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Recent studies have confirmed that monovision treatment degrades stereopsis but it is not clear if these effects are limited to fine disparity processing, or how they are affected by viewing distance or age. Given the link between stereopsis and postural stability, it is important that we have full understanding of the impact of monovision on binocular function. In this study we assessed the short-term effects of optically induced monovision on a depth-discrimination task for young and older (presbyopic) adults. In separate sessions, the upper limits of stereopsis were assessed with participants' best optical correction and with monovision (-1D and +1D lenses in front of the dominant and non-dominant eyes respectively), at both near (62 cm) and far (300 cm) viewing distances. Monovision viewing resulted in significant reductions in the upper limit of stereopsis or more generally in discrimination performance at large disparities, in both age groups at a viewing distance of 300 cm. Dynamic photorefraction performed on a sample of four young observers revealed that they tended to accommodate to minimize blur in one eye at the expense of blur in the other. Older participants would have experienced roughly equivalent blur in the two eyes. Despite this difference, both groups displayed similar detrimental effects of monovision. In addition, we find that discrimination accuracy was worse with monovision at the 3 m viewing distance which involves fixation distances that are typical during walking. These data suggest that stability during locomotion may be compromised, a factor that is of concern for our older participants.

Background Flickering light has been shown to sensitize the migraine visual system at high stimulus contrast while elevating thresholds at low contrast. The present study employs a dichoptic psychophysical paradigm to ask whether the abnormal adaptation to flicker in migraine occurs before or after the binocular combination of inputs from the two eyes in the visual cortex. Methods Following adaptation to high contrast flicker presented to one eye only, flicker contrast increment thresholds were measured in each eye separately using dichoptic viewing. Results Modest interocular transfer of adaptation was seen in both migraine and control groups at low contrast. Sensitization at high contrast in migraine relative to control participants was seen in the adapted eye only, and an unanticipated threshold elevation occurred in the non-adapted eye. Migraineurs also showed significantly lower aversion thresholds to full field flicker than control participants, but aversion scores and increment thresholds were not correlated. Conclusions The results are simulated with a three-stage neural model of adaptation that points to strong adaptation at monocular sites prior to binocular combination, and weaker adaptation at the level of cortical binocular neurons. The sensitization at high contrast in migraine is proposed to result from stronger adaptation of inhibitory neurons, which act as a monocular normalization pool.

Background: Patients with migraine are averse to certain visual stimuli, such as flicker and striped patterns that evoke paroxysmal EEG activity in patients with photosensitive epilepsy. Migraineurs demonstrate a hyper-responsiveness to such stimuli, and there is debate as to whether the aversion and hyper-responsiveness are due to a hyperexcitability of the cortex similar to that in patients with photosensitive epilepsy. In these patients grating patterns with certain spatial characteristics can be epileptogenic, depending critically on their movement. If the contours of the grating drift continually, the grating is not epileptogenic, but if the contours are static or if their direction is repeatedly and rapidly reversed so as to vibrate, the grating then becomes highly epileptogenic. Methods: We compared aversion to vibrating, drifting and static gratings in migraineurs and controls. The contrast of each grating was gradually increased, but only until the participant felt discomfort, so as to obtain a contrast threshold for aversion with minimal exposure. Results: Migraineurs had lower thresholds than the control group, indicating greater aversion. For both groups the threshold was higher (aversion was lower) for static than for both types of moving gratings. The drifting gratings were more aversive than the vibrating gratings when both groups were combined. Conclusion: The findings suggest that the aversion shown by migraineurs is not attributable to a cortical hyperexcitability similar to that in photosensitive epilepsy.

Migraine is a very common disorder occurring in 20% of women and 6% of men. Central neuronal hyperexcitability is proposed to be the putative basis for the physiological disturbances in migraine. Since there are no consistent structural disturbances in migraine, physiological and psychophysical studies have provided insight into the underlying mechanisms. This is a review of the neurophysiological studies which have provided an insight to migraine pathogenesis supporting the theory of hyperexcitability.

