensional (1D) quantum fluid at finite energy and momentum, at which the particle's wavelength provides the length scale. We present experimental evidence for this new type of power law in the spectral function of interacting electrons in a quantum wire using a transport-spectroscopy technique. The observed momentum dependence of the power law in the high-energy region matches the theoretical predictions, supporting not only the 1D theory of interacting particles beyond the linear regime but also the existence of a new type of universality that emerges at finite energy and momentum.
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- 2019
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16. Extracting pure absorbance spectra in infrared microspectroscopy by modeling absorption bands as Fano resonances.
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Schofield AJ, Blümel R, Kohler A, Lukacs R, and Hirschmugl CJ
- Abstract
Midinfrared absorbance spectra obtained from spatially inhomogeneous and finite samples often contain scattering effects characterized by derivative-like bands with shifted peak positions. Such features may be interpreted and accurately modeled by Fano theory when the imaginary part of the complex dielectric function is small and Lorentzian in nature-as is the case for many biological media. Furthermore, by fitting Fano line shapes to isolated absorbance bands, recovery of the peak position and pure absorption strength can be obtained with high accuracy. Additionally, for small and optically soft spherical scatterers, recovery of one or the other of constant refractive index or radius (given approximate knowledge of the other) is possible.
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- 2019
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17. Temporal diabetes-induced biochemical changes in distinctive layers of mouse retina.
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Aboualizadeh E, Sorenson CM, Schofield AJ, Unger M, Sheibani N, and Hirschmugl CJ
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- Animals, Biomarkers, Diabetic Retinopathy diagnostic imaging, Disease Models, Animal, Disease Progression, Male, Mice, Photoreceptor Cells metabolism, Spectroscopy, Fourier Transform Infrared, Tomography, Optical Coherence, Diabetic Retinopathy metabolism, Diabetic Retinopathy pathology, Retina metabolism, Retina pathology
- Abstract
To discover the mechanisms underlying the progression of diabetic retinopathy (DR), a more comprehensive understanding of the biomolecular processes in individual retinal cells subjected to hyperglycemia is required. Despite extensive studies, the changes in the biochemistry of retinal layers during the development of DR are not well known. In this study, we aimed to determine a more detailed understanding of the natural history of DR in Akita/+ (type 1 diabetes model) male mice with different duration of diabetes. Employing label-free spatially resolved Fourier transform infrared (FT-IR) chemical imaging engaged with multivariate analysis enabled us to identify temporal-dependent reproducible biomarkers of the individual retinal layers from mice with 6 weeks,12 weeks, 6 months, and 10 months of age. We report, for the first time, the nature of the biochemical alterations over time in the biochemistry of distinctive retinal layers namely photoreceptor retinal layer (PRL), inner nuclear layer (INL), and plexiform layers (OPL, IPL). Moreover, we present the molecular factors associated with the changes in the protein structure and cellular lipids of retinal layers induced by different duration of diabetes. Our paradigm provides a new conceptual framework for a better understanding of the temporal cellular changes underlying the progression of DR.
- Published
- 2018
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18. Reduced sensitivity for visual textures affects judgments of shape-from-shading and step-climbing behaviour in older adults.
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Schofield AJ, Curzon-Jones B, and Hollands MA
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- Adult, Aged, Female, Humans, Male, Middle Aged, Photic Stimulation, Surface Properties, Young Adult, Aging physiology, Contrast Sensitivity physiology, Judgment physiology, Stereognosis physiology
- Abstract
Falls on stairs are a major hazard for older adults. Visual decline in normal ageing can affect step-climbing ability, altering gait and reducing toe clearance. Here we show that a loss of fine-grained visual information associated with age can affect the perception of surface undulations in patterned surfaces. We go on to show that such cues affect the limb trajectories of young adults, but due to their lack of sensitivity, not that of older adults. Interestingly neither the perceived height of a step nor conscious awareness is altered by our visual manipulation, but stepping behaviour is, suggesting that the influence of shape perception on stepping behaviour is via the unconscious, action-centred, dorsal visual pathway.
- Published
- 2017
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19. Binocular functional architecture for detection of contrast-modulated gratings.
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Georgeson MA and Schofield AJ
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- Adult, Discrimination, Psychological, Humans, Perceptual Masking physiology, Sensory Thresholds physiology, Contrast Sensitivity physiology, Vision, Binocular physiology
- Abstract
Combination of signals from the two eyes is the gateway to stereo vision. To gain insight into binocular signal processing, we studied binocular summation for luminance-modulated gratings (L or LM) and contrast-modulated gratings (CM). We measured 2AFC detection thresholds for a signal grating (0.75c/deg, 216ms) shown to one eye, both eyes, or both eyes out-of-phase. For LM and CM, the carrier noise was in both eyes, even when the signal was monocular. Mean binocular thresholds for luminance gratings (L) were 5.4dB better than monocular thresholds - close to perfect linear summation (6dB). For LM and CM the binocular advantage was again 5-6dB, even when the carrier noise was uncorrelated, anti-correlated, or at orthogonal orientations in the two eyes. Binocular combination for CM probably arises from summation of envelope responses, and not from summation of these conflicting carrier patterns. Antiphase signals produced no binocular advantage, but thresholds were about 1-3dB higher than monocular ones. This is not consistent with simple linear summation, which should give complete cancellation and unmeasurably high thresholds. We propose a three-channel model in which noisy monocular responses to the envelope are binocularly combined in a contrast-weighted sum, but also remain separately available to perception via a max operator. Vision selects the largest of the three responses. With in-phase gratings the binocular channel dominates, but antiphase gratings cancel in the binocular channel and the monocular channels mediate detection. The small antiphase disadvantage might be explained by a subtle influence of background responses on binocular and monocular detection., (Copyright © 2016 Elsevier Ltd. All rights reserved.)
