Distinct neural processes in grapheme-colour synaesthetes and semantic controls.

In synaesthetes, stimulation of one sensory pathway provokes a sensory experience (e.g. a colour concurrent) in a different sensory modality or sub-modality.

Synaesthetic perception of colour and visual space in a blind subject: an fMRI case study.

In spatial sequence synaesthesia (SSS) ordinal stimuli are perceived as arranged in peripersonal space. Using fMRI, we examined the neural bases of SSS and colour synaesthesia for spoken words in a late-blind synaesthete, JF. He reported days of the week and months of the year as both coloured and spatially ordered in peripersonal space; parts of the days and festivities of the year were spatially ordered but uncoloured.

Modality and variability of synesthetic experience.

In synesthesia, stimulation of one sensory or cognitive pathway leads to additional, involuntary experiences in a second sensory or cognitive pathway. We here review previous surveys on this neurologically based phenomenon and report the results of 63 synesthetes who completed our Internet and paper questionnaire on synesthesia. In addition to asking for personal data and information on the participant's synesthesia, the questionnaire focused on the components of the inducer that elicit or modulate synesthesia.

Task-irrelevant blindsight and the impact of invisible stimuli.

Despite their subjective invisibility, stimuli presented within regions of absolute cortical blindness can both guide forced-choice behavior when they are task-relevant and modulate responses to visible targets when they are task-irrelevant. We here tested three hemianopic patients to learn whether their performance in an attention-demanding rapid serial visual presentation task would be affected by task-irrelevant stimuli. Per trial, nine black letters and one white target letter appeared briefly at fixation; the white letter was to be named at the end of each trial.

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.

Cueless blindsight.

The term blindsight describes the non-reflexive visual functions that remain or recover in fields of absolute cortical blindness. As visual stimuli confined to such fields are subjectively invisible, they are customarily announced by visible or audible cues that inform the patients when to respond. The pervasive use of cueing has spawned the widely held assumption that sight and blindsight differ in that only blindsight requires cueing.

Functional rehabilitation of partial cortical blindness?

The current doctrine regards fields of partial cortical blindness as permanent once a temporally restricted window for spontaneous recovery has passed. Accordingly, neuropsychological rehabilitation mainly applies compensatory procedures that train patients to make better use of their sighted field. The more ambitious goal of functional recovery depends on the survival of pathways that continue to transmit retinal information from the blind field.

Seeing 'where' through the ears: effects of learning-by-doing and long-term sensory deprivation on localization based on image-to-sound substitution.

Background: Sensory substitution devices for the blind translate inaccessible visual information into a format that intact sensory pathways can process. We here tested image-to-sound conversion-based localization of visual stimuli (LEDs and objects) in 13 blindfolded participants.

Methods And Findings: Subjects were assigned to different roles as a function of two variables: visual deprivation (blindfolded continuously (Bc) for 24 hours per day for 21 days; blindfolded for the tests only (Bt)) and system use (system not used (Sn); system used for tests only (St); system used continuously for 21 days (Sc)).

A blindsight conundrum: how to respond when there is no correct response.

Whereas research on blindsight customarily defines the correct responses to all visual stimuli presented to the cortically blind field, we here introduced a small number of unexpected 'no stimulus' trials in a localization task, to discover whether they would elicit the same responses as blind field targets. As no correct responses existed for the blank stimuli, our subjects, three hemianopic and one normal monkey, and one human hemianope who was aware of many blind-field targets, could either respond to these catch trials as to a target or refrain from responding. Visual stimuli were presented singly at four possible positions, two in the blind field of the hemianopes, and all subjects correctly localized the vast majority of targets in either hemifield.

ERP--correlates of response selection in a response conflict paradigm.

Neuroimaging and electrophysiological studies suggest that the anterior cingulate cortex (ACC) is involved in the cognitive control of response related action. A frontocentral negative ERP-component, the N2, which probably originates from the ACC, is usually enhanced in conflict-trials that demand an unexpected response. We here used stepped adjustment of response expectation in a response-cueing task, and measured how the N2 varied with global and local cue validity.

Blindsight, conscious vision, and the role of primary visual cortex.

What is the role the primary visual cortex (V1) in vision? Is it necessary for conscious sight, as indicated by the cortical blindness that results from V1 destruction? Is it even necessary for blindsight, the nonreflexive visual functions that can be evoked with stimuli presented to cortically blind fields? In the context of this controversial issue, I present evidence indicating that not only is blindsight possible, but that conscious vision may, to a varying degree, return to formerly blind fields with time and practice even in cases where functional neuroimaging reveals no V1 activation.

Image-to-sound conversion: experience-induced plasticity in auditory cortex of blindfolded adults.

The ability to adapt to environmental changes is based on the impressive capacity of the central nervous system for plasticity changes. A better understanding of the requirements of neuroplasticity will help to apprehend and predict the success of sensory prostheses. To investigate neuroplastic changes associated with (1) blindfolding and (2) the use of a mobile visual-auditory substitution system, five normally sighted adults underwent weekly measurements of neuromagnetic activity using a 122-channel whole head neuromagnetometer.

Stimulus cueing in blindsight.

When visual stimuli are presented in the cortically blind visual field of patients or monkeys with verified destruction of striate cortex, many subjects can voluntarily respond to them. In studies of this blindsight, the on- and/or offset of the visual stimulus is usually known to the subject, either because it is signaled in some way or because the subject can present the stimulus himself. To study the effect of stimulus uncertainty on the responses of four hemianopic monkeys and one human hemianope, we compared trials on which the subjects themselves could instantly trigger the stimulus with trials on which the same stimulus appeared 1-7 s after the start-light that normally served as the trigger was first touched.