This study aimed to develop a clinical test of face perception which is applicable to a wide range of patients and can capture normal variability. The Caledonian face test utilises synthetic faces which combine simplicity with sufficient realism to permit individual identification. Face discrimination thresholds (i.e. minimum difference between faces required for accurate discrimination) were determined in an "odd-one-out" task. The difference between faces was controlled by an adaptive QUEST procedure. A broad range of face discrimination sensitivity was determined from a group (N=52) of young adults (mean 5.75%; SD 1.18; range 3.33-8.84%). The test is fast (3-4 min), repeatable (test-re-test r(2)=0.795) and demonstrates a significant inversion effect. The potential to identify impairments of face discrimination was evaluated by testing LM who reported a lifelong difficulty with face perception. While LM's impairment for two established face tests was close to the criterion for significance (Z-scores of -2.20 and -2.27) for the Caledonian face test, her Z-score was -7.26, implying a more than threefold higher sensitivity. The new face test provides a quantifiable and repeatable assessment of face discrimination ability. The enhanced sensitivity suggests that the Caledonian face test may be capable of detecting more subtle impairments of face perception than available tests.

Spatial context can alter perceived shape, and temporal context can influence the perception of a stimulus. We sought to determine the time course of shape interactions by using a paradigm in which closed shape contours are laterally displaced over space and time. Target and masks are separated by various stimulus onset asynchrony (SOA) values, yielding forward, backward, and simultaneous masking conditions. Results indicate that spatial lateral interactions of shape are amplified by temporal asynchrony, reaching a peak at SOAs of 80–110ms. Mask amplitude scales all effects and masking is shape specific. When a single mask follows the target, both spatial configuration and mask onset transient are critical in determining depth of masking. When the target is followed by two sequential masks, the possibility of apparent motion determines whether one or both masks drive masking. These findings suggest that temporal interactions of shape are dependent on an interactive combination of shape specificity and transients, that apparent motion plays a modulatory role, and that target shape is determined after a temporal window, not at its onset.

The perception of a stimulus can be impaired when presented in the context of a masking pattern. To determine the timing and the nature of face processing, the effect of various masks on the discriminability of faces was investigated. Results reveal a strong configural effect: the magnitude of masking depends on the similarity between mask and target. Masking is absent for non-face masks (noise, houses), modest for scrambled and inverted faces and strongest for upright faces, even when they differ in size, gender or viewpoint from the targets. This suggests an extra-striate location for the masking (possibly FFA). Reduced but significant masking for isolated face parts (internal features or head shape) is consistent with holistic computations in face perception. The duration over which a face mask can impair face discrimination (130ms) is markedly longer than previously assumed and is sufficient for iterative and feedback computations to be part of face processing.

In the primate visual system relatively complex patterns such as curved shapes are first represented at intermediate levels of the ventral pathway. Furthermore, there is now evidence for the existence of curvature population coding in primate V4. We sought to determine whether similar encoding occurs in the human visual system by using a context-dependent lateral masking paradigm. In this paradigm a central closed contour comprising the test pattern is masked by surrounding larger or smaller patterns with various configurations. Results indicate that test thresholds are not affected by a circular control mask, and that elevations are greatest when curvature extrema of the mask are aligned with those of the target. These lateral interactions extend over greater than 1° and are tuned for target shape. Masking increases with the number of local curvature extrema aligned with the target. Finally, masking persists when target and mask have orthogonal local orientations and increases with mask amplitude. These findings are incompatible with local orientation-selective interactions (V1-mediated) but are consistent with the existence of population codes based on curvature maxima at intermediate levels of processing (presumably V4) in human vision. The paradigm we introduce provides a new tool for evaluating the representation of complex percepts.

Humans are remarkably sensitive in detecting small deviations from circularity. In tasks involving discrimination between closed contours, either circular in shape or defined by sinusoidal modulations of the circle radius, human performance has been shown to be limited by global processing. We assessed the amount of global pooling for different pattern shapes (different radial modulation frequencies, RF) when circular deformation was restricted to a fraction of the contour. The results show that the improvement in performance depends on the modulation frequency (the pattern shape) when increasing the number of cycles of an RF pattern. Global processing only extends up to modulation frequencies between 5 and 10. For higher frequencies, performance can be predicted by probability summation. Position uncertainty cannot explain these effects. In a circumstance where global pooling exceeds probability summation (RF=5), we split the pattern up into five identical segments conserving the total amount of information presented. Thresholds are significantly affected by different global arrangements of these segments: (a) Occluding small parts of the pattern shows a significant effect on the position of occluders with performance lowest when gaps are placed at the points of maximum curvature. (b) Shifting segments away from the pattern centre (exploded condition) or displaying them out of concentric context (spiral condition) shuts down global processing. (c) Jittering segments radially disrupts both global and local processing. We conclude that RF patterns in the global processing range are analysed by detecting the points of maximum curvature and that, in this range, the visual system can only reliably process up to about 5 local curvature extrema.

