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Introduction Source localization of interictal epileptic discharges (IEDs) is clinically useful in the presurgical workup of epilepsy patients. Recently, we have demonstrated that distributed magnetic source imaging (dMSI) has better accuracy than clinically approved equivalent current dipole method (ECD). Here, we aimed to compare the performance of four different dMSI techniques: Minimum Norm Estimate (MNE), dynamic Statistical Parametric Mapping (dSPM), standardized Low-Resolution Electromagnetic Tomography (sLORETA) and coherent Maximum Entropy on the Mean (cMEM, an entropy-based technique). Methods We analyzed dMSI results of 206 IEDs derived from MEG recordings in 28 focal epilepsy patients who had a well-defined focus determined through intracranial EEG, epileptogenic MRI lesions or surgical resection. dMSI accuracy and spatial properties were quantitatively estimated as: (a) minimum distance between the source peak and the focus; (b) within-subject reproducibility; (c) spatial dispersion of the source map outside the focus; (d) extension of cortical map; (e) effect of thresholding on map size and properties. Results Distance between the map peak and epilepsy focus as well as within subject reproducibility were clinically comparable across methods (median distance from the focus around 1 cm). Spatial dispersion was significantly lower for cMEM. cMEM maps display typically higher contrast between the source maximum and surrounding regions, being therefore less sensitive to map thresholding. Conclusions All dMSI techniques under investigation provided excellent performance in localizing the epileptic focus. cMEM provides the lowest amount of spurious activity, while obtaining similar localization accuracy compared to other techniques. dMSI techniques currently available for clinical use have the potential to significantly improve identification of intracranial EEG targets and to guide surgical planning.

Background: The precise cortical origins of the electrophysiological and behavioral effects of transcranial magnetic stimulation (TMS) remain largely unclear. Addressing this question is further impeded by substantial inter-individual response variability to TMS. Objective: We present a novel method to reliably and user-independently determine the effectively stimulated cortical site at the individual subject level. This generic approach combines physiological measurements with electric field simulations and leverages information from random coil positions, electric field estimations, and electromyography. Methods: We applied ~1000 single biphasic TMS pulses with standard TMS hardware to 13 subjects with random coil positions & orientations over the primary motor hand area. Motor evoked potentials (MEPs) of three finger muscles were recorded concurrently. We calculated the corresponding electric fields for all TMS pulses and regressed them against the elicited MEPs in each cortical element. This yields a cortical map of congruency between induced field strength and generated response. Results: We observed high congruence between the electric fields and the elicited MEPs in hotspots located primarily on the crowns and rims of the precentral gyrus. The three cortical digit representations could be distinguished at the individual subject level with a high spatial resolution. A post-hoc convergence analysis revealed a possible lower bound of only 180 pulses to obtain qualitatively identical results. Conclusions: Leveraging information from many different TMS pulses significantly reduces the number of necessary stimulations and mapping time. The protocol is easy to implement due to the realization of arbitrary coil positions & orientations and is suitable for practical and clinical use such as preoperative mapping.

Hearing loss is the biggest risk factor for tinnitus, and hearing-loss-related pathological changes in the auditory pathway have been hypothesized as the mechanism underlying tinnitus. However, due to the comorbidity of tinnitus and hearing loss, it has been difficult to differentiate between neural correlates of tinnitus and consequences of hearing loss. In this study, we dissociated tinnitus and hearing loss in FVB mice, which exhibit robust resistance to tinnitus following monaural noise-induced hearing loss. Furthermore, knock-down of glutamate decarboxylase 65 (GAD65) expression in auditory cortex (AI) by RNA interference gave rise to tinnitus in normal-hearing FVB mice. We found that tinnitus was significantly correlated with downregulation of GAD65 in the AI. By contrast, cortical map distortions, which have been hypothesized as a mechanism underlying tinnitus, were correlated with hearing loss but not tinnitus. Our findings suggest new strategies for the rehabilitation of tinnitus and other phantom sensation, such as phantom pain.SIGNIFICANCE STATEMENTHearing loss is the biggest risk factor for tinnitus in humans. Most animal models of tinnitus also exhibit comorbid hearing loss, making it difficult to dissociate the mechanisms underlying tinnitus from mere consequences of hearing loss. Here we show that, although both C57BL/6 and FVB mice exhibited similar noise-induced hearing threshold increase, only C57BL/6, but not FVB, mice developed tinnitus following noise exposure. Although both strains showed frequency map reorganization following noise-induced hearing loss, only C57BL/6 mice had reduced glutamate decarboxylase 65 (GAD65) expression in the auditory cortex (AI). Knocking down GAD65 expression in the AI resulted in tinnitus in normal-hearing FVB mice. Our results suggest that reduced inhibitory neuronal function, but not sensory map reorganization, underlies noise-induced tinnitus.

Key points Partial sensory deprivation (deafferentation) by removing whiskers from the rat snout resulted in a reduced responsiveness of related cortical representations.Repetitive transcranial magnetic stimulation (three blocks of intermittent theta‐burst) applied for 5 days in combination with sensory exploration restored the normal responsiveness level of the deafferented barrel cortex.However, intracortical inhibition (lateral and recurrent) appeared to be reduced after repetitive transcranial magnetic stimulation, probably as the cause of improved responsiveness.Repetitive transcranial magnetic stimulation also reduced the asymmetry of the lateral spread of sensory activity. Abstract Repetitive transcranial magnetic stimulation (rTMS) modulates human cortical excitability. It has the potential to support recovery to normal cortical function when the excitation–inhibition balance is altered (e.g. after a stroke or loss of sensory input). We tested cortical map plasticity on the basis of sensory responses (local field potentials, LFPs) and expression of neuronal activity marker proteins within the barrel cortex of rats receiving either active or sham rTMS after selective unilateral deafferentation by whiskers plucking. Rats received daily rTMS [intermittent theta‐burst (iTBS), active or sham] for 5 days before exploring an enriched environment. Our previous studies indicated a disinhibitory effect of iTBS on cortical activity. Therefore, we also expected disinhibitory effects if deafferentation causes depression of sensory responses. Deafferentation resulted in an acute general reduction of sensory responsiveness and enhanced expression of inhibitory activity markers (GAD67, parvalbumin) in the deafferented hemisphere. Active but not sham‐iTBS‐rTMS normalized these measures. The stronger caudal‐to‐frontal horizontal spread of activity across barrels was reduced after deafferentation but not restored after active iTBS, despite generally increased responses. Fitting the LFP data with a computational model of different strengths and types of excitatory and inhibitory connections further revealed an iTBS‐induced reduction of lateral and recurrent inhibition as the most probable scenario. Whether the disinhibitory effect of iTBS for the restoration of normal cortical function in the acute phase of depression after deafferentiation is also beneficial in humans remains to be demonstrated. As recently discussed, disinhibition appears to be required to open a window for neuronal plasticity., Key points Partial sensory deprivation (deafferentation) by removing whiskers from the rat snout resulted in a reduced responsiveness of related cortical representations.Repetitive transcranial magnetic stimulation (three blocks of intermittent theta‐burst) applied for 5 days in combination with sensory exploration restored the normal responsiveness level of the deafferented barrel cortex.However, intracortical inhibition (lateral and recurrent) appeared to be reduced after repetitive transcranial magnetic stimulation, probably as the cause of improved responsiveness.Repetitive transcranial magnetic stimulation also reduced the asymmetry of the lateral spread of sensory activity.