- Published
- 2016
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20. Nonlinear spectra of spinons and holons in short GaAs quantum wires.
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Moreno M, Ford CJ, Jin Y, Griffiths JP, Farrer I, Jones GA, Ritchie DA, Tsyplyatyev O, and Schofield AJ
- Abstract
One-dimensional electronic fluids are peculiar conducting systems, where the fundamental role of interactions leads to exotic, emergent phenomena, such as spin-charge (spinon-holon) separation. The distinct low-energy properties of these 1D metals are successfully described within the theory of linear Luttinger liquids, but the challenging task of describing their high-energy nonlinear properties has long remained elusive. Recently, novel theoretical approaches accounting for nonlinearity have been developed, yet the rich phenomenology that they predict remains barely explored experimentally. Here, we probe the nonlinear spectral characteristics of short GaAs quantum wires by tunnelling spectroscopy, using an advanced device consisting of 6000 wires. We find evidence for the existence of an inverted (spinon) shadow band in the main region of the particle sector, one of the central predictions of the new nonlinear theories. A (holon) band with reduced effective mass is clearly visible in the particle sector at high energies.
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- 2016
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21. Publisher's Note: Dichotomy between the Hole and Electron Behavior in Multiband Superconductor FeSe Probed by Ultrahigh Magnetic Fields [Phys. Rev. Lett. 115, 027006 (2015)].
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Watson MD, Yamashita T, Kasahara S, Knafo W, Nardone M, Béard J, Hardy F, McCollam A, Narayanan A, Blake SF, Wolf T, Haghighirad AA, Meingast C, Schofield AJ, V Löhneysen H, Matsuda Y, Coldea AI, and Shibauchi T
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- 2015
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22. Dichotomy between the Hole and Electron Behavior in Multiband Superconductor FeSe Probed by Ultrahigh Magnetic Fields.
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Watson MD, Yamashita T, Kasahara S, Knafo W, Nardone M, Béard J, Hardy F, McCollam A, Narayanan A, Blake SF, Wolf T, Haghighirad AA, Meingast C, Schofield AJ, Löhneysen Hv, Matsuda Y, Coldea AI, and Shibauchi T
- Abstract
Magnetoresistivity ρ(xx) and Hall resistivity ρ(xy) in ultrahigh magnetic fields up to 88 T are measured down to 0.15 K to clarify the multiband electronic structure in high-quality single crystals of superconducting FeSe. At low temperatures and high fields we observe quantum oscillations in both resistivity and the Hall effect, confirming the multiband Fermi surface with small volumes. We propose a novel approach to identify from magnetotransport measurements the sign of the charge carriers corresponding to a particular cyclotron orbit in a compensated metal. The observed significant differences in the relative amplitudes of the quantum oscillations between the ρ(xx) and ρ(xy) components, together with the positive sign of the high-field ρ(xy), reveal that the largest pocket should correspond to the hole band. The low-field magnetotransport data in the normal state suggest that, in addition to one hole and one almost compensated electron band, the orthorhombic phase of FeSe exhibits an additional tiny electron pocket with a high mobility.
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- 2015
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23. Hierarchy of modes in an interacting one-dimensional system.
- Author
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Tsyplyatyev O, Schofield AJ, Jin Y, Moreno M, Tan WK, Ford CJ, Griffiths JP, Farrer I, Jones GA, and Ritchie DA
- Abstract
Studying interacting fermions in one dimension at high energy, we find a hierarchy in the spectral weights of the excitations theoretically, and we observe evidence for second-level excitations experimentally. Diagonalizing a model of fermions (without spin), we show that levels of the hierarchy are separated by powers of R^{2}/L^{2}, where R is a length scale related to interactions and L is the system length. The first-level (strongest) excitations form a mode with parabolic dispersion, like that of a renormalized single particle. The second-level excitations produce a singular power-law line shape to the first-level mode and multiple power laws at the spectral edge. We measure momentum-resolved tunneling of electrons (fermions with spin) from or to a wire formed within a GaAs heterostructure, which shows parabolic dispersion of the first-level mode and well-resolved spin-charge separation at low energy with appreciable interaction strength. We find structure resembling the second-level excitations, which dies away quite rapidly at high momentum.
- Published
- 2015
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24. Texture variations suppress suprathreshold brightness and colour variations.