To compare the effects of early monocular versus early binocular deprivation on the perception of global form, we assessed sensitivity to global concentric structure in Glass patterns with varying ratios of paired signal dots to noise dots. Children who had been deprived by dense congenital cataracts in one (n=10) or both (n=8) eyes performed significantly worse than comparably aged children without eye problems. Consistent with previous results on sensitivity to global motion [Vision Research 42 (2002) 169], thresholds in the deprived eyes were significantly better after monocular deprivation than after binocular deprivation of comparable duration, even when there had been little patching of the nondeprived eye after monocular deprivation. Together, the results indicate that the competitive interactions between a deprived and nondeprived eye evident in the primary visual cortex can co-occur with complementary interactions in extrastriate cortex that enable a relative sparing of some visual functions after early monocular deprivation.

The well-known instances of reversing figures (e.g., Necker cube – Figure 1) have intrigued scientists and philosophers for generations. These images provide a route to observing the brain’s intrinsic competitive operations, mechanisms likely involved in choice and decision-making at many neural levels. At lower levels in the visual pathway, this can involve rivalry between the two eyes when their inputs are conflicting in ways that cannot be reconciled (e.g. vertical lines in one eye, horizontal lines in the other); at higher cortical levels it entails rivalry between two or more perceptual organizations or interpretations as in the Necker cube, the face/vase illusion and the young lady/old woman illusion. Measurements of the rate at which these perceptual switches occur, and the distribution of the intervals between switches has provided the basis for neural network models of the possible underlying brain mechanisms. While varied in detail, these models generally involve some form of reciprocal inhibition between the two organizations coupled with some component of adaptation that leads to the gradual weakening of inhibitory domination of one organization over the other (1). Figure 1 Necker cube. If the viewer fixates the dot, it will appear to lie either in the lower left corner of the back wall of the cube, or in the lower left corner of the closest side of the cube to the viewer, which faces slightly up and to the right. With extended ... The relevance of these phenomena to the study of migraine is that they provide one route to examining possible intrinsic differences in the balance between excitation and inhibition in the human neocortex, an issue which has attracted considerable attention in migraine. Abnormally weak inhibition, which has been suggested to occur in migraine (2), should lead to abnormally rapid reversals. In the first attempt to examine this, using binocular rivalry, reversal rates were marginally slower than normal, ruling out any loss of inhibition at least at this level (3). Other neuromodulatory influences on this basic switching network could also affect the reversal rate. The observation that patients with clinical anxiety show more rapid than normal reversal rates has been interpreted as supporting an abnormal modulatory role for serotonin (4). In the present issue, McKendrick et al. (5) have applied this experimental approach in a very ingenious fashion to ask two questions. The first is whether the trend toward slower oscillations seen in binocular rivalry (presumably occurring in striate cortex (V1)) is also present and perhaps even stronger in perceptual tasks known to depend on extrastriate regions higher up the visual cortical processing hierarchy. Secondly, they ask whether similar effects can be seen in a second sensory system – audition. In this, their work was motivated by the observation that electrophysiological abnormalities following habituation occur not only in vision, but also in audition and somatosensation (6). The two tasks chosen by McKendrick et al. (5) are well known in the visual and auditory fields respectively, but have rarely been examined in the same experimental subjects. The visual task involves two sets of overlaid dots moving in different directions (upward to the left and upward to the right). The two percepts one can experience are either of two transparent sheets of dots, one sliding over the other, moving in different directions, or alternatively, the whole field of dots appears glued together and moving as one along a new trajectory (in this case directly upward) that is different from the trajectories of either of the dot sets. The extraction of global motion direction is a well established property of the MT/MT+ complex (7). The auditory task is analogous, and was introduced by Bregman to study auditory stream segregation (8). A set of alternating high and low tones (high-low-high; high-low high) are heard either as a single stream with a galloping rhythm or as two separate streams of low notes and high notes playing in parallel. In both tasks, participants track the alternations between the two organizations by pressing one of two buttons depending on which organization is currently experienced. The results in the present paper are focused largely on length of the interval to the first reversal; however, similar results are seen for mean reversal time over the entire 30 s tested. As found earlier for binocular rivalry (3), intervals between these perceptual switches were longer in migraineurs than in controls and, in this case, a correlation was found between headache frequency and interval to first reversal: those with most frequent headaches show the greatest slowing in perceptual reversal, independent of their classification as migraine with or without aura. Furthermore, the visual and auditory results show a significant positive correlation lending stronger credence to the argument that this represents a global cortical abnormality affecting multiple systems similarly within an individual. The authors’ preferred explanation for their findings is that this reflects reduced levels of serotonin leading to lower cortical pre-activation interictally, a concept first introduced in the migraine field by Schoenen and his collaborators (6, 9). While this is an appealing notion, it is far from providing a detailed mechanistic understanding of the underlying processes involved. “Lower pre-activation” is a term that has not been well defined in the literature. It could mean a cortex that is maintained under an enhanced level of inhibition. It could reflect a cortex with reduced glutamate stores. Or it could mean a cortex, the excitability of which has been down-regulated in some way by one of the many neuromodulators acting upon it, the role suggested for serotonin (6). Serotonin has widespread and often apparently antagonistic effects in the nervous system. For example, in the lateral geniculate nucleus (the visual way-station in the thalamus), at least three synaptic roles for serotonin (5HT) have been described (10, 11), and the picture is similar in the cortex where location, density and type of 5HT receptors vary across cytoarchitectural areas. A recent study by Watakabe (12) has provided some fascinating insight into the complementary roles played by two 5HT receptor types in cortical area V1; the modulatory effect of serotonin is activity-dependent at both receptors but in opposite directions. Findings of this sort will provide a basis on which more detailed models may be built. A caveat is that most of our knowledge of serotonin’s actions at the cortical level of the visual pathway is restricted to V1, whereas the phenomena described in the present work certainly have their bases at a level beyond the primary visual and auditory cortices. As pointed out by the authors, one must also remember that the neocortex is innervated by multiple modulatory systems, and serotonin need not be the critical system at play in slowing perceptual switching in the case of migraine. Histamine, for example, exerts exert widespread effects across all levels of the visual cortical hierarchy (13). Another possibility was raised in a recent study using an ambiguous visual stimulus somewhat similar to McKendrick et al’s visual motion stimulus: perceptual switching may be mediated by a separate region of neocortex within the parietal lobe (14). Whether correlations between switching rate and parietal lobe structure will also be found to apply to auditory stimuli remains to be tested. The point is that a single cortical locus could act as the oscillator which modulates the activity of numerous cortical sensory regions through feedback pathways. In any event, the study in this issue by McKendrick et al. (5) provides a valuable new tool through which behavioral variation in migraine may eventually be linked to electrophysiological and/or structural changes in the brain.