Cortical demyelination occurs early in multiple sclerosis (MS) and relates to disease outcome. The brain cortex has endogenous propensity for remyelination as proven from histopathology study. In this study, we aimed at characterizing cortical microstructural abnormalities related to myelin content by applying a novel quantitative MRI technique in early MS. A combined myelin estimation (CME) cortical map was obtained from quantitative 7-Tesla (7T) T2* and T1 acquisitions in 25 patients with early MS and 19 healthy volunteers. Cortical lesions in MS patients were classified based on their myelin content by comparison with CME values in healthy controls as demyelinated, partially demyelinated, or non-demyelinated. At follow-up, we registered changes in cortical lesions as increased, decreased, or stable CME. Vertex-wise analysis compared cortical CME in the normal-appearing cortex in 25 MS patients vs. 19 healthy controls at baseline and investigated longitudinal changes at 1 year in 10 MS patients. Measurements from the neurite orientation dispersion and density imaging (NODDI) diffusion model were obtained to account for cortical neurite/dendrite loss at baseline and follow-up. Finally, CME maps were correlated with clinical metrics. CME was overall low in cortical lesions (p = 0.03) and several normal-appearing cortical areas (p < 0.05) in the absence of NODDI abnormalities. Individual cortical lesion analysis revealed, however, heterogeneous CME patterns from extensive to partial or absent demyelination. At follow-up, CME overall decreased in cortical lesions and non-lesioned cortex, with few areas showing an increase (p < 0.05). Cortical CME maps correlated with processing speed in several areas across the cortex. In conclusion, CME allows detection of cortical microstructural changes related to coexisting demyelination and remyelination since the early phases of MS, and shows to be more sensitive than NODDI and relates to cognitive performance.

BACKGROUND: Machine learning (ML) can distinguish cases with psychotic disorder from healthy controls based on magnetic resonance imaging (MRI) data, but it is not yet clear which MRI metrics are the most informative for case-control ML, or how ML algorithms relate to the underlying biology.METHODS: We analyzed multimodal MRI data from 2 independent case-control studies of psychotic disorders (cases, n = 65, 28; controls, n = 59, 80) and compared ML accuracy across 5 selected MRI metrics from 3 modalities. Cortical thickness, mean diffusivity, and fractional anisotropy were estimated at each of 308 cortical regions, as well as functional and structural connectivity between each pair of regions. Functional connectivity data were also used to classify nonpsychotic siblings of cases (n = 64) and to distinguish cases from controls in a third independent study (cases, n = 67; controls, n = 81).RESULTS: In both principal studies, the most informative metric was functional MRI connectivity: The areas under the receiver operating characteristic curve were 88% and 76%, respectively. The cortical map of diagnostic connectivity features (ML weights) was replicable between studies (r = .27, p < .001); correlated with replicable case-control differences in functional MRI degree centrality and with a prior cortical map of adolescent development of functional connectivity; predicted intermediate probabilities of psychosis in siblings; and was replicated in the third case-control study.CONCLUSIONS: ML most accurately distinguished cases from controls by a replicable pattern of functional MRI connectivity features, highlighting abnormal hubness of cortical nodes in an anatomical pattern consistent with the concept of psychosis as a disorder of network development.

Highlights • Cortical signatures selective to AD could provide an early MRI biomarker. • Autosomal dominant Alzheimer disease (ADAD) may model an ideal AD signature. • ADAD and late-onset maps overlap in parietal cortex but contain unique features. • Signatures predicted increasing amyloid within their own, but not across cohorts. • These results indicate atrophy in AD can take multiple spatial patterns., Defining a signature of cortical regions of interest preferentially affected by Alzheimer disease (AD) pathology may offer improved sensitivity to early AD compared to hippocampal volume or mesial temporal lobe alone. Since late-onset Alzheimer disease (LOAD) participants tend to have age-related comorbidities, the younger-onset age in autosomal dominant AD (ADAD) may provide a more idealized model of cortical thinning in AD. To test this, the goals of this study were to compare the degree of overlap between the ADAD and LOAD cortical thinning maps and to evaluate the ability of the ADAD cortical signature regions to predict early pathological changes in cognitively normal individuals. We defined and analyzed the LOAD cortical maps of cortical thickness in 588 participants from the Knight Alzheimer Disease Research Center (Knight ADRC) and the ADAD cortical maps in 269 participants from the Dominantly Inherited Alzheimer Network (DIAN) observational study. Both cohorts were divided into three groups: cognitively normal controls (nADRC = 381; nDIAN = 145), preclinical (nADRC = 153; nDIAN = 76), and cognitively impaired (nADRC = 54; nDIAN = 48). Both cohorts underwent clinical assessments, 3T MRI, and amyloid PET imaging with either 11C-Pittsburgh compound B or 18F-florbetapir. To generate cortical signature maps of cortical thickness, we performed a vertex-wise analysis between the cognitively normal controls and impaired groups within each cohort using six increasingly conservative statistical thresholds to determine significance. The optimal cortical map among the six statistical thresholds was determined from a receiver operating characteristic analysis testing the performance of each map in discriminating between the cognitively normal controls and preclinical groups. We then performed within-cohort and cross-cohort (e.g. ADAD maps evaluated in the Knight ADRC cohort) analyses to examine the sensitivity of the optimal cortical signature maps to the amyloid levels using only the cognitively normal individuals (cognitively normal controls and preclinical groups) in comparison to hippocampal volume. We found the optimal cortical signature maps were sensitive to early increases in amyloid for the asymptomatic individuals within their respective cohorts and were significant beyond the inclusion of hippocampus volume, but the cortical signature maps performed poorly when analyzing across cohorts. These results suggest the cortical signature maps are a useful MRI biomarker of early AD-related neurodegeneration in preclinical individuals and the pattern of decline differs between LOAD and ADAD.