- Author
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Schofield AJ and Kingdom FA
- Subjects
- Cues, Humans, Photic Stimulation, Psychometrics, Color Perception physiology, Contrast Sensitivity physiology, Sensory Thresholds physiology, Vision, Ocular physiology
- Abstract
Discriminating material changes from illumination changes is a key function of early vision. Luminance cues are ambiguous in this regard, but can be disambiguated by co-incident changes in colour and texture. Thus, colour and texture are likely to be given greater prominence than luminance for object segmentation, and better segmentation should in turn produce stronger grouping. We sought to measure the relative strengths of combined luminance, colour and texture contrast using a suprathreshhold, psychophysical grouping task. Stimuli comprised diagonal grids of circular patches bordered by a thin black line and contained combinations of luminance decrements with either violet, red, or texture increments. There were two tasks. In the Separate task the different cues were presented separately in a two-interval design, and participants indicated which interval contained the stronger orientation structure. In the Combined task the cues were combined to produce competing orientation structure in a single image. Participants had to indicate which orientation, and therefore which cue was dominant. Thus we established the relative grouping strength of each cue pair presented separately, and compared this to their relative grouping strength when combined. In this way we observed suprathreshold interactions between cues and were able to assess cue dominance at ecologically relevant signal levels. Participants required significantly more luminance and colour compared to texture contrast in the Combined compared to Separate conditions (contrast ratios differed by about 0.1 log units), showing that suprathreshold texture dominates colour and luminance when the different cues are presented in combination.
- Published
- 2014
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25. Perceptual integration for qualitatively different 3-D cues in the human brain.
- Author
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Dövencioğlu D, Ban H, Schofield AJ, and Welchman AE
- Subjects
- Adult, Algorithms, Brain blood supply, Brain Mapping, Female, Humans, Image Processing, Computer-Assisted, Magnetic Resonance Imaging, Male, Oxygen blood, Photic Stimulation, Predictive Value of Tests, Probability, Psychophysics, Young Adult, Brain physiology, Cues, Depth Perception physiology
- Abstract
The visual system's flexibility in estimating depth is remarkable: We readily perceive 3-D structure under diverse conditions from the seemingly random dots of a "magic eye" stereogram to the aesthetically beautiful, but obviously flat, canvasses of the Old Masters. Yet, 3-D perception is often enhanced when different cues specify the same depth. This perceptual process is understood as Bayesian inference that improves sensory estimates. Despite considerable behavioral support for this theory, insights into the cortical circuits involved are limited. Moreover, extant work tested quantitatively similar cues, reducing some of the challenges associated with integrating computationally and qualitatively different signals. Here we address this challenge by measuring fMRI responses to depth structures defined by shading, binocular disparity, and their combination. We quantified information about depth configurations (convex "bumps" vs. concave "dimples") in different visual cortical areas using pattern classification analysis. We found that fMRI responses in dorsal visual area V3B/KO were more discriminable when disparity and shading concurrently signaled depth, in line with the predictions of cue integration. Importantly, by relating fMRI and psychophysical tests of integration, we observed a close association between depth judgments and activity in this area. Finally, using a cross-cue transfer test, we found that fMRI responses evoked by one cue afford classification of responses evoked by the other. This reveals a generalized depth representation in dorsal visual cortex that combines qualitatively different information in line with 3-D perception.
- Published
- 2013
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26. Perceptual learning of second order cues for layer decomposition.
- Author
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Dövencioğlu DN, Welchman AE, and Schofield AJ
- Subjects
- Adult, Analysis of Variance, Depth Perception physiology, Female, Humans, Luminescence, Male, Photic Stimulation methods, Sensory Thresholds physiology, Cues, Discrimination Learning physiology, Lighting, Visual Perception physiology
- Abstract
Luminance variations are ambiguous: they can signal changes in surface reflectance or changes in illumination. Layer decomposition-the process of distinguishing between reflectance and illumination changes-is supported by a range of secondary cues including colour and texture. For an illuminated corrugated, textured surface the shading pattern comprises modulations of luminance (first order, LM) and local luminance amplitude (second-order, AM). The phase relationship between these two signals enables layer decomposition, predicts the perception of reflectance and illumination changes, and has been modelled based on early, fast, feed-forward visual processing (Schofield et al., 2010). However, while inexperienced viewers appreciate this scission at long presentation times, they cannot do so for short presentation durations (250 ms). This might suggest the action of slower, higher-level mechanisms. Here we consider how training attenuates this delay, and whether the resultant learning occurs at a perceptual level. We trained observers to discriminate the components of plaid stimuli that mixed in-phase and anti-phase LM/AM signals over a period of 5 days. After training, the strength of the AM signal needed to differentiate the plaid components fell dramatically, indicating learning. We tested for transfer of learning using stimuli with different spatial frequencies, in-plane orientations, and acutely angled plaids. We report that learning transfers only partially when the stimuli are changed, suggesting that benefits accrue from tuning specific mechanisms, rather than general interpretative processes. We suggest that the mechanisms which support layer decomposition using second-order cues are relatively early, and not inherently slow., (Copyright © 2012 Elsevier Ltd. All rights reserved.)
- Published
- 2013
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27. A cue-free method to probe human lighting biases.
- Author
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Mazzilli G and Schofield AJ
- Subjects
- Cues, Depth Perception physiology, Humans, Judgment physiology, Students, Light, Lighting methods, Photic Stimulation methods, Visual Perception physiology
- Abstract
People readily perceive patterns of shading as 3-D shapes. Owing to the generalised bas-relief ambiguity when extracting shape from shading, people must simultaneously estimate the shape of the surface and the nature of the light source. In many cases cues in the image will be insufficient to resolve all of the ambiguities present, and in such cases the human visual system may employ one of a number of prior assumptions based on ecology and experience. One such assumption is the lighting-from-above prior. Here, in the absence of extrinsic cues to lighting direction, ambiguous shading patterns are interpreted as if lit by a light source that is above the observer's head. Studies of this prior typically use ambiguous stimuli and observe perceptual biases. A degree of cueing is inherent to such methods. Participants see the shaded stimuli repeatedly and are asked to make shape judgments about them regardless of whether or not they actually perceive any 3-D shape. We wanted to access people's lighting prior more directly by establishing the template they would employ to detect a shaded object in the absence of any visual cue to object shape. To this end, we adopted a classification image approach.