In the classic oblique effect contrast detection thresholds, orientation discrimination thresholds, and other psychophysical measures are found to be smallest for vertical or horizontal stimuli and significantly higher for stimuli near the +/-45 degrees obliques. Here we report a novel inverse oblique effect in which thresholds for detecting translational structure in random dot patterns [Glass, L. (1969). Moiré effect from random dots. Nature, 223, 578-580] are lowest for obliquely oriented structure and higher for either horizontal or vertical structure. Area summation experiments provide evidence that this results from larger pooling areas for oblique orientations in these patterns. The results can be explained quantitatively by a model for complex cells in which the final filtering stage in a filter-rectify-filter sequence is of significantly larger area for oblique orientations.

There are two visual components to gaze: head orientation and orientation of the eyes relative to the head. This study explores the accuracy with which subjects can discriminate head orientation when the eyes are centered in the head. Discrimination thresholds averaged 1.9 degrees of head rotation for base head orientations of 0 degree and 15 degrees, but discrimination was markedly poorer around a 30 degrees head orientation. Results were independent of spatial frequency and size over a 4-fold range. Neither negative contrast nor head inversion affected discrimination. Experiments dissociating the internal features from head outline revealed the presence of two main cues to discrimination: deviation of the head profile from bilateral symmetry, and deviation of nose orientation from vertical. Simulations show that model V4 units revealed in previous experiments with Glass patterns can extract the relevant head orientation information. The data are consistent with neurological data indicating a selective loss of face recognition in prosopagnosia with spared gaze discrimination.