Corporeal awareness of body unity, continuity, and integrity is hardwired in the brain, even following massive deafferentation. Following peripheral limb injury, referred phantom sensations are reported frequently on the cheek and, rarely, on the ear. Here, we explore how brain plasticity mechanisms induced by multisensory stimulation of different facial regions (cheek and ear) modulate the feeling that a complete missing limb is still attached to the body. We applied the modified rubber hand illusion (RHI) paradigm following synchronous and asynchronous stimulation of the face-hand and ear-hand in the unusual case of a patient with a brachial plexus lesion, who had lost upper-left limb sensation and developed a phantom sensation of the arm restricted to the ear. He experienced a strong illusion of ownership of the rubber hand during synchronous stroking of the ear but not the cheek and reported more defined tactile sensations in his previously numb body part during the illusion than when simply touching the ear. Phantom experiences are not exclusively based on sensory memories of the once-present body periphery, they are organized into a topographic cortical map with the ear-hand area adjoining but separate from the face. Multimodal experiences specifically modulate possible remapping of ear-hand representations and generate a more defined connection between the brain's memory of the body and what one feels of the actual physical body. We suggest that RHI is a form of sensory intervention that makes the best use of residual signals from disconnected body parts after peripheral injury, evoking and controlling the limb sensations.

Echolocating bats use echoes of their sonar emissions to determine the position and distance of objects or prey. Target distance is represented as a map of echo delay in the auditory cortex (AC) of bats. During a bat’s flight through a natural complex environment, echo streams are reflected from multiple objects along its flight path. Separating such complex streams of echoes or other sounds is a challenge for the auditory system of bats as well as other animals. We investigated the representation of multiple echo streams in the AC of anesthetized bats ( Phyllostomus discolor) and tested the hypothesis that neurons can lock on echoes from specific objects in a complex echo-acoustic pattern while the representation of surrounding objects is suppressed. We combined naturalistic pulse/echo sequences simulating a bat’s flight through a virtual acoustic space with extracellular recordings. Neurons could selectively lock on echoes from one object in complex echo streams originating from two different objects along a virtual flight path. The objects were processed sequentially in the order in which they were approached. Object selection depended on sequential changes of echo delay and amplitude, but not on absolute values. Furthermore, the detailed representation of the object echo delays in the cortical target range map was not fixed but could be dynamically adapted depending on the temporal pattern of sonar emission during target approach within a simulated flight sequence. NEW & NOTEWORTHY Complex signal analysis is a challenging task in sensory processing for all animals, particularly for bats because they use echolocation for navigation in darkness. Recent studies proposed that the bat’s perceptional system might organize complex echo-acoustic information into auditory streams, allowing it to track specific auditory objects during flight. We show that in the auditory cortex of bats, neurons can selectively respond to echo streams from specific objects.

BackgroundMachine learning (ML) can distinguish cases with psychotic disorder from healthy controls based on magnetic resonance imaging (MRI) data, with reported accuracy in the range 60-100%. It is not yet clear which MRI metrics are the most informative for case-control ML.MethodsWe analysed multi-modal MRI data from two independent case-control studies of patients with psychotic disorders (cases, N = 65, 28; controls, N = 59, 80) and compared ML accuracy across 5 MRI metrics. Cortical thickness, mean diffusivity and fractional anisotropy were estimated at each of 308 cortical regions, as well as functional and structural connectivity between each pair of regions. Functional connectivity data were also used to classify non-psychotic siblings of cases (N=64) and to distinguish cases from controls in a third independent study (cases, N=67; controls, N = 81).ResultsIn both principal studies, the most diagnostic metric was fMRI connectivity: the areas under the receiver operating characteristic curve were 92% and 77%, respectively. The cortical map of diagnostic connectivity features was replicable between studies (r = 0.31, P < 0.001); correlated with replicable case-control differences in fMRI degree centrality, and with prior cortical maps of aerobic glycolysis and adolescent development of functional connectivity; predicted intermediate probabilities of psychosis in siblings; and replicated in the third case-control study.ConclusionsML most accurately distinguished cases from controls by a replicable pattern of fMRI connectivity features, highlighting abnormal hubness of cortical nodes in an anatomical pattern consistent with the concept of psychosis as a disorder of network development.

During echolocation, bats continuously perform audio-motor adjustments to optimize detection efficiency. It has been demonstrated that bats adjust the amplitude of their biosonar vocalizations (known as 'pulses') to stabilize the amplitude of the returning echo. Here, we investigated this echo-level compensation behaviour by swinging mustached bats on a pendulum towards a reflective surface. In such a situation, the bats lower the amplitude of their emitted pulses to maintain the amplitude of incoming echoes at a constant level as they approach a target. We report that cortical auditory neurons that encode target distance have receptive fields that are optimized for dealing with echo-level compensation. In most cortical delay-tuned neurons, the echo amplitude eliciting the maximum response matches the echo amplitudes measured from the bats' biosonar vocalizations while they are swung in a pendulum. In addition, neurons tuned to short target distances are maximally responsive to low pulse amplitudes while neurons tuned to long target distances respond maximally to high pulse amplitudes. Our results suggest that bats dynamically adjust biosonar pulse amplitude to match the encoding of target range and to keep the amplitude of the returning echo within the bounds of the cortical map of echo delays.

In this mini-review I have proposed that there are two kinds of position sense, one a sense of the position of one part of the body relative to another, the other a sense of the location in space of our body and its limbs. A common method used to measure position sense is to ask subjects to match with one arm the position adopted by the other. Here all of the evidence points to muscle spindles as the major proprioceptors, with cutaneous receptors acting as proprioceptors providing a supporting role. Other senses such as vision do not play a major role. The sense of localisation in space measured by pointing to the arm, rather than matching its position, I propose, is not served by proprioceptors but by exteroceptors, vision, touch and hearing. Here the afferent input is relayed to sensory areas of the brain, to address the postural schema, a cortical map of the body and limbs, specifying its size and shape. It is here that spatial location is computed. This novel interpretation of position sense as two separate entities has the advantage of proposing new, future experiments and if it is supported by the findings, it will represent an important step forward in our understanding of the central processing of spatial information.

Repeatedly pairing vagus nerve stimulation (VNS) with a tone or movement drives highly specific and long-lasting plasticity in auditory or motor cortex, respectively. Based on this robust enhancement of plasticity, VNS paired with rehabilitative training has emerged as a potential therapy to improve recovery, even when delivered long after the neurological insult. Development of VNS delivery paradigms that reduce therapy duration and maximize efficacy would facilitate clinical translation. The goal of the current study was to determine whether primary auditory cortex (A1) plasticity can be generated more quickly by shortening the interval between VNS-tone pairing events or by delivering fewer VNS-tone pairing events. While shortening the inter-stimulus interval between VNS-tone pairing events resulted in significant A1 plasticity, reducing the number of VNS-tone pairing events failed to alter A1 responses. Additionally, shortening the inter-stimulus interval between VNS-tone pairing events failed to normalize neural and behavioral responses following acoustic trauma. Extending the interval between VNS-tone pairing events yielded comparable A1 frequency map plasticity to the standard protocol, but did so without increasing neural excitability. These results indicate that the duration of the VNS-event pairing session is an important parameter that can be adjusted to optimize neural plasticity for different clinical needs.