- Published
- 2013
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28. Two operational modes in the perception of shape from shading revealed by the effects of edge information in slant settings.
- Author
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Sun P and Schofield AJ
- Subjects
- Humans, Lighting, Surface Properties, Contrast Sensitivity physiology, Form Perception physiology, Models, Neurological, Photic Stimulation methods
- Abstract
The perception of shape from shading (SFS) has been an active research topic for more than two decades, yet its quantitative description remains poorly specified. One obstacle is the variability typically found between observers during SFS tasks. In this study, we take a different view of these inconsistencies, attributing them to uncertainties associated with human SFS. By identifying these uncertainties, we are able to probe the underlying computation behind SFS in humans. We introduce new experimental results that have interesting implications for SFS. Our data favor the idea that human SFS operates in at least two distinct modes. In one mode, perceived slant is linear to luminance or close to linear with some perturbation. Whether or not the linear relationship is achieved is influenced by the relative contrasts of edges bounding the luminance variation. This mode of operation is consistent with collimated lighting from an oblique angle. In the other mode, recovered surface height is indicative of a surface under lighting that is either diffuse or collimated and frontal. Shape estimates under this mode are partially accounted for by the "dark-is-deep" rule (height ∝ luminance). Switching between these two modes appears to be driven by the sign of the edges at the boundaries of the stimulus. Linear shading was active when the boundary edges had the same contrast polarity. Dark-is-deep was active when the boundary edges had opposite contrast polarity. When both same-sign and opposite-sign edges were present, observers preferred linear shading but could adopt a combination of the two computational modes.
- Published
- 2012
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29. Sun and sky: Does human vision assume a mixture of point and diffuse illumination when interpreting shape-from-shading?
- Author
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Schofield AJ, Rock PB, and Georgeson MA
- Subjects
- Cues, Depth Perception physiology, Humans, Models, Theoretical, Pattern Recognition, Visual physiology, Photic Stimulation methods, Form Perception physiology, Lighting
- Abstract
People readily perceive smooth luminance variations as being due to the shading produced by undulations of a 3-D surface (shape-from-shading). In doing so, the visual system must simultaneously estimate the shape of the surface and the nature of the illumination. Remarkably, shape-from-shading operates even when both these properties are unknown and neither can be estimated directly from the image. In such circumstances humans are thought to adopt a default illumination model. A widely held view is that the default illuminant is a point source located above the observer's head. However, some have argued instead that the default illuminant is a diffuse source. We now present evidence that humans may adopt a flexible illumination model that includes both diffuse and point source elements. Our model estimates a direction for the point source and then weights the contribution of this source according to a bias function. For most people the preferred illuminant direction is overhead with a strong diffuse component., (Copyright © 2011 Elsevier Ltd. All rights reserved.)
- Published
- 2011
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30. The efficacy of local luminance amplitude in disambiguating the origin of luminance signals depends on carrier frequency: further evidence for the active role of second-order vision in layer decomposition.
- Author
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Sun P and Schofield AJ
- Subjects
- Humans, Models, Theoretical, Perceptual Masking physiology, Psychophysics, Lighting, Pattern Recognition, Visual physiology, Visual Perception physiology
- Abstract
When an undulating surface bearing a painted texture is illuminated the resulting shading pattern produces in-phase modulations of the mean luminance (LM) and luminance amplitude (AM) of the texture. Experimentally, in-phase combinations of LM and AM (LM+AM) are seen as undulating surfaces whereas anti-phase combinations (LM-AM) are more ambiguous; being seen as undulating when presented alone but as flat when presented in a plaid with LM+AM. AM is a second-order cue and its influence on shape-from-shading can be explained with a bottom-up layer decomposition model containing second-order mechanisms. However, the role of second-order vision in layer decomposition has not been established. If second-order vision is involved in layer decomposition then the perceptual differences between LM+AM and LM-AM should depend on the properties of the carrier texture in a way that is consistent with the known properties of second-order vision. Here we find a preference for carrier frequencies 3 octaves above the modulation frequency and take this as an indication that second-order (filter-rectify-filter) mechanisms are involved in processing our LM/AM mixes. We introduce a modified model which takes into account the selectivity of second-order vision for carrier frequency., (Published by Elsevier Ltd.)
- Published
- 2011
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31. What is second-order vision for? Discriminating illumination versus material changes.
