Phosphene Perception Relates to Visual Cortex Glutamate Levels and Covaries with Atypical Visuospatial Awareness.

Phosphenes are illusory visual percepts produced by the application of transcranial magnetic stimulation to occipital cortex. Phosphene thresholds, the minimum stimulation intensity required to reliably produce phosphenes, are widely used as an index of cortical excitability. However, the neural basis of phosphene thresholds and their relationship to individual differences in visual cognition are poorly understood.

Reorganization of retinotopic maps after occipital lobe infarction.

We studied patient JS, who had a right occipital infarct that encroached on visual areas V1, V2v, and VP. When tested psychophysically, he was very impaired at detecting the direction of motion in random dot displays where a variable proportion of dots moving in one direction (signal) were embedded in masking motion noise (noise dots). The impairment on this motion coherence task was especially marked when the display was presented to the upper left (affected) visual quadrant, contralateral to his lesion.

Early auditory processing in area V5/MT+ of the congenitally blind brain.

Previous imaging studies of congenital blindness have studied individuals with heterogeneous causes of blindness, which may influence the nature and extent of cross-modal plasticity. Here, we scanned a homogeneous group of blind people with bilateral congenital anophthalmia, a condition in which both eyes fail to develop, and, as a result, the visual pathway is not stimulated by either light or retinal waves. This model of congenital blindness presents an opportunity to investigate the effects of very early visual deafferentation on the functional organization of the brain.

Isoluminant coloured stimuli are undetectable in blindsight even when they move.

Moving stimuli are the most effective of all in eliciting blindsight. The detection of static luminance-matched coloured stimuli is negligible or even impossible in blindsight. However, moving coloured stimuli on an achromatic background have not been tested.

Modulation of cortical excitability can speed up blindsight but not improve it.

Blindsight has been widely investigated and its properties documented. One property still debated and contested is the puzzling absence of phenomenal visual percepts of visual stimuli that can be detected with perfect accuracy. We investigated the possibility that phenomenal visual percepts of exogenous visual stimuli in patient GY might be induced by using transcranial direct current stimulation.

The fate of the oculomotor system in clinical bilateral anophthalmia.

The interdependence of the development of the eye and oculomotor system during embryogenesis is currently unclear. The occurrence of clinical anophthalmia, where the globe fails to develop, permits us to study the effects this has on the development of the complex neuromuscular system controlling eye movements. In this study, we use very high-resolution T2-weighted imaging in five anophthalmic subjects to visualize the extraocular muscles and the cranial nerves that innervate them.

Are hemianopic monkeys and a human hemianope aware of visual events in the blind field?

In three hemianopic monkeys and one normal monkey who subsequently became hemianopic and in one human hemianope we measured reaction times to touch the remembered position of a brief visual target presented in the normal hemifield or in the blind hemifield, or on the blank trials where no visual target occurred and the correct response was to touch a separate and permanently outlined part of the display. This is the same procedure as first used to demonstrate blindsight in these hemianopic monkeys. In the present experiment physically salient high-contrast (0.

Language networks in anophthalmia: maintained hierarchy of processing in 'visual' cortex.

Imaging studies in blind subjects have consistently shown that sensory and cognitive tasks evoke activity in the occipital cortex, which is normally visual. The precise areas involved and degree of activation are dependent upon the cause and age of onset of blindness. Here, we investigated the cortical language network at rest and during an auditory covert naming task in five bilaterally anophthalmic subjects, who have never received visual input.

Enhanced cortical excitability in grapheme-color synesthesia and its modulation.

Synesthesia is an unusual condition characterized by the over-binding of two or more features and the concomitant automatic and conscious experience of atypical, ancillary images or perceptions. Previous research suggests that synesthetes display enhanced modality-specific perceptual processing, but it remains unclear whether enhanced processing contributes to conscious awareness of color photisms. In three experiments, we investigated whether grapheme-color synesthesia is characterized by enhanced cortical excitability in primary visual cortex and the role played by this hyperexcitability in the expression of synesthesia.

On the usefulness of 'what' and 'where' pathways in vision.

The primate visual brain is classically portrayed as a large number of separate 'maps', each dedicated to the processing of specific visual cues, such as colour, motion or faces and their many features. In order to understand this fractionated architecture, the concept of cortical 'pathways' or 'streams' was introduced. In the currently prevailing view, the different maps are organised hierarchically into two major pathways, one involved in recognition and memory (the ventral stream or 'what' pathway) and the other in the programming of action (the dorsal stream or 'where' pathway).

Detection of first- and second-order coherent motion in blindsight.

Blindsight patients can detect fast moving stimuli presented within their blind field even when they deny any phenomenal visual experience. Although mounting evidence suggests the presence of different mechanisms and separate neural substrates underlying the processing of first-order (luminance-defined) and second-order (contrast-defined) motion, the perception of second-order motion in blindsight has scarcely been explored. In the present study, we investigated whether two blindsighted patients (GY and MS) can detect a variety of first- and second-order moving stimuli, and by using repetitive transcranial magnetic stimulation (rTMS), we assessed the role of V5/MT(+) and V3(+) in coherent motion processing.

Awareness-related activity in prefrontal and parietal cortices in blindsight reflects more than superior visual performance.

Many imaging studies report activity in the prefrontal and parietal cortices when subjects are aware as opposed to unaware of visual stimuli. One possibility is that this activity simply reflects higher signal strength or the superior task performance that is associated with awareness. To find out, we studied the hemianope GY who has unilateral destruction of almost all primary visual cortices.