The system described in this paper combines virtual reality with haptic feedback to increase the level of immersion and invoke the sense of agency in patients with phantom limb pain with the aim of reducing perceived pain. This paper presents three case studies of an on-going clinical study. The initial results suggest an increased sense of embodiment of the virtual limb promotes a decrease in perceived levels of pain. The results strengthen the view that the cortical map does not fully “disappear” yet lays dormant.

Significance statementFunctional topography is present throughout the cerebral cortex, often in the form of columns or clusters of neurons with similar functional properties within identified cortical regions. Most of the evidence for these structures comes from work with non-human species. Using high-field strength magnetic resonance imaging in living human cortex, the search for this structure is frustrated by the presence of a background of local spatial correlations in the signal across the cortical surface. This structured background, which we term ‘neural dust’, is a form of noise that imposes a fundamental limit on the detection of clustering and topography. We apply a novel analysis approach that quantifies the background of spatial correlations, so that we achieve reliable identification of high-signal clusters of activation in the human cortex at a scale previously only visible with invasive optical imaging in animals. We searched specifically in visual cortex for correlates of binocular stereoscopic depth. We show both that signals for stereoscopic depth are clustered into specific zones within the cortex and that these signals occur within spatially extended cortical domains, which have similar preferences for the stereoscopic depth. The revealed structures are reliably identified in all subjects tested and in repeated testing of individual subjects. Our methods provide an objective approach for defining the size and locations of clusters of activation within functional images of neural activity.AbstractA characteristic principle of the organization of cerebral neocortex is the presence of clusters or columns of neurons with similar functional properties. Animals with forward-facing eyes exploit slight differences between the images in the two eyes to determine binocular depth using stereopsis. Evidence for an organized structure in human visual cortex for the representation of stereoscopic depth has proved elusive. Using 7-tesla functional MRI, with gradient-echo echo-planar imaging at 0.75 mm isotropic resolution and a novel analytical approach based on geospatial mapping methods, we find that clustered responses for disparity-defined depth can be clearly segregated from a background of spatially correlated signals in all subjects tested. High-signal clusters are associated with cortical domains as large as 12-15mm across the cortical surface, in which nearby points in the cortical map tend to respond to the same stereoscopic depth. These domains are found predominantly within visual cortical area V3A.

The purpose of this study was to determine the size and location of the representations of the anterior thigh muscles on the human motor cortex in the dominant and non-dominant hemispheres. Motor-evoked potentials (MEPs) induced by transcranial magnetic stimulation were recorded from the right and left vastus lateralis (rVL, lVL) muscles. A total of ten right-handed healthy volunteers participated in the study. In a single session experiment, we investigated VL muscle corticospinal excitability (motor threshold, MEP size, short interval intracortical inhibition, intracortical facilitation) and cortical representation (map area, volume, and location) in the dominant and non-dominant hemispheres. The motor threshold, MEPs, and intracortical excitability did not differ significantly between the hemispheres (P > 0.05). Furthermore, no difference between sides was found in the location of VL motor representation (mediolateral and anteroposterior axis) or in map area and volume (P > 0.05). Vastus lateralis muscle corticospinal excitability and cortical map were symmetrical in right-handed subjects. Future studies on patients with unilateral lower extremity injuries could examine side-to-side plastic reorganization in corticomotor output and map location in both hemispheres. Clin. Anat. 27:1053–1057, 2014. © 2014 Wiley Periodicals, Inc.

The EPIONE consortium aims to evaluate the effect of artificial sensory stimulation using an operant conditioning paradigm as a therapy for Phantom Limb Pain (PLP). A uniform therapy /assessment protocol was developed for a multi-center study to evaluate the effect of the therapy on their (1) Phantom Limb Pain, (2) psychological state and (3) sensory motor cortical map. Participating subjects complete several phases of assessment. Post hoc analyses will compare results from testing done before, during, and after therapy. Outcomes of the intervention aim to improve the current understanding of pain and the psychophysical effects of pain management, which will progress the pilot therapy program toward a fully developed PLP solution.

The relationships between structural and functional measures of the human brain remain largely unknown. A majority of our limited knowledge regarding structure–function associations has been obtained through comparisons between specific groups of patients and healthy controls. Unfortunately, a direct and complete view of the associations across multiple structural and functional metrics in normal population is missing. We filled this gap by learning cross-individual co-variance among structural and functional measures using large-scale neuroimaging datasets. A discover-confirm scheme was applied to two independent samples (N = 184 and N = 340) of multi-modal neuroimaging datasets. A data mining tool, gRAICAR, was employed in the discover stage to generate quantitative and unbiased hypotheses of the co-variance among six functional and six structural imaging metrics. These hypotheses were validated using an independent dataset in the confirm stage. Fifteen multi-metric co-variance units, representing different co-variance relationships among the 12 metrics, were reliable across the two sets of neuroimaging datasets. The reliable co-variance units were summarized into a database, where users can select any location on the cortical map of any metric to examine the co-varying maps with the other 11 metrics. This database characterized the six functional metrics based on their co-variance with structural metrics, and provided a detailed reference to connect previous findings using different metrics and to predict maps of unexamined metrics. Gender, age, and handedness were associated to the co-variance units, and a sub-study of schizophrenia demonstrated the usefulness of the co-variance database. Electronic supplementary material The online version of this article (doi:10.1007/s00429-015-1177-6) contains supplementary material, which is available to authorized users.

Phase-encoded designs advanced the early development of functional MRI, enabling the “killer app” of retinotopic mapping, which helped demonstrate fMRI's value to a skeptical scientific public. The design, also called “the traveling wave”, remains in wide use today, due to its ability to easily measure neural activity in a parameterized set of experimental conditions. In phase-encoded designs, stimuli defined by a numerical parameter, for example visual eccentricity, are presented continuously in the order specified by the parameter. The stimulus parameter that produces maximum response can be recovered from the timing of neural activity, i.e. its phase. From the outset, phase-encoded designs were used for two related, but complementary purposes: 1) to measure aggregate response properties of neurons in a voxel, for example the average visual field location of receptive fields, and 2) to segregate the set of voxels that corresponds to an organized cortical region, for example a retinotopically mapped visual area. This short review will cover the history and current uses of phase-encoded fMRI, while noting the ongoing tension in the field between the brain mapping and computational neuroimaging approaches.