- Author
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Schofield AJ, Rock PB, Sun P, Jiang X, and Georgeson MA
- Subjects
- Cues, Discrimination, Psychological physiology, Humans, Orientation physiology, Photic Stimulation methods, Surface Properties, Contrast Sensitivity physiology, Depth Perception physiology, Lighting, Models, Neurological
- Abstract
The human visual system is sensitive to second-order modulations of the local contrast (CM) or amplitude (AM) of a carrier signal. Second-order cues are detected independently of first-order luminance signals; however, it is not clear why vision should benefit from second-order sensitivity. Analysis of the first- and second-order contents of natural images suggests that these cues tend to occur together, but their phase relationship varies. We have shown that in-phase combinations of LM and AM are perceived as a shaded corrugated surface whereas the anti-phase combination can be seen as corrugated when presented alone or as a flat material change when presented in a plaid containing the in-phase cue. We now extend these findings using new stimulus types and a novel haptic matching task. We also introduce a computational model based on initially separate first- and second-order channels that are combined within orientation and subsequently across orientation to produce a shading signal. Contrast gain control allows the LM + AM cue to suppress responses to the LM - AM when presented in a plaid. Thus, the model sees LM - AM as flat in these circumstances. We conclude that second-order vision plays a key role in disambiguating the origin of luminance changes within an image.
- Published
- 2010
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32. Condensed-matter physics: The emergent and hidden unveiled.
- Author
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Schofield AJ
- Published
- 2010
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33. Probing spin-charge separation in a Tomonaga-Luttinger liquid.
- Author
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Jompol Y, Ford CJ, Griffiths JP, Farrer I, Jones GA, Anderson D, Ritchie DA, Silk TW, and Schofield AJ
- Abstract
In a one-dimensional (1D) system of interacting electrons, excitations of spin and charge travel at different speeds, according to the theory of a Tomonaga-Luttinger liquid (TLL) at low energies. However, the clear observation of this spin-charge separation is an ongoing challenge experimentally. We have fabricated an electrostatically gated 1D system in which we observe spin-charge separation and also the predicted power-law suppression of tunneling into the 1D system. The spin-charge separation persists even beyond the low-energy regime where the TLL approximation should hold. TLL effects should therefore also be important in similar, but shorter, electrostatically gated wires, where interaction effects are being studied extensively worldwide.
- Published
- 2009
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34. Depth propagation and surface construction in 3-D vision.
- Author
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Georgeson MA, Yates TA, and Schofield AJ
- Subjects
- Computer Graphics, Contrast Sensitivity physiology, Depth Perception physiology, Fixation, Ocular, Form Perception physiology, Humans, Psychophysics, Sensory Thresholds, Cues, Optical Illusions, Vision, Binocular physiology, Visual Perception physiology
- Abstract
In stereo vision, regions with ambiguous or unspecified disparity can acquire perceived depth from unambiguous regions. This has been called stereo capture, depth interpolation or surface completion. We studied some striking induced depth effects suggesting that depth interpolation and surface completion are distinct stages of visual processing. An inducing texture (2-D Gaussian noise) had sinusoidal modulation of disparity, creating a smooth horizontal corrugation. The central region of this surface was replaced by various test patterns whose perceived corrugation was measured. When the test image was horizontal 1-D noise, shown to one eye or to both eyes without disparity, it appeared corrugated in much the same way as the disparity-modulated (DM) flanking regions. But when the test image was 2-D noise, or vertical 1-D noise, little or no depth was induced. This suggests that horizontal orientation was a key factor. For a horizontal sine-wave luminance grating, strong depth was induced, but for a square-wave grating, depth was induced only when its edges were aligned with the peaks and troughs of the DM flanking surface. These and related results suggest that disparity (or local depth) propagates along horizontal 1-D features, and then a 3-D surface is constructed from the depth samples acquired. The shape of the constructed surface can be different from the inducer, and so surface construction appears to operate on the results of a more local depth propagation process.
- Published
- 2009
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35. Discriminating depth in corrugated stereo surfaces: facilitation by a pedestal is explained by removal of uncertainty.
- Author
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Georgeson MA, Yates TA, and Schofield AJ
- Subjects
- Humans, Male, Monte Carlo Method, Photic Stimulation methods, Sensory Thresholds physiology, Signal Detection, Psychological, Vision Disparity physiology, Vision, Binocular physiology, Depth Perception physiology, Discrimination, Psychological physiology, Pattern Recognition, Visual physiology
- Abstract
With luminance gratings, psychophysical thresholds for detecting a small increase in the contrast of a weak 'pedestal' grating are 2-3 times lower than for detection of a grating when the pedestal is absent. This is the 'dipper effect'--a reliable improvement whose interpretation remains controversial. Analogies between luminance and depth (disparity) processing have attracted interest in the existence of a 'disparity dipper'. Are thresholds for disparity modulation (corrugated surfaces), facilitated by the presence of a weak disparity-modulated pedestal? We used a 14-bit greyscale to render small disparities accurately, and measured 2AFC discrimination thresholds for disparity modulation (0.3 or 0.6 c/deg) of a random texture at various pedestal levels. In the first experiment, a clear dipper was found. Thresholds were about 2x lower with weak pedestals than without. But here the phase of modulation (0 or 180 deg) was varied from trial to trial. In a noisy signal-detection framework, this creates uncertainty that is reduced by the pedestal, which thus improves performance. When the uncertainty was eliminated by keeping phase constant within sessions, the dipper effect was weak or absent. Monte Carlo simulations showed that the influence of uncertainty could account well for the results of both experiments. A corollary is that the visual depth response to small disparities is probably linear, with no threshold-like nonlinearity.
- Published
- 2008
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36. A tale of two agnosias: distinctions between form and integrative agnosia.