Certain periodic dot patterns (Marroquin patterns) generate a percept of dynamically oscillating circles, and analogous effects were explored by op artists in the 1960s. Here we show psychophysically that circles are perceived in these patterns only around specific points that are quantitatively predicted by a neural model of configural units hypothesized to reside in cortical area V4. Circles superimposed on the pattern mask perception of illusory circles. A neural model of lateral inhibitory interactions among V4 configural units showing spike-frequency adaptation quantitatively accounts for the human data. The model is consistent with ideas on the neural basis of attention in V4, and it suggests that attention may be biased via neuromodulation of slow hyperpolarizing potentials in cortical neurons.

Using a task which relied upon the detection of sinusoidal deformations from circularity, we show that strabismic amblyopes exhibit deficits which are not critically dependent on either the scale of deformation or the spatial frequency characteristics of the stimulus (circular D4) itself. We show that this loss is not due to the restricted passband of the amblyopic eye. Furthermore, in a pedestal distortion experiment, we show that the suprathreshold form of this loss is consistent with an elevated level of ‘intrinsic noise’ rather than a loss in ‘sampling efficiency’.

Casual observation suggests that when the elements of a visual array are packed at a sufficiently high density they cohere to generate the percept of a texture. This ‘texture-coherence limit’ has been quantified by using arrays composed of Gabor functions, sixth Gaussian derivatives, or differences of Gaussians. In all cases the texture-coherence limit was a power-law function of the size of the elements as quantified by their space constants with an exponent averaging 0.7. Furthermore, the texture-coherence limit was independent of both element spatial frequency and contrast over a considerable range. A quantitative fit to the data is provided by a model in which the texture-coherence limit is determined by activation of complex cells, which pool a spatial range of subunit inputs, throughout the stimulus region. Possible extensions to two dimensions are considered.

Psychophysical data demonstrate that orientation information in concentric, random-dot Glass patterns is summed linearly to extract a global form percept. Surprisingly, no such global pooling was found for Glass patterns with parallel structure. A simple neural model explains these results and agrees with recent V4 single unit physiology. As V4 provides the major input to IT, global concentric units may play an important role in analyzing complex images such as faces. In support of this possibility, deficits in the perception of concentric Glass patterns have recently been linked to prosopagnosia.

By the 1960s it was evident from neuroanatomy that there were extensive recurrent interactions, both excitatory and inhibitory, among visual cortical neurons. Nevertheless, the psychophysical discovery of ‘spatial-frequency channels’ gave rise to a decade in which parallel, independent channels were thought to subserve early spatial vision. Recent work, however, has clearly demonstrated that early visual channels do not perform a Fourier or wavelet decomposition of the image. Instead, they interact through a variety of nonlinear pooling mechanisms. Such nonlinear interactions perform important computations in texture perception, stereopsis, and motion and form vision.

Texture segmentation was studied developmentally in kittens using a two-alternative forced choice jumping stand paradigm. None of 15 kittens tested solved a texture segmentation task based on orientation contrast prior to 83 days of age, despite their rapid acquisition age, despite their rapid acquisition of an analogous luminance-based image segmentation problem. However, three kittens showed rapid acquisition of the texture segmentation task when the textures were composed of non-oriented elements (dots and annuli), reaching criterion performance by 52-59 days of age. A control experiment demonstrated that kittens can discriminate between vertical and horizontal gratings comparable in line width to the oriented texture elements as early as 53 days of age. The surprisingly late appearance of orientation-based texture segmentation is considered in the context of current models of texture segmentation and is compared to recent reports of a similar finding in human infants [3,54].

Periodic trajectories are an important component of biological motion. Or, Thabet, Wilkinson, and Wilson (2011) studied radial frequency (RF) motion trajectory detection and concluded that, for RF2-5 trajectories, the threshold function paralleled that of static RF patterns. We have extended Or et al.'s (2011) findings to a broader range of RFs (three to 24 cycles) and across a 4-fold range of radii (1°-4°). We report that (a) thresholds for RF trajectories decrease as a power function of RF for low RF trajectories (three to six cycles) before approaching an asymptote at high RFs (12-24 cycles); (b) detection thresholds for RF trajectories scale proportionally with radius; and (c) there is no lower versus upper field advantage in the parafoveal field for stimuli displaced from fixation on the vertical midline. The results are compared to earlier findings for static RF thresholds, and we argue that our findings support the existence of parallel spatial and temporal processing channels that may contribute to both action perception and production.