To investigate motor cortical map patterns in children with diplegic and hemiplegic cerebral palsy (CP), and the relationships between motor cortical geometry and motor function in CP.Transcranial magnetic stimulation (TMS) was used to map motor cortical representations of the first dorsal interosseus (FDI) and tibialis anterior (TA) muscles in 13 children with CP (age 9-16 years, 6 males.) The Gross Motor Function Measure (GMFM) and Melbourne upper extremity function were used to quantify motor ability.In the hemiplegic participants (N = 7), the affected (right) FDI cortical representation was mapped on the ipsilateral (N = 4), contralateral (N = 2), or bilateral (N = 1) cortex. Participants with diplegia (N = 6) showed either bilateral (N = 2) or contralateral (N = 4) cortical hand maps. The FDI and TA motor map center-of-gravity mediolateral location ranged from 2-8 cm and 3-6 cm from the midline, respectively. Among diplegics, more lateral FDI representation locations were associated with lower Melbourne scores, i.e. worse hand motor function (Spearman's rho = -0.841, p = 0.036).Abnormalities in TMS-derived motor maps cut across the clinical classifications of hemiplegic and diplegic CP. The lateralization of the upper and lower extremity motor representation demonstrates reorganization after insults to the affected hemispheres of both diplegic and hemiplegic children.The current study is a step towards defining the relationship between changes in motor maps and functional impairments in CP. These results suggest the need for further work to develop improved classification schemes that integrate clinical, radiologic, and neurophysiologic measures in CP.

A striking feature of cortical organization is that the encoding of many stimulus features, for example orientation or direction selectivity, is arranged into topographic maps. Functional imaging methods such as optical imaging of intrinsic signals, voltage sensitive dye imaging or functional magnetic resonance imaging are important tools for studying the structure of cortical maps. As functional imaging measurements are usually noisy, statistical processing of the data is necessary to extract maps from the imaging data. We here present a probabilistic model of functional imaging data based on Gaussian processes. In comparison to conventional approaches, our model yields superior estimates of cortical maps from smaller amounts of data. In addition, we obtain quantitative uncertainty estimates, i.e. error bars on properties of the estimated map. We use our probabilistic model to study the coding properties of the map and the role of noise-correlations by decoding the stimulus from single trials of an imaging experiment.

Functional mapping of eloquent cortex is often necessary prior to invasive brain surgery, but current techniques that derive this mapping have important limitations. In this article, we demonstrate the first comprehensive evaluation of a rapid, robust, and practical mapping system that uses passive recordings of electrocorticographic signals. This mapping procedure is based on the BCI2000 and SIGFRIED technologies that we have been developing over the past several years. In our study, we evaluated 10 patients with epilepsy from four different institutions and compared the results of our procedure with the results derived using electrical cortical stimulation (ECS) mapping. The results show that our procedure derives a functional motor cortical map in only a few minutes. They also show a substantial concurrence with the results derived using ECS mapping. Specifically, compared with ECS maps, a next-neighbor evaluation showed no false negatives, and only 0.46 and 1.10% false positives for hand and tongue maps, respectively. In summary, we demonstrate the first comprehensive evaluation of a practical and robust mapping procedure that could become a new tool for planning of invasive brain surgeries.

The processing deficit in amblyopia is not restricted to just high spatial frequencies but also involves low‐medium spatial frequency processing, for suprathreshold stimuli with a broad orientational bandwidth. This is the case in all three forms of amblyopia; strabismic, anisometropic, and deprivation. Here we use both a random block design and a phase‐encoded design to ascertain (1) the extent to which fMRI activation is reduced at low‐mid spatial frequencies in different visual areas, (2) how accurately spatial frequency is mapped across the amblyopic cortex. We report a loss of function to suprathreshold low‐medium spatial frequency stimuli that involves more than just area V1, suggesting a diffuse loss in spatial frequency processing in a number of different cortical areas. An analysis of the fidelity of the spatial frequency cortical map reveals that many voxels lose their spatial frequency preference when driven by the amblyopic eye, suggesting a broader tuning for spatial frequency for neurons driven by the amblyopic eye within this low‐mid spatial frequency range. Hum Brain Mapp, 2009. © 2009 Wiley‐Liss, Inc.

Functional maps arise in developing visual cortex as response selectivities become organized into columnar patterns of population activity. Recent studies of developing orientation and direction maps indicate that both are sensitive to visual experience, but not to the same degree or duration. Direction maps have a greater dependence on early vision, while orientation maps remain sensitive to experience for a longer period of cortical maturation. There is also a darker side to experience: abnormal vision through closed lids produces severe impairments in neuronal selectivity, rendering these maps nearly undetectable. Thus, the rules that govern their formation and the construction of the underlying neural circuits are modulated-for better or worse-by early vision. Direction maps, and possibly maps of other properties that are dependent upon precise conjunctions of spatial and temporal signals, are most susceptible to the potential benefits and maladaptive consequences of early sensory experience.

In the adult visual cortex, multiple feature maps exist and have characteristic spatial relationships with one another. The relationships can be reproduced by “dimension-reduction” computational models, suggesting that the principles of continuity and coverage may underlie cortical map organization. However, the mechanisms responsible for establishing these relationships are unknown. We explored whether removing one feature map during development causes a coordinated reorganization of the remaining maps or whether the remaining maps are unaffected. We removed the ocular dominance map by monocular enucleation in newborn ferrets, so that single eye stimulation drove the cortex in a more spatially uniform manner in adult monocular animals compared with normal animals. Maps of orientation, spatial frequency, and retinotopy formed in monocular ferrets, but their structures and spatial relationships differed from those in normal ferrets. The wavelength of the orientation map increased, so that the average orientation gradient across the cortex decreased. The decrease in the orientation gradient in monocular animals was most prominent in the high gradient regions of the spatial frequency map, indicating a coordinated reorganization between these two maps. In monocular animals, the orthogonal relationship between the orientation and spatial frequency maps was preserved, and the orthogonal relationship between the orientation and retinotopic maps became more pronounced. These results were consistent with detailed predictions of a dimension-reduction model of cortical organization. Thus, the number of feature maps in a cortical area influences the relationships between them, and inputs to the cortex have a significant role in generating these relationships.

The primary somatosensory cortex contains a topographic map of the body surface, with two notable discontinuities-the representation of the face is next to that of the hands, and that of the feet is next to the genitals. Farah [Why does the somatosensory homunculus have hands next to face and feet next to genitals? a hypothesis. Neural Computation 10(8) (1998) 1983-1985] has suggested that these discontinuities are due to the mechanisms of self-organisation which underlie cortical map development. The typical position of the foetus in the womb means that these two pairs of body parts will often touch and hence their representations will be simultaneously co-active, even though they are distal in terms of the body surface. We use the Kohonen self-organising map algorithm to provide an existence proof of the plausibility of Farah's hypotheses. We then use the model to test the viability of other possible causes of the known map structure and to explore the limitations of self-organisation for explaining the features of the somatosensory map. The model shows that (a) the Kohonen algorithm requires high frequencies of co-activation to introduce a selective discontinuity into the map, and (b) that higher frequency of separate activation of the critical patterns alone is not sufficient to generate the selective discontinuity and (c) the consistency of near-optimal map formation, and in particular the medial-lateral ordering, cannot be reliably generated by a simple Kohonen algorithm.