- Author
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Riddoch MJ, Humphreys GW, Akhtar N, Allen H, Bracewell RM, and Schofield AJ
- Subjects
- Aged, 80 and over, Agnosia physiopathology, Agnosia psychology, Attention physiology, Brain Mapping, Contrast Sensitivity physiology, Depth Perception physiology, Discrimination Learning, Dyslexia diagnosis, Dyslexia physiopathology, Dyslexia psychology, Female, Field Dependence-Independence, Humans, Magnetic Resonance Imaging, Male, Middle Aged, Neuropsychological Tests, Pattern Recognition, Visual physiology, Prosopagnosia diagnosis, Prosopagnosia physiopathology, Prosopagnosia psychology, Psychomotor Performance physiology, Reaction Time physiology, Size Perception physiology, Stroke complications, Stroke physiopathology, Visual Acuity physiology, Visual Cortex physiopathology, Visual Pathways physiopathology, Agnosia diagnosis
- Abstract
The performance of two patients with visual agnosia was compared across a number of tests examining visual processing. The patients were distinguished by having dorsal and medial ventral extrastriate lesions. While inanimate objects were disadvantaged for the patient with a dorsal extrastriate lesion, animate items are disadvantaged for the patient with the medial ventral extrastriate lesion. The patients also showed contrasting patterns of performance on the Navon Test: The patient with a dorsal extrastriate lesion demonstrated a local bias while the patient with a medial ventral extrastriate lesion had a global bias. We propose that the dorsal and medial ventral visual pathways may be characterized at an extrastriate level by differences in local relative to more global visual processing and that this can link to visually based category-specific deficits in processing.
- Published
- 2008
- Full Text
- View/download PDF
37. Asymmetric transfer of the dynamic motion aftereffect between first- and second-order cues and among different second-order cues.
- Author
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Schofield AJ, Ledgeway T, and Hutchinson CV
- Subjects
- Adult, Contrast Sensitivity, Discrimination, Psychological, Female, Humans, Light, Male, Orientation, Photic Stimulation methods, Space Perception, Time Perception, Visual Perception, Cues, Figural Aftereffect physiology, Motion Perception physiology
- Abstract
Recent work on motion processing has suggested a distinction between first-order cues (such as luminance modulation [LM]) and second-order cues (such as local contrast modulation [CM]). We studied interactions between moving LM, CM, and orientation modulation (OM) first comparing their spatial- and temporal-frequency sensitivity. We then tested for the transfer of the dynamic motion aftereffect (dMAE) between the three cues, matched for visibility. Observers adapted to moving, 0.5-c/deg horizontal modulations for 2 min (with 10 s top-ups). Relatively strong dMAEs were found when the adaptation and test patterns were defined by the same cue (i.e., both LM, both CM, or both OM); these effects were tuned for spatial frequency in the case of LM and CM. There was a partial transfer of the dMAE from LM to CM and OM; this transferred effect seemed to lose its tuning. The aftereffect transferred well from CM to OM and retained its tuning. There was little or no transfer from CM to LM or from OM to CM or LM. This asymmetric transfer of the dMAE between first- and second-order cues and between the second-order cues suggests some degree of separation between the mechanisms that process them.
- Published
- 2007
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38. Physics. Two for the price of one.
- Author
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Schofield AJ
- Published
- 2007
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39. Local luminance amplitude modulates the interpretation of shape-from-shading in textured surfaces.
- Author
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Schofield AJ, Hesse G, Rock PB, and Georgeson MA
- Subjects
- Contrast Sensitivity physiology, Cues, Humans, Pattern Recognition, Visual physiology, Photic Stimulation methods, Scattering, Radiation, Form Perception physiology, Lighting
- Abstract
The pattern of illumination on an undulating surface can be used to infer its 3-D form (shape-from-shading). But the recovery of shape would be invalid if the luminance changes actually arose from changes in reflectance. So how does vision distinguish variation in illumination from variation in reflectance to avoid illusory depth? When a corrugated surface is painted with an albedo texture, the variation in local mean luminance (LM) due to shading is accompanied by a similar modulation in local luminance amplitude (AM). This is not so for reflectance variation, nor for roughly textured surfaces. We used depth mapping and paired comparison methods to show that modulations of local luminance amplitude play a role in the interpretation of shape-from-shading. The shape-from-shading percept was enhanced when LM and AM co-varied (in-phase) and was disrupted when they were out of phase or (to a lesser degree) when AM was absent. The perceptual differences between cue types (in-phase vs out-of-phase) were enhanced when the two cues were present at different orientations within a single image. Our results suggest that when LM and AM co-vary (in-phase) this indicates that the source of variation is illumination (caused by undulations of the surface), rather than surface reflectance. Hence, the congruence of LM and AM is a cue that supports a shape-from-shading interpretation.
- Published
- 2006
- Full Text
- View/download PDF
40. Oscillatory motion induces change blindness.
- Author
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Schofield AJ, Bishop NJ, and Allan J
- Subjects
- Adolescent, Adult, Analysis of Variance, Female, Humans, Male, Models, Psychological, Motion Perception, Psychophysiology, Reaction Time, Vibration, Eye Movements, Motion, Perceptual Masking physiology, Recognition, Psychology, Visual Perception physiology
- Abstract
Change blindness is the relative inability of normally sighted observers to detect large changes in scenes when the low-level signals associated with those changes are either masked or of extremely low magnitude. Change detection can be inhibited by saccadic eye movements, artificial saccades or blinks, and 'mud splashes'. We now show that change detection is also inhibited by whole image motion in the form of sinusoidal oscillations. The degree of disruption depends upon the frequency of oscillation, which at 3 Hz is equivalent to that produced by artificial blinks. Image motion causes the retinal image to be blurred and this is known to affect object recognition. However, our results are inconsistent with good change detection followed by a delay due to poor recognition of the changing object. Oscillatory motion can induce eye movements that potentially mask or inhibit the low-level signals related to changes in the scene, but we show that eye movements promote rather than inhibit change detection when the image is moving.