According to feature integration theory, conjunction search is conducted via a serial self-terminating search. However, effects attributed to search processes operating on the entire display may actually reflect search restricted to elements defined by a single feature. In experiment 1 this question is addressed in a reaction-time (RT) paradigm by varying distractor ratios within an array of fixed size. For trials in which the target was present in the array, RT functions were roughly symmetric, the shortest RTs being for extreme distractor ratios, and the longest RTs being for arrays in which there were an equal number of each distractor type. This result is superficially consistent with Zohary and Hochstein's interpretation that subjects search for only one distractor type and are able to switch search strategy from trial to trial. However, negative-trial data from experiment 1 cast doubt on this interpretation. In experiment 2 the possible role of ‘pop out’ and of distractor grouping in visual conjunction search is investigated. Results of experiment 2 suggest that grouping may play a more important role than does distractor ratio, and point to the importance of the spatial layout of the target and of the distractor elements in visual conjunction search. Results of experiment 2 also provide clear evidence that groups of spatially adjacent homogeneous elements may be processed as a unit.

Cortical hyperexcitability in migraine could arise from abnormally weak inhibition or from strengthened intracortical excitatory mechanisms. The present study employed binocular rivalry to differentiate between these possibilities. Rivalry between static oriented grating patterns was examined in migraine with aura (MA), migraine without aura (MoA) and headache-free control participants. A non-significant trend toward elevated mean dominance intervals (monocular percepts, in seconds) was seen in both migraine groups at all contrasts. Second, significant interocular differences in rivalry dominance durations were seen in the MoA group compared with controls; this difference also approached significance in the MA group. Finally, both MA and MoA exhibited significantly greater visual discomfort than the control group in the presence of both static stripes and flickering visual stimuli. The rivalry results provide no support for weakened intracortical inhibition in migraine. Optical or neural precortical differences in the eyes' input strengths paired with enhanced recurrent cortical excitation can explain these findings.

The ventral or form vision hierarchy comprises a sequence of cortical areas in which successively more complex visual attributes are extracted, beginning with contour orientations in V1 and culminating in face and object representations at the highest levels. In addition, ventral areas exhibit increasing receptive field diameter by a factor of approximately three from area to area, and conversely neuron density decreases. We argue here that this is consistent with configural combination of adjacent orientations to form curves or angles, followed by combination of these to form descriptions of object shapes. Substantial data from psychophysics, functional magnetic resonance imaging (fMRI), and neurophysiology support this organization, and computational models consistent with it have also been proposed. We further argue that a key to the role of the ventral stream is dimensionality reduction in object representations.

The purpose of this study was to examine basic ocular motor function in individuals with migraine. We used an infrared eye-tracking system to measure horizontal smooth pursuit to a sinusoidal target, saccades to horizontal target displacements of 5–20°, and the stability of fixation in 19 migraine without aura (MoA), 19 migraine with aura (MA) and 19 headache-free control (C) subjects. Eye movement measurements were made at two target displacement rates and against both homogeneous grey and patterned backgrounds. We found no statistically significant differences between migraine and control subjects in any of the eye movement parameters measured, but did find highly significant effects of both target speed and background pattern in all groups. Our results do not provide support for subclinical cerebellar impairment in migraineurs, and do provide evidence that previously described visual abnormalities in migraine are not artefacts of abnormal fixation or eye movements.

Mechanisms selective for complex shape are vulnerable to adaptation techniques historically used to probe those underlying performance in lower-level visual tasks. We explored the nature of these shape after-effects using radial frequency patterns. Adapting to a radial frequency pattern resulted in a strong and systematic after-effect of a pattern that was 180° out of phase with the adapting pattern. This after-effect was characterized as both a shift in the point of subjective equality and an increase in response uncertainty. The after-effect transferred across adapting pattern contrast and adaptor amplitude, suggesting an involvement from shape-specific mechanisms located at higher processing stages along the visual pathway. Moreover, our results suggested that the shift in the point of subjective equality was guided by global processing mechanisms, whereas the increase in uncertainty reflected activity from local processing mechanisms. Together, these results suggest that shape-specific after-effects reflect gain control processes at various stages of processing along the ventral pathway.

fMRI (functional magnetic resonance imaging) studies on humans have shown a cortical area, the fusiform face area, that is specialized for face processing. An important question is how faces are represented within this area. This study provides direct evidence for a representation in which individual faces are encoded by their direction (facial identity) and distance (distinctiveness) from a prototypical (mean) face. When facial geometry (head shape, hair line, internal feature size and placement) was varied, the fMRI signal increased with increasing distance from the mean face. Furthermore, adaptation of the fMRI signal showed that the same neural population responds to faces falling along single identity axes within this space.