Visual cortical neurons are sensitive to visual stimulus contrast and most cells adapt their sensitivity to the prevailing visual environment. Specifically, they match the steepest region of their contrast response function to the prevailing contrast (contrast gain control), and reduce spike rates to limit saturation (response gain control). Most neurons are also tuned for stimulus orientation, and neurons with similar orientation preference are clustered together into iso-orientation zones arranged around pinwheels, i.e. points where all orientations are represented. Here we investigated the relationship between the contrast adaptation properties of neurons and their location relative to pinwheels in the orientation preference map. We measured orientation preference maps in cat cortex using optical intrinsic signal imaging. We then characterized the contrast adaptation properties of single neurons located close to pinwheels, in iso-orientation zones, and at regions in between. We found little evidence of differential contrast sensitivity of neurons adapted to zero contrast. However, after adaptation to their preferred orientation at high contrast, changes in both contrast and response gain were greater for neurons near pinwheels compared with other map regions. Therefore, in the adapted state, which is probably typical during natural viewing, there is a spatial map of contrast sensitivity that is associated with the orientation preference map. This differential adaptation revealed a new dimension of cortical functional organization, linking the contrast adaptation of cells with the orientation preference of their nearest neighbours.

The mammalian visual system has been extensively studied since Hubel and Wiesel’s work on cortical feature maps in the 1960s. Feature maps representing the cortical neurons’ ocular dominance, orientation and direction preferences have been well explored experimentally and computationally. The predominant view has been that direction selectivity (DS) in particular, is a feature entirely dependent upon visual experience and as such does not exist prior to eye opening (EO). However, recent experimental work has shown that there is in fact a DS bias already present at EO. In the current work we use a computational model to reproduce the main results of this experimental work and show that the DS bias present at EO could arise purely from the cortical architecture without any explicit coding for DS and prior to any self-organising process facilitated by spontaneous activity or training. We explore how this latent DS (and its corresponding cortical map) is refined by training and that the time-course of development exhibits similar features to those seen in the experimental study. In particular we show that the specific cortical connectivity or ‘proto-architecture’ is required for DS to mature rapidly and correctly with visual experience.

Phase-encode mapping has been used as the primary functional magnetic resonance imaging (fMRI) technique to reveal spatial variation of response properties in both visual and auditory cortex (e.g., retinotopy and tonotopy). Spatial variation in the time of response produces a map of cortical sensitivity to a time-varying property of the presented stimulus. Inherently, this technique assumes that response latency is invariant across cortex. However, the latency of the fMRI response has been found to exhibit both temporal and spatial non-stationarity, with variance across cortex increasing with longer stimulus duration--a critical concern given the typically long duration (e.g., 48-64 s) of stimuli presented in phase-encode mapping experiments. This study addresses two issues--assessment of error rates caused by latency variance and reliability of maps identified using phase-encode mapping. Our findings suggest that the latency variance was found to have a greater effect on the type II error (missed detection) rate than on the type I error (false alarm) rate, with the size of the cortical map controlling the type I error rate. Moreover, empirical determination of false alarm rates provides the first approximation of the statistical significance (i.e., P values) of observed maps in both retinotopic and tonotopic studies, with maps as short as three voxels in length achieving the P < 0.05 significance level for a single subject.

While previous studies showed that intrinsic optical signals spatially correspond with electrophysiological responses in mammalian visual cortex, the quantitative correspondence of their response strengths is open to question. Measurement of both signals' strength as functions of visual stimulus contrast provides an opportunity for quantitative comparison. Towards that end, the spatial and temporal properties of the optical signal impose important constraints upon quantification of its strength. We used intrinsic optical signal imaging and single unit recording to measure responses to drifting gratings at contrasts ranging from 10-80% in cat area 18. We calculated the average difference images for pairs of oppositely moving, or orthogonally oriented, gratings at each contrast and evaluated three different methods for quantifying optical signal strength. After about 2.5 s, the spatial patterns of optical images and the time course of their strength were contrast-invariant. This "space-time-contrast separability" for optical response implies a spatial uniformity of the optical contrast response functions, provides an objective basis to guide temporal averaging of optical signals, and validates a scalar metric of optical signal strength. Optically measured contrast response functions increase monotonically and saturate at high contrasts, qualitatively resembling those from single units. However, quantitative comparison reveals a nonlinear relationship with neural firing, such that the optical response reaches half of its maximum when the neural response has reached only around 20% of its maximum. This relationship suggests that intrinsic optical signals are relatively more sensitive to weak signals than neural firing.

Exposure to sound of sufficient duration and level causes permanent damage to the peripheral auditory system, which results in the reorganization of the cortical tonotopic map. The changes are such that neurons with pre-exposure tuning to frequencies in the hearing loss range now become tuned to frequencies near the near-normal lower boundary of the hearing loss range, which thus becomes over represented. However, cats exposed to a traumatizing noise and immediately thereafter placed for a few weeks in an enriched acoustic environment presented a much-restricted hearing loss compared with similarly exposed cats that were placed for the same time in a quiet environment. The enriched environment spectrally matched the expected hearing loss range and was ∼40 dB above the level of the expected hearing loss. The hearing loss in the quiet environment-reared cats ranged from 6 to 32 kHz with the largest loss (on average, 40 dB) ranging from 24 to 32 kHz. In contrast, the hearing loss in the enriched-environment cats was restricted to 6-8 kHz at a level of, on average, 35 dB and with 16-32 kHz having normal thresholds. Despite the remaining hearing loss for the enriched-environment cats in the 6-8 kHz range, plastic tonotopic map changes in primary auditory cortex could no longer be demonstrated, suggesting that the enriched acoustic environment prevents this reorganization. This finding has implications for the treatment of hearing disorders, such as tinnitus, that have been linked to cortical tonotopic map reorganization.

The goal of this study was to determine whether transcranial magnetic stimulation (TMS) measures of the corticomotor pathways might reflect and predict functional improvements during rehabilitation of a hemiparetic upper extremity. Ten patients with variable levels of functional impairment in upper extremity use were enrolled at least 3 months after a stroke. TMS was used to obtain serial measures of motor thresholds and motor-evoked potential (MEP) size for a muscle from both hands before and after each session of an intervention aimed at improving functional motor control. Functional ability and cortical map area and volume were measured before each therapy session. At intake, all TMS measures from the affected side were impaired compared with the unaffected side but they did not reliably predict the level of functional improvement. Motor thresholds decreased, whereas MEP amplitude and map size increased with treatment. The amount of change in affected side measures was correlated with the amount of improvement in hand/arm function. Normalization of MEP amplitude asymmetry in response to the 1st 2 therapy sessions predicted long-term improvement in Fugl-Meyer motor score. Within limits, TMS appears to be useful in both moderate and more impaired patients as a physiological assay of treatment-induced plasticity and behavioral gains. Methodological differences in the literature, however, currently obscure a full understanding of the potential contributions of TMS to rehabilitation research.