Transneuronal retrograde degeneration of retinal ganglion cells and optic tract in hemianopic monkeys and humans.

Transneuronal retrograde degeneration of retinal ganglion cells after removal of primary visual cortex (area V1) is well established by quantitative neurohistological analysis of the ganglion cell layer in monkeys, but remains controversial in human patients. Therefore, we first histologically examined retinal degeneration in sectioned archived retinae of 26 macaque monkeys with unilateral V1 ablation and post-surgical survival times ranging from 3 months to 14.3 years.

TMS-induced blinking assessed with high-speed video: optical disruption of visual perception.

It is known that TMS can induce blinking, but it is unknown to what extent and at what time TMS-induced blinking can cover the pupil. We applied single-pulse TMS with a leftward and rightward monophasic current through a round coil over the occipital pole in 8 healthy subjects, using high-speed video to monitor left or right eye with a spatial resolution of 0.1 mm and a temporal resolution of 2 ms.

Visual activation of extra-striate cortex in the absence of V1 activation.

When the primary visual cortex (V1) is damaged, there are a number of alternative pathways that can carry visual information from the eyes to extrastriate visual areas. Damage to the visual cortex from trauma or infarct is often unilateral, extensive and includes gray matter and white matter tracts, which can disrupt other routes to residual visual function. We report an unusual young patient, SBR, who has bilateral damage to the gray matter of V1, sparing the adjacent white matter and surrounding visual areas.

Visual system: how does blindsight arise?

Some patients can discriminate unseen visual stimuli within a field defect caused by damage to the primary visual cortex. The pathways for this 'blindsight' have never been established, but recent studies implicate hitherto overlooked cells in the thalamic LGN.

Similar effects of repetitive transcranial magnetic stimulation of MT+ and a dorsomedial extrastriate site including V3A on pattern detection and position discrimination of rotating and radial motion patterns.

Our recent psychophysical experiments have identified differences in the spatial summation characteristics of pattern detection and position discrimination tasks performed with rotating, expanding, and contracting stimuli. Areas MT and MST are well established to be involved in processing these stimuli. fMRI results have shown retinotopic activation of area V3A depending on the location of the center of radial motion in vision.

Edges, colour and awareness in blindsight.

It remains unclear what is being processed in blindsight in response to faces, colours, shapes, and patterns. This was investigated in two hemianopes with chromatic and achromatic stimuli with sharp or shallow luminance or chromatic contrast boundaries or temporal onsets. Performance was excellent only when stimuli had sharp spatial boundaries.

Imaging studies in congenital anophthalmia reveal preservation of brain architecture in 'visual' cortex.

The functional specialization of the human brain means that many regions are dedicated to processing a single sensory modality. When a modality is absent, as in congenital total blindness, 'visual' regions can be reliably activated by non-visual stimuli. The connections underlying this functional adaptation, however, remain elusive.

Functional and anatomical profile of visual motion impairments in stroke patients correlate with fMRI in normal subjects.

We used six psychophysical tasks to measure sensitivity to different types of global motion in 45 healthy adults and in 57 stroke patients who had recovered from the initial results of the stroke, but a large subset of them had enduring deficits on selective visual motion perception tasks. The patients were divided into four groups on the basis of the location of their cortical lesion: occipito-temporal, occipito-parietal, rostro-dorsal parietal, or frontal-prefrontal. The six tasks were: direction discrimination, speed discrimination, motion coherence, motion discontinuity, two-dimensional form-from-motion, and motion coherence - radial.

The blindsight saga.

Blindsight is the ability of patients with clinically blind field defects, caused by damage to the primary visual cortex V, to detect, localise and even discriminate visual stimuli that they deny seeing. Blindsight tells us much about the nature of visual processing in the absence of the primary visual cortex and is a paradigmatic example of implicit knowledge. It has attracted widespread interest and debate amongst philosophers, cognitive neuropsychologists and visual neuroscientists.

Human frontal eye fields and target switching.

The frontal eye fields (FEF) have typically been predominantly investigated in terms of their role in the generation of eye movements. Lesions to this area, either accidental or experimental, disrupt saccades and electrical stimulation elicits eye movements. Recently there has been increasing interest in the involvement of this area in visual processes, including in tasks where eye movements were either not required or were precluded.

Near space functioning of the human angular and supramarginal gyri.

The posterior parietal cortex (PPC) has been associated with the encoding of events in peripersonal space, but little is known about the precise segregation of parietal areas involved specifically in 'near-space' visuospatial processing. This study applied transcranial magnetic stimulation (TMS) to two parietal areas: the right angular gyrus (ANG) and the right supramarginal gyrus (SMG) in addition to a control site, cortical visual area V5, while subjects performed symmetry judgements on lines presented simultaneously in the left and right visual fields. Eight subjects performed the task with the stimuli presented either in peripersonal or extra personal space.

Human dorsolateral prefrontal cortex is involved in visual search for conjunctions but not features: a theta TMS study.

Functional neuroimaging studies have shown that the detection of a target defined by more than one feature (for example, a conjunction of colour and orientation) amongst distractors is associated with the activation of a network of brain areas. Dorsolateral prefrontal cortex (DLPFC), along with areas such as the frontal eye fields (FEF) and posterior parietal cortex (PPC), is a component of this network. While transcranial magnetic stimulation (TMS) had shown that both FEF and PPC are necessary for, and not just correlated with, successful conjunction search, this is not the case for DLPFC.