Pathological perceptual completion in hemianopia extends to the control of reach-to-grasp movements

The neuropsychological phenomenon of blindsight is observed when patients who are cortically blind exhibit residual visual processing capabilities for stimuli presented within their scotoma to which they are otherwise unaware. Cortically blind patients may also exhibit the phenomenon of pathological visual completion in which, paradoxically, they can become aware of a complete visual stimulus even when a significant portion of that stimulus falls within their blind hemifield. In this study, the ability of a blindsight patient (G.Y.) to use visual information to control reach-to-grasp movements to static objects presented within his blind hemifield was investigated. The results indicate that while G.Y. was insensitive to variations in object size when reaching for objects presented entirely within his blind hemifield, his ability to accurately grasp objects located within his blind field was vastly improved if part of the object to be grasped extended into his seeing hemifield. This finding demonstrates that visual awareness can facilitate the visuomotor processing of object form within G.Y.'s apparently blind field, and suggests that the primary deficit in blindsight may be an impairment of visual consciousness rather than an absolute loss of visual function.     

Direction discrimination of moving gratings and plaids and coherence in dot displays without primary visual cortex (V1)

We present new experimental observations of G.Y., a well-tested patient with unilateral loss of primary visual cortex. We stimulated G.Y.'s blind hemifield using first- and second-order motion stimuli at velocities around psychophysical threshold. Using a dual response paradigm (awareness level of visual motion, motion direction discrimination) psychophysical performance improved with increasing velocity and dot coherence. We were also able to influence directly G.Y.'s performance for the better and at will, by placing the emphasis solely on direction discrimination. In the absence of V1, graduated detection and discrimination of stimuli known to activate both V1 and extrastriate motion areas MT/V5 and MST is still possible. These results are in line with residual visual processing but did not show evidence of unconscious processing of motion stimuli characteristic of ‘blindsight’.     

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. On blank trials, the monkeys, but not the human, often failed to respond, and when they did respond, all hemianopes almost invariably touched a target position in the blind field. Analysis of reaction times showed that necessarily false responses to blank stimuli took longer than responses to blind field targets. However, apart from one hemianopic monkey, incorrect target responses took as long as responses to blank stimuli. The human hemianope showed the same pattern of reaction times as the hemianopic monkeys unless he had to report on stimulus awareness and confidence. Then, his confidence reports and response times mirrored his awareness of the stimuli, but neither differed for correct versus false responses once these were separated for 'aware' versus 'unaware' trials. The hemianopic monkeys' response probability and reaction time data indicate that they, implicitly or explicitly, registered differences between target and blank stimuli and, in one case, even between false responses to blind-field and blank stimuli.     

Spatial and temporal processing in a subject with cortical blindness following occipital surgery

Blindsight subjects are typically better at discriminating rapid, transient visual events than those with gradual on/off-sets. Surprisingly, the detailed investigation of temporal characteristics of mechanisms mediating blindsight is only reported in one subject (GY). It is of interest to establish whether these characteristics are similar to those in other cases of blindsight. Here, we report on a systematic study of spatio-temporal properties of mechanisms mediating blindsight in a subject VN. VN has a lower right quadranopia following surgical removal of the left occipital cortex above the calcarine sulcus, therefore, there are no remaining islands of intact visual cortex within this area. Similar to GY, the blindsight mechanisms in VN have narrowly tuned band-pass temporal characteristics with a peak sensitivity at 20Hz and above chance performance at temporal frequencies >/=10 and 相似文献   

Neural activity within area V1 reflects unconscious visual performance in a case of blindsight

Although lesions of the striate (V1) cortex disrupt conscious vision, patients can demonstrate surprising residual abilities within their affected visual field, a phenomenon termed blindsight. The relative contribution of spared "islands" of functioning striate cortex to residual vision, versus subcortical pathways to extrastriate areas, has implications for the role of early visual areas in visual awareness and performance. Here we describe the behavioral and neural features of residual cortical function in Patient M.C., who sustained a posterior cerebral artery stroke at the age of 15 years. Within her impaired visual field, we found preserved visual abilities characteristic of blindsight, including superior detection of motion, and above-chance discrimination of shape, color, and motion direction. Functional magnetic resonance imaging demonstrated a retinotopically organized representation of M.C.'s blind visual field within the lesioned occipital lobe, specifically within area V1. The incongruity of a well-organized cortex and M.C.'s markedly impaired vision was resolved by measurement of functional responses within her damaged occipital lobe. Attenuated neural contrast-response functions were found to correlate with M.C.'s impaired psychophysical performance. These results demonstrate that the behavioral features of blindsight may arise in the presence of residual striate responses that are spatially organized and sensitive to contrast variation.     