The recovery of coordinated motor function after stroke onset has been associated with the practice of upper limb movements that required the activation of homologous muscles. This pilot study investigated whether repetitive bimanual coordinated movements enhanced upper limb corticomotor (CM) excitability and motor function poststroke. Patients practiced driving their paretic wrist through passive rhythmical flexion-extension by active flexion-extension of their unaffected wrist using purpose-built manipulanda over a 4-week period. Both preintervention and postintervention motricity was assessed using the upper limb Fugl-Meyer rating scale, and cortical maps of wrist flexor and extensor representations were derived from potentials evoked by transcranial magnetic stimulation. Five of nine subjects improved upper limb motricity in response to this novel active-passive bimanual movement therapy (APBT). Unaffected cortical map volume decreased, especially for a subgroup of five patients who had a postintervention increase in motricity. No change in unaffected map volume was revealed for the four patients who did not improve their postintervention motricity. No consistent shifts in cortical map center of gravity were revealed. These findings suggest that APBT can initiate an improvement in motricity that is accompanied by a balancing of between-hemisphere CM excitability. The findings justify the assessment of the rehabilitative effects of APBT in a homogeneous sample of patients poststroke.

Jaw-deviation dystonia is characterized by the lateral shift of the mandible due to involuntary masticatory muscle contraction, causing difficulties in speech or mastication. We evaluated a patient with jaw-deviation dystonia by recording movement-related cortical potentials (MRCPs) and treated with muscle afferent block (MAB). MRCP associated with mandibular movements (mouth opening, closing, and left and right lateral movements) was recorded in the patient and ten age-matched healthy subjects. In the control subjects, the amplitude of Bereitschaftspotential (BP)/negative slope (NS') was significantly higher for left lateral movements than for the mouth closing. The cortical map of BP/NS' prior to mouth opening and closing showed symmetric distribution, whereas those of lateral movements showed a slight predominance in the ipsilateral hemisphere. The patient showed lower amplitude as compared with control subjects. The right lateral movement (homonymous task) showed task-specific markedly reduced potentials. After MAB by intramuscular injection of lidocaine and ethanol to the inferior head of the left lateral pterygoid muscle, the deviation abolished and severity in speech and mastication was significantly improved. This study suggests that jaw-deviation dystonia might have the same etiology as other focal dystonias.

Using a delay adaptation learning rule, we model the activity-dependent development of directionally selective cells in the primary visual cortex. Based on input stimuli, a learning rule shifts delays to create synchronous arrival of spikes at cortical cells. As a result, delays become tuned creating a smooth cortical map of direction selectivity. This result demonstrates how delay adaption can serve as a powerful abstraction for modeling temporal learning in the brain.

Exposure to a high-contrast visual stimulus causes adaptation, a psychophysical phenomenon that is quite selective for stimulus orientation. Its mechanism is largely cortical but the underlying circuitry is still not unambiguously resolved. It has been suggested that adaptation could be the result of integration of inputs from cells within a large local pool, effectively scaling their outputs with respect to local contrast. In this case, orientation selectivity of neuronal adaptation should depend on the location of neurons within the cortical map of orientation preference. We tested this hypothesis by quantifying adaptation to optimally oriented and to orthogonal-to-optimum gratings among neurons recorded either from iso-orientation domains or orientation pinwheel centres, as identified by optical imaging of cat visual cortex. We did not find a significant difference in adaptation characteristics for these two populations of cells, implying that these characteristics do not depend on the local functional architecture. Surprisingly, however, we additionally observed that under isoflurane (but not halothane) anaesthesia, most neurons exhibited adaptation by cross-oriented gratings, regardless of their location within the orientation map. It seems likely that, under isoflurane, inputs became visible that were masked by the commonly used, deeper halothane anaesthesia. For individual cells, the presence of these inputs was independent of their location within the cortical orientation map.

Peripheral nerves have a potency to regenerate vigorously after injury, but often it is difficult to achieve a sufficient functional recovery. Although advances in microsurgery and a better understanding of nerve regeneration have improved the outcomes of nerve repair, a delayed and misdirected reinnervation still be a problem hindering functional restoration. Rehabilitation managements of peripheral nerve injury, such as electrical stimulation, exercise training and other approaches, have been in clinical use, but their effects on promoting reinnervation remain controversial. In this paper we review the basic studies which have investigated the mechanisms and efficacies of electrical stimulation, exercise training and motor/sensory reeducation. Electrical stimulation could elicit effects on the regenerated nerves, but inappropriate stimulation conditions might disturb regeneration. Exercise training often improves functional recovery after nerve injury, but overloaded and forced exercise might have a detrimental effect. The peripheral axonal misdirection is reflected in a cortical reorganizational changes. Some studies have revealed beneficial changes in the cortical map after motor/sensory reeducation programs in monkeys with nerve injury, which may explain the improved functional outcomes after rehabilitation in humans.

Magnetic resonance imaging can now provide human brain images of structure, function, and connectivity with isotropic voxels smaller than one millimeter, and thus much smaller than the cortical thickness. This resolution, achievable in a scan time of less than 1 h, enables visualization of myeloarchitectural layer structure, intracortical variations in functional activity--recorded in changes in blood oxygenation level dependent signal or cerebral blood volume CBV--and intracortical axonal orientational structure via diffusion-weighted magnetic resonance imaging. While recent improvements in radiofrequency receiver coils now enable excellent image data to be obtained at 3T, scanning at the ultra-high field of 7 T offers further gains in signal-to-noise ratio and speed of image acquisition, with a structural image resolution of about 300 μm. These improvements throw into sharp question the strategies that have become conventional for the analysis of functional imaging data, especially the practice of spatial smoothing of raw functional data before further analysis. Creation of a native cortical map for each human subject that provides a reliable individual parcellation into cortical areas related to Brodmann Areas enables a strikingly different approach to functional image analysis. This proposed approach involves surface registration of the cortices of groups of subjects using maps of the longitudinal relaxation time T1 as an index of myelination, and methods for inferring statistical significance that do not entail spatial smoothing. The outcome should be a far more precise comparison of like-with-like cortical areas across subjects, with the potential to greatly increase experimental power, to discriminate activity in neighboring cortical areas, and to enable correlation of function and connectivity with specific cytoarchitecture. Such analyses should enable a far more convincing modeling of brain mechanisms than current graph-based methods that require gross over-simplification of brain activity patterns in order to be computationally tractable.

Echolocating bats use the delay between their sonar emissions and the reflected echoes to measure target range, a crucial parameter for avoiding collisions or capturing prey. In many bat species, target range is represented as an orderly organized map of echo delay in the auditory cortex. Here we show that the map of target range in bats is dynamically modified by the continuously changing flow of acoustic information perceived during flight (echo-acoustic flow). Combining dynamic acoustic stimulation in virtual space with extracellular recordings, we found that neurons in the auditory cortex of the bat Phyllostomus discolor encode echo-acoustic flow information on the geometric relation between targets and the bats flight trajectory, rather than echo delay per se. Specifically, the cortical representation of close-range targets is enlarged when the lateral passing distance of the target decreases. This flow-dependent enlargement of target representation may trigger adaptive behaviours such as vocal control or flight manoeuvres.