Chromatic priming in hemianopic visual fields.

In three monkeys made hemianopic by unilateral striate cortical ablation, in one normal monkey and in a human hemianope (GY), we measured reaction times to chromatic targets presented in the normal hemifield as a function of prior chromatic primes in the blind field. The first of our three tasks showed an unspecific priming effect in that the colour of the here unpredictive prime was irrelevant. However, when contingencies were changed in the second task so that the prime was usually valid, its colour did significantly influence reaction times in two of the hemianopic monkeys as well as in the human subject.

Aware or unaware: assessment of cortical blindness in four men and a monkey.

In four patients and one monkey with unilateral visual field defects caused by retro-geniculate lesions we measured forced-choice localization of square-wave gratings as a function of contrast, and compared results from the patients' absolutely and relatively blind fields. In addition, the patients indicated verbally whether they were aware of the stimuli. We then switched to a signal detection task in which the subjects had to signal a stimulus as in the localization task, by touching it, no matter whether it appeared in the good or bad hemifield, and in addition to signal a blank trial by touching an outlined square now constantly present on the monitor, and designated the no-stimulus response area.

Change blindness and time to consciousness.

Detection of changes in a visual scene can be substantially delayed when the original and the modified image are separated by a brief screen flicker. We used this phenomenon of "change blindness" to find when the brain detects the mismatch in relation to when the observer reports it, and whether changes in identity and position are processed similarly. Event-related brain potentials (ERPs) recorded while the subjects searched for the change in alternating series of images showed that the epoch during which they indicated detection was characterized by a marked positivity from 300 to 700 ms.

Low-level phenomenal vision despite unilateral destruction of primary visual cortex.

GY, an extensively studied human hemianope, is aware of salient visual events in his cortically blind field but does not call this "vision." To learn whether he has low-level conscious visual sensations or whether instead he has gained conscious knowledge about, or access to, visual information that does not produce a conscious phenomenal sensation, we attempted to image process a stimulus s presented to the impaired field so that when the transformed stimulus T(s) was presented to the normal hemifield it would cause a sensation similar to that caused by s in the impaired field. While degradation of contrast, spatio-temporal filtering, contrast reversal, and addition of smear and random blobs all failed to match the response to a flashed bar s(f), moving textures of low contrast were accepted to match the response to a moving contrast-defined bar, s(m).

Detection and discrimination of chromatic targets in hemianopic macaque monkeys and humans.

We measured the ability of three macaque monkeys with unilateral removal of primary visual cortex to detect 9 degrees, 200-ms targets presented at random in the upper or lower quadrants of the normal and hemianopic visual fields. The white or variously coloured target could differ from the background in both colour and luminance, or in either of them. Blue and red targets were detectable at any luminance contrast, but green and white targets were barely or not at all detectable at and near isoluminance in the hemianopic field.

The neuroanatomy of phenomenal vision: a psychological perspective.

Somewhere in the visual system, phenomenal vision--the seeing of colors, brightness, depths, shades, and motion--is generated not only from the distribution of light on the retina, but also when the eyes are closed, in dreams, hallucinations, phosphenes, and (possibly) imagery. Whether these different forms of phenomenal vision share a common substrate although their origins are different (optical, mechanical, electrical, endogenous) is discussed in the light of evidence from neuropsychological and functional imaging studies. Whereas extrastriate visual cortical areas appear to be involved in all types of phenomenal vision that have been studied, the necessity of a contribution from primary visual cortex is demonstrated by the loss of conscious vision that follows its destruction.

Neural correlates of religious experience.

The commonsense view of religious experience is that it is a preconceptual, immediate affective event. Work in philosophy and psychology, however, suggest that religious experience is an attributional cognitive phenomenon. Here the neural correlates of a religious experience are investigated using functional neuroimaging.

Sustained extrastriate cortical activation without visual awareness revealed by fMRI studies of hemianopic patients.

Patients with lesions in the primary visual cortex (V1) may show processing of visual stimuli presented in their field of cortical blindness even when they report being unaware of the stimuli. To elucidate the neuroanatomical basis of their residual visual functions, we used functional magnetic resonance imaging in two hemianopic patients, FS and GY. In the first experiment, a rotating spiral stimulus was used to assess the responsiveness of dorsal stream areas.

Is V1 necessary for conscious vision in areas of relative cortical blindness?

Visual field defects result from postgeniculate lesions. It is generally assumed that absolute defects are caused by total destruction or denervation of primary visual cortex (V1) and that the degraded but conscious vision that remains or returns in relative or partial defects is mediated by compromised V1 cortex that retains a sufficiently large population of functional neurons. We here report the results of three patients with long-standing postgeniculate lesions who underwent functional magnetic resonance imaging while their partial defect was stimulated with high-contrast reversing checkerboard stimuli.

The retinal projection to the pregeniculate nucleus in normal and destriate monkeys.

Taking advantage of the segregation of different classes of ganglion cell fibres in the optic tract, we investigated the ganglion cell class that projects to the pregeniculate nucleus (PGN) in the normal macaque monkey (Macaca mulatta and Macaca fascicularis) and following long-standing removal of striate cortex in one hemisphere. Confining small pellets of horseradish peroxidase and biocytin into ventral or dorsal parts of the tract unilaterally, or placing several pellets throughout the tract, we labelled the retina retrogradely and the PGN anterogradely. Classification of ganglion cells according to soma size and dendritic morphology showed that implants in the dorsal part of the tract labelled predominantly, and perhaps exclusively, P beta cells, and produced dense anterograde label in the parvocellular lateral geniculate nucleus as well in the PGN.