Blindsight after optic nerve injury indicates functionality of spared fibers

Some patients with lesions in the geniculostriate pathway (GSP) can respond to visual stimuli in the blind field without conscious acknowledgement. The substrate for this "blindsight" is controversial: whether it is the uninjured extrastriate pathway (EXP), which bypasses the lesion site, or residual fibers within damaged visual cortex ("islands of vision"). Using stimulus detection, localization, and spatial summation tasks, we have found blindsight in patients with damage both in the optic nerve (ON) and EXP. The prevalence and functional characteristics of their blindsight are indistinguishable from that in patients with GSP lesions, so blindsight does not require a completely intact EXP. The present findings support the view that a few surviving ON axons within an area of primary damage are sufficient to mediate blindsight: Several combinations of partially intact pathways can transmit information to the extrastriate cortex and the sum of activation of all visual fibers surviving the injury determines if and to what extent blindsight occurs.     

Blindsight in action: what can the different sub-types of blindsight tell us about the control of visually guided actions?

Blindsight broadly refers to the paradoxical neurological condition where patients with a visual field defect due to a cortical lesion nevertheless demonstrate implicit residual visual sensitivity within their field cut. The aim of this paper is twofold. First, through a selective review of the blindsight literature we propose a new taxonomy for the subtypes of residual abilities described in blindsight. Those patients able to accurately act upon blind field stimuli (e.g. by pointing or saccading towards them) are classified as having 'action-blindsight', those whose residual functions can be said to rely to some extent upon attentive processing of blind field stimuli are classified as demonstrating 'attention-blindsight', while finally, patients who have somewhat accurate perceptual judgements for blind field stimuli despite a complete lack of any conscious percept, are classified as having 'agnosopsia'--literally meaning 'not knowing what one sees'. We also address the possible neurological substrates of these residual sensory processes. Our second aim was to investigate the most striking subtype of blindsight, action-blindsight. We review the data relevant to this subtype and the hypotheses proposed to account for it, before speculating on how action-blindsight may inform our normal models of visuomotor control.     

Temporal properties of spatial channel of processing in hemianopia

The term blindsight, coined by Larry Weiskrantz, describes those discrimination abilities that can be elicited with visual stimuli restricted to the blindfield of a patient with occipital brain lesion or damaged optic radiation. Over the past 3 decades, many aspects of blindsight have been investigated including detection of basic stimulus attributes such as structure, colour and movement as well as more complex tasks such as discrimination of facial expressions and semantic processing. The neuronal mechanisms mediating blindsight rely on processing in subcortical and/or extrastriate areas. It appears that following the occipital brain damage, there is a restricted "window of processing" and stimulus parameters mainly outside this window may not lead to blindsight performance. Here we report how the restricted "window of processing" appears to have a specific spatio-temporal response profile, mainly tuned to low spatial frequencies and intermediate temporal frequencies. In addition, in a group of blindsight patients, we demonstrate that above chance detection performance is related to the target size. The findings have implications both for the reported incidence of blindsight and development of rehabilitation strategies.     

Form discrimination in a case of blindsight

DB, the first extensively tested blindsight case, has demonstrated the ability to detect and discriminate a range of visual stimuli presented within his perimetrically blind visual field [Weiskrantz L. (1986). Blindsight: A case study and implications. Oxford: Oxford University Press]. After initial reports of basic form discrimination (Weiskrantz, 1986), it later emerged that this ability to discriminate single stimuli presented to the blind field was probably based on the discrimination of orientation cues [Weiskrantz, L. (1987). Residual vision in a scotoma. A follow-up study of 'form' discrimination. Brain, 110, 77-92]. Even so, DB found it impossible to make a 'same/different' discrimination for pairs of stimuli in the blind field (Weiskrantz, 1986). In the current study, DB's discrimination and identification of stimuli on the basis of their form was tested (in 2AFC and FR paradigms). We have demonstrated that DB could successfully identify outline low contrast images of objects, make successful 'same/different' discriminations for pairs of stimuli presented in his blind field and identify complex images (digital photographs) presented entirely within his cortically blind field. The results are discussed in relation to the issues of likely neuronal mediation, the potential for improvement in cases of blindsight and the potential relevance of these findings in relation to theories of normal visual perception.     