- Published
- 2006
- Full Text
- View/download PDF
41. Breakdown of weak-field magnetotransport at a metallic quantum critical point.
- Author
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Fenton J and Schofield AJ
- Abstract
We show how the collapse of an energy scale in a quantum critical metal can lead to physics beyond the weak-field limit usually used to compute transport quantities. For a density-wave transition we show that the presence of a finite magnetic field at the critical point leads to discontinuities in the transport coefficients as temperature tends to zero. The origin of these discontinuities lies in the breakdown of the weak-field Jones-Zener expansion which has previously been used to argue that magnetotransport coefficients are continuous at simple quantum critical points. The presence of potential scattering and magnetic breakdown rounds the discontinuities over a window determined by tauDelta < 1 where Delta is the order parameter and tau is the quasiparticle elastic lifetime.
- Published
- 2005
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42. Quantum criticality.
- Author
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Coleman P and Schofield AJ
- Abstract
As we mark the centenary of Albert Einstein's seminal contribution to both quantum mechanics and special relativity, we approach another anniversary--that of Einstein's foundation of the quantum theory of solids. But 100 years on, the same experimental measurement that puzzled Einstein and his contemporaries is forcing us to question our understanding of how quantum matter transforms at ultra-low temperatures.
- Published
- 2005
- Full Text
- View/download PDF
43. Transfer of tilt after-effects between second-order cues.
- Author
-
Cruickshank AG and Schofield AJ
- Subjects
- Humans, Posture physiology, Adaptation, Ocular physiology, Contrast Sensitivity physiology, Cues, Figural Aftereffect physiology, Orientation
- Abstract
Second-order cues are visual stimuli that are detectable by human observers, without eliciting a peak in Fourier energy that corresponds to their perceptual properties. The most commonly studied exemplars of second-order cues are those defined by modulation of local contrast (CM). It is widely accepted that such cues are initially detected separately from first-order, luminance modulated (LM), cues. However, after-effects have been shown to transfer between first- and second-order cues (LM and CM, respectively). This suggests the existence of a late link in the mechanisms that subserve their processing. To extend the investigation of the mechanisms for processing second-order cues we consider cues defined by modulations in local orientation (OM). Using a tilt-after-effect (TAE) paradigm, we found partial transfer of adaptation between LM and OM cues, confirming the presence of a link between first and second-order cues. Furthermore, we found a partial transfer of TAE between OM and CM cues. These results suggest that, at or before the site of adaptation, information from all visual cues is combined. However, as transfer of adaptation is below 100% in all cases, this is only a partial integration of information.
- Published
- 2005
- Full Text
- View/download PDF
44. Interactions between orientation and contrast modulations suggest limited cross-cue linkage.
- Author
-
Schofield AJ and Yates TA
- Subjects
- Computational Biology, Contrast Sensitivity, Humans, Pattern Recognition, Visual, Perceptual Masking, Psychophysics, Sensory Thresholds, Cues, Form Perception physiology, Models, Psychological, Orientation
- Abstract
Recent studies of texture segmentation and second-order vision have proposed very similar models for the detection of orientation modulation and contrast modulation (OM and CM). From the similarity of the models it is tempting to assume that the two cues might be processed by a single generalised texture mechanism; however, recent results (Kingdom et al, 2003 Visual Neuroscience 2 65-76) have suggested that these cues are detected independently, or at least in a mechanism that is able to maintain an apparent independence between the cues. We tested new combinations of OM and CM and found that CM at 0.4 cycle deg(-1) facilitates the detection of OM at 0.2 cycle deg(-1) when the peaks of contrast align with the extremes of orientation. There is also some evidence of weak facilitation of CM by OM under the same conditions. Further, this facilitation can be predicted by filter--rectify--filter channels optimised for the detection of each cue, adding weight to the argument that texture cues are processed in a single generalised mechanism that nonetheless achieves cue independence or near-independence in many circumstances. We also found that the amount of suprathreshold masking produced by an orientation cue depends on the overall percept formed by that cue.
- Published
- 2005
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- View/download PDF
45. Disorder-sensitive phase formation linked to metamagnetic quantum criticality.
- Author
-
Grigera SA, Gegenwart P, Borzi RA, Weickert F, Schofield AJ, Perry RS, Tayama T, Sakakibara T, Maeno Y, Green AG, and Mackenzie AP
- Abstract
Condensed systems of strongly interacting electrons are ideal for the study of quantum complexity. It has become possible to promote the formation of new quantum phases by explicitly tuning systems toward special low-temperature quantum critical points. So far, the clearest examples have been appearances of superconductivity near pressure-tuned antiferromagnetic quantum critical points. We present experimental evidence for the formation of a nonsuperconducting phase in the vicinity of a magnetic field-tuned quantum critical point in ultrapure crystals of the ruthenate metal Sr3Ru2O7, and we discuss the possibility that the observed phase is due to a spin-dependent symmetry-breaking Fermi surface distortion.