Hallucinations in psychologically normal individuals provide a valuable route to studying the neural mechanisms of visual awareness. Migraine auras, epileptic auras and the hallucinations of Charles Bonnet Syndrome are examined in this context. Both similarities and striking differences in content are noted and the extent to which we are currently able to localize the source of these forms of endogenously driven visual awareness is discussed.

Hallucinations in psychologically normal individuals provide a valuable route to studying the neural mechanisms of visual awareness. Migraine auras, epileptic auras and the hallucinations of Charles Bonnet Syndrome are examined in this context. Both similarities and striking differences in content are noted and the extent to which we are currently able to localize the source of these forms of endogenously driven visual awareness is discussed.

Objective.—To measure the perceived rate of flicker (temporal frequency) observed during visual auras. Background.—The flickering or scintillating quality of aura elements is a commonly described characteristic of visual migraine auras. Hypotheses about the neural mechanisms involved in aura have rarely taken this feature into account, perhaps because of a lack of quantitative data on this aspect of the aura. While a rate of 10 Hertz had been suggested in the literature, estimates have been speculative due to the difficulty of judging temporal frequencies subjectively. Methods.—Eleven participants were given portable devices that contained an adjustable light-emitting diode with which to match the flickering of their auras. Observers were asked to make flicker matches at two time points so that rate change during aura progression could be analyzed. Results.—Data were obtained for 36 aura episodes. The mean rate of flicker across individuals was 17.8 Hertz. Rates varied widely between individuals, but were more consistent across multiple episodes in the same observer. Rate of flicker did not appear to relate to aura side or type, or to individual characteristics such as migraine history. When episodes were analyzed for change in flicker rate over time, patterns of increase (n = 7), decrease (n = 4), and no change (n = 22) were all observed. Conclusions.—When measured with an objective task, aura scintillation rates were found to be somewhat higher than previous anecdotal observations had suggested. These data are discussed in the context of two competing hypotheses concerning the neural mechanism underlying the flicker percept during migraine aura.

One of the major advances in neuroscience over the past 40 years has been the anatomical unravelling of the primate visual pathway, particularly its cortical components. From the vantage point of the year 2000, it is difficult to realize how little was known of the territory beyond the striate cortex (V1) when Hubel and Wiesel began their single unit exploration of primate V1 in 1968 (Hubel and Wiesel, 1968), and Gross described object specific neurons in inferotemporal cortex in 1969 (Gross, Bender, and Rocha-Miranda, 1969). This despite the fact that fully half of the macaque neocortex is in some way involved in visual function. However, by 1991 over two decades of intensive anatomical and electrophysiological investigation by many investigators culminated in the publication by Felleman and Van Essen of a comprehensive representation of the parallel and hierarchical relationships among the more than 30 cortical areas with visual functions in the macaque brain (Felleman and Van Essen, 1991).

The present study examined the extent to which migraineurs demonstrate interictal visual cortical hyperexcitability as a result of poor inhibitory control in the visual system. We employed a well-established psychophysical measure of inhibition, visual contrast gain control. The task involved detecting a briefly presented target that was superimposed on a highly excitable high contrast masking pattern. The strength of inhibition was assessed by comparing target detection thresholds with and without the operation of gain controls. Migraineurs with and without aura ( n = 25, n = 22, respectively) were compared with those with no history of migraine ( n = 25). Our results do not indicate a loss of inhibition in migraine; the strength of inhibitory feedback contrast gain controls was similar between migraineurs and controls. We did however, find a statistically greater masking effect in migraineurs compared with controls in the zero delay condition, suggesting cortical hyperexcitability in migraine. Possible mechanisms of cortical hyperexcitability are discussed in light of the results.

Orientation discrimination is a visual task dependent on inhibitory mechanisms in the visual cortex. In this study, orientation discrimination thresholds for bar and grating patterns were measured at two visual field locations in subjects with migraine with ( n = 20) and without aura ( n = 20) and in migraine-free control subjects ( n = 20). No statistically significant differences were found between migraine groups and the control group on either task at foveal or peripheral visual field locations. No significant correlations were found between psychophysical thresholds and age, total lifetime auras or total lifetime migraine episodes. However, a trend was seen toward slightly impaired performance on the two foveal tasks in a subgroup of subjects with the highest total lifetime aura count. Thus we have found no convincing evidence that impaired cortical inhibitory mechanisms are a predisposing characteristic in migraine, but cannot rule out the possibility that cortical inhibitory mechanisms may be adversely affected by repeated visual auras.