Topographic maps are an often-encountered feature in the brains of many species, yet there are no standard, objective procedures for quantifying topography. Topographic maps are typically identified and described subjectively, but in cases where the scale of the map is close to the resolution limit of the measurement technique, identifying the presence of a topographic map can be a challenging subjective task. In such cases, an objective topography detection test would be advantageous. To address these issues, we assessed seven measures (Pearson distance correlation, Spearman distance correlation, Zrehen's measure, topographic product, topological correlation, path length and wiring length) by quantifying topography in three classes of cortical map model: linear, orientation-like, and clusters. We found that all but one of these measures were effective at detecting statistically significant topography even in weakly-ordered maps, based on simulated noisy measurements of neuronal selectivity and sparse sampling of the maps. We demonstrate the practical applicability of these measures by using them to examine the arrangement of spatial cue selectivity in pallid bat A1. This analysis shows that significantly topographic arrangements of interaural intensity difference and azimuth selectivity exist at the scale of individual binaural clusters.

Self-organising maps can recreate many of the essential features of the known functional organisation of primary cortical areas in the mammalian brain. According to such models, cortical maps represent the spatial-temporal structure of sensory and/or motor input patterns registered during the early development of an animal, and this structure is determined by interactions between the neural control architecture, the body morphology, and the environmental context in which the animal develops. We present a minimal model of pseudo-physical interactions between an animat body and its environment, which includes each of these elements, and show how cortical map self-organisation is affected by manipulations to each element in turn. Initial simulation results suggest that maps robustly self-organise to reveal a homuncular organisation, where nearby body parts tend to be represented by adjacent neurons.

The motor maps in the cortex are topologically organized, just like the sensory cortical maps, where nearby locations in the map represent behaviors of similar kinds. However, there is not much research on how such motor maps are formed. In this paper, as a first step in this direction, we developed a target reaching gesture map using a self-organizing map model of cortical development (the GCAL model, a simplified yet enhanced version of the LISSOM model). The inputs were target reaching behavior of a two-joint arm on a 2D plane (2 DOF), encoded as a time-lapse image where time was encoded as the pixel intensity. For training, 20,000 random arm movements were generated where each arm movement started at a random initial location and moved toward one of 24 predefined target locations. The resulting gesture map showed global topological order where nearby neurons represented gestures toward nearby target locations, comparable to the motor map reported in the experimental literature. Although our simulations were based on a sensory cortical map development framework, the results suggest that it could be easily adapted to transition into motor map development. Our work is an important first step toward a fully motor-based motor map development (e.g. using proprioceptive input), and we expect the findings reported in this paper to help us better understand the general nature of cortical map development, not just for the sensory but also for the motor modality.

International audience; Recently, an important aspect of human visual word recognition has been characterized. The letter position is encoded in our brain using an explicit representation of order based on letter pairs: the open-bigram coding [15]. We hypothesize that spelling has evolved in order to minimize reading errors. Therefore, word recognition using bigrams — instead of letters — should be more efficient. First, we study the influence of the size of the neighborhood, which defines the number of bigrams per word, on the performance of the matching between bigrams and word. Our tests are conducted against one of the best recognition solutions used today by the industry, which matches letters to words. Secondly, we build a cortical map representation of the words in the bigram space — which implies numerous experiments in order to achieve a satisfactory projection. Third, we develop an ultra-fast version of the self-organizing map in order to achieve learning in minutes instead of months.

Hindlimb sensory deprivation is known to induce a decrease in the cortical representation of hindpaw, and an increase in the size of the cutaneous receptive fields. The aim of the present study was to determine (i) the time-course of recovery when the rat retrieves a normal use of its limbs after a 14-day period of sensory disruption and (ii) whether a 1-day period of sensory deprivation is sufficient to induce a plasticity. Our results indicate that the remodelling of the cortical map was not observed after 1 day of sensory deprivation. On the other hand, the recovery was achieved after 6 h. These findings suggest that a procedure reducing sensory function resulted in reversible changes in the somatosensory cortex. The recovery was more rapid than the induction of plasticity.

A hierarchical dynamical map is proposed to be the basic frame of sensory cortical maps. We modeled a cortical neural network and carried out a cognitive process known as “priming” in which the identification of a stimulus is facilitated as a consequence of prior exposure to it. We demonstrate that even a single cortical map can have a hierarchical function based on the dynamics of the cortical network. It has also been demonstrated that the hierarchical dynamical map plays an essential role in binding different sensory modalities simultaneously present in the same visual scene.

We examine the dynamics of development of cortical maps of spatial position, ocular dominance and orientation preference in the elastic net model. Analysis and simulation results are presented for the order in which each of these maps first emerges as a function of the parameters of the feature space.

Cortical maps of orientation preference in cats, ferrets and monkeys contain numerous half-rotation point singularities. Experimental data have shown that direction preference also has a smooth representation in these maps, with preferences being for the most part orthogonal to the axis of preferred orientation. As a result, the orientation singularities induce an extensive set of linear fractures in the direction map. These fractures run between and connect nearby point orientation singularities. Their existence appears to pose a puzzle for theories that postulate that cortical maps maximize continuity of representation, because the fractures could be avoided if the orientation map contained full-rotation singularities. Here we show that a dimension-reduction model of cortical map formation, which implements principles of continuity and completeness, produces an arrangement of linear direction fractures connecting point orientation singularities which is similar to that observed experimentally. We analyse the behaviour of this model and suggest reasons why the model maps contain half-rotation rather than full-rotation orientation singularities.

Cerebral lateralization refers to the poorly understood fact that some functions are better controlled by one side of the brain than the other (e.g. handedness, language). Of particular concern here are the asymmetries apparent in cortical topographic maps that can be demonstrated electrophysiologically in mirror-image locations of the cerebral cortex. In spite of great interest in issues surrounding cerebral lateralization, methods for measuring the degree of organization and asymmetry in cortical maps are currently quite limited. In this paper, several measures are developed and used to assess the degree of organization, lateralization, and mirror symmetry in topographic map formation. These measures correct for large constant displacements as well as curving of maps. The behavior of the measures is tested on several topographic maps obtained by self-organization of an initially random artificial neural network model of a bihemispheric brain, and the results are compared with subjective assessments made by humans.

Three computational rules are sufficient to generate model cortical maps that simulate the interrelated structure of cortical ocular dominance and orientation columns: a noise input, a spatial band pass filter, and competitive normalization across all feature dimensions. The data of Blasdel from optical imaging experiments reveal cortical map fractures, singularities, and linear zones that are fit by the model. In particular, singularities in orientation preference tend to occur in the centers of ocular dominance columns, and orientation contours tend to intersect ocular dominance columns at right angles. The model embodies a universal computational substrate that all models of cortical map development and adult function need to realize in some form.