- Published
- 2004
- Full Text
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46. Ferromagnetic superconductivity driven by changing Fermi surface topology.
- Author
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Sandeman KG, Lonzarich GG, and Schofield AJ
- Abstract
We introduce a simple but powerful zero temperature Stoner model to explain the unusual phase dia-gram of the ferromagnetic superconductor, UGe2. Triplet superconductivity is driven in the ferromagnetic phase by tuning the majority spin Fermi level through one of two peaks in the paramagnetic density of states (DOS). Each peak is associated with a metamagnetic jump in magnetization. The twin-peak DOS may be derived from a tight-binding, quasi-one-dimensional band structure, inspired by previous band-structure calculations.
- Published
- 2003
- Full Text
- View/download PDF
47. Sensitivity to contrast modulation: the spatial frequency dependence of second-order vision.
- Author
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Schofield AJ and Georgeson MA
- Subjects
- Artifacts, Humans, Lighting, Male, Motion Perception, Perceptual Masking, Photic Stimulation methods, Psychophysics, Contrast Sensitivity
- Abstract
We consider the overall shape of the second-order modulation sensitivity function (MSF). Because second-order modulations of local contrast or orientation require a carrier signal, it is necessary to evaluate modulation sensitivity against a variety of carriers before reaching a general conclusion about second-order sensitivity. Here we present second-order sensitivity functions for new carrier types (low pass (1/f) noise, and high pass noise) and demonstrate that, when first-order artefacts have been accounted for, the shape of the resulting MSFs are similar to one another and to those for white and broad band noise. They are all low pass with a likely upper frequency limit in the range 10-20 c/deg, suggesting that detection of second-order stimuli is relatively insensitive to the structure of the carrier signal. This result contrasts strongly with that found for (first-order) luminance modulations of the same noise types. Here the noise acts as mask and each noise type masks most those frequencies that are dominant in its spectrum. Thus the shape of second-order MSFs are largely independent of the spectrum of their noise carrier, but first-order CSFs depend on the spectrum of an additive noise mask. This provides further evidence for the separation of first- and second-order vision and characterises second-order vision as a low pass mechanism.
- Published
- 2003
- Full Text
- View/download PDF
48. Metamagnetic quantum criticality in metals.
- Author
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Millis AJ, Schofield AJ, Lonzarich GG, and Grigera SA
- Abstract
We present a renormalization group treatment of metamagnetic quantum criticality in metals. We show that for clean systems the universality class is that of the overdamped, conserving (dynamical exponent z = 3) Ising type. We obtain detailed results for the field and temperature dependence of physical quantities including the differential susceptibility, resistivity, and specific heat. Our results are shown to be in quantitative agreement with data on Sr3Ru2O7 except very near to the critical point itself.
- Published
- 2002
- Full Text
- View/download PDF
49. Sensitivity to disorder of the metallic state in the ruthenates.
- Author
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Capogna L, Mackenzie AP, Perry RS, Grigera SA, Galvin LM, Raychaudhuri P, Schofield AJ, Alexander CS, Cao G, Julian SR, and Maeno Y
- Abstract
We report the results of transport measurements on SrRuO3, Sr3Ru2O7, and CaRuO3. In SrRuO3 and Sr3Ru2O7, our findings are consistent with the predictions of Fermi liquid theory, in contrast to previous reports based on samples with much shorter mean free paths. In CaRuO3, however, a T1.5 power law is seen in the resistivity in the high purity samples studied here. Our work gives concrete evidence that even the metallic state of the ruthenates is highly sensitive to disorder.
- Published
- 2002
- Full Text
- View/download PDF
50. Shading and texture: separate information channels with a common adaptation mechanism?
- Author
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Georgeson MA and Schofield AJ
- Subjects
- Figural Aftereffect physiology, Humans, Motion Perception, Photic Stimulation methods, Vision, Binocular, Adaptation, Ocular physiology, Contrast Sensitivity physiology, Pattern Recognition, Visual physiology
- Abstract
We outline a scheme for the way in which early vision may handle information about shading (luminance modulation, LM) and texture (contrast modulation, CM). Previous work on the detection of gratings has found no sub-threshold summation, and no cross-adaptation, between LM and CM patterns. This strongly implied separate channels for the detection of LM and CM structure. However, we now report experiments in which adapting to LM (or CM) gratings creates tilt aftereffects of similar magnitude on both LM and CM test gratings, and reduces the perceived strength (modulation depth) of LM and CM gratings to a similar extent. This transfer of aftereffects between LM and CM might suggest a second stage of processing at which LM and CM information is integrated. The nature of this integration, however, is unclear and several simple predictions are not fulfilled. Firstly, one might expect the integration stage to lose identity information about whether the pattern was LM or CM. We show instead that the identity of barely detectable LM and CM patterns is not lost. Secondly, when LM and CM gratings are combined in-phase or out-of-phase we find no evidence for cancellation, nor for 'phase-blindness'. These results suggest that information about LM and CM is not pooled or merged--shading is not confused with texture variation. We suggest that LM and CM signals are carried by separate channels, but they share a common adaptation mechanism that accounts for the almost complete transfer of perceptual aftereffects.
- Published
- 2002
- Full Text
- View/download PDF
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