Modulation frequency and orientation tuning of second-order mechanisms underlying the detection of modulation in local spatial-frequency information are assessed by using an oblique-masking paradigm. Stimuli were Gabor-filtered noise patterns in which the local carrier spatial frequency was modulated about an average value of 4.7 cycles per degree (cpd) according to sinusoidal function. Thresholds were determined for spatial-frequency modulated test patterns (0.2 and 0.8 cpd) with fixed vertical carrier and modulation orientations presented alone and in the presence of spatiotemporally superimposed masks. Mask modulation frequency (0.1, 0.2, 0.4, 0.8, or 1.6 cpd), modulation orientation (0 degree, 45 degrees, or 90 degrees relative to vertical), and carrier orientation (18.5 degrees or 90 degrees relative to vertical) were manipulated independently while the mask modulation amplitude remained fixed at 0.25. Manipulating the modulation frequency of the mask revealed some modulation frequency specificity, particularly at lower test modulation frequencies. Spatial-frequency modulated masks produced threshold elevations regardless of the local carrier orientation. However, there was no evidence of threshold elevation when the mask modulation orientation was orthogonal to that of the test pattern. These results suggest a second-order texture mechanism that is tuned to both modulation frequency and modulation orientation but is not selective in terms of the orientation of first-order inputs.

We measured the apparent contrast and spatial frequency of a parafoveal Gabor signal located at the center of an array of Gabor signals as a function of both element density and the direction of contrast and spatial frequency of the surrounding elements. The target Gabor appeared lower in contrast and higher in spatial frequency when the elements were in close proximity, regardless of the direction of contrast and spatial frequency of the surrounding elements. Overall, the evidence suggests that the appearance of a parafoveal target is strongly affected by its visual context. These findings provide additional support for the existence of spatial interactions among neurons implicated in textural processing.

A horizontal array of vertically oriented Gabor elements was used to examine lateral masking in the near periphery (1.9 degrees-5.7 degrees eccentricity). Thresholds were assessed for detecting changes in the contrast, the spatial frequency, and the orientation of the central element within the array. The presence of surround elements induced marked threshold elevations that increased in strength as interelement spacing decreased and as retinal eccentricity increased. A model incorporating spatial summation by complex cells and reciprocal inhibition between simple and complex cells is shown to provide a quantitative fit to the data. This model suggests that complex cells analyze highly redundant textures, whereas simple cells function predominantly in the presence of isolated contours.

The ability of kittens (45–135 days of age) to segment images based on textural differences was examined using a two-alternative forced-choice procedure on the jumping stand. Tasks based on 3 textural cues—element size, element density and element orientation—were presented concurrently in a within-subject design. Texture segmentation based on element size appeared as early as 47 days of age, and segmentation based on element density as early as 57 days. In both cases, onset age varied with the specific stimulus parameters. Segmentation based on a 90 deg difference in element orientation did not appear until after 90 days and its time of appearance was independent of element size over a 2 octave range. For all segmentation cues, age was a more powerful determinant of when a task would be solved than was amount of training. The late onset of segmentation based on orientation, relative to other cues, closely parallels recent findings in human infants. This evidence of differences in developmental time course provides strong support for the idea that texture segmentation based on orientation differences does not share a common neural substrate with texture segmentation based on other visual cues.

Perception of periodic or closed-circuit motion trajectories plays a crucial role in our ability to learn and perform many common skilled actions. For example, periodic trajectories are a key component of many types of biological movements when viewed relative to body translation. In the current fMRI study, we used a novel visual stimulus consisting of a target moving along a closed trajectory defined by a radial frequency (RF) pattern (i.e., a sinusoidal variation of trajectory radius relative to a circular trajectory) to determine which brain regions encode these periodic movement paths. Multivoxel pattern analyses permitted prediction of the shapes of different periodic trajectories within regions V2 and V3 indicating that these regions play a role in the processing of periodic visual motion. In addition, blood oxygen level dependent (BOLD) responses associated with the presentation of targets moving along RF trajectories compared with nonperiodic motion and static RF shapes revealed significantly greater activity in visual areas V1, V2, V3, V3A, and V4. To our knowledge, the results of this study represent the first examination of the functional brain activity underlying periodic motion processing and should inform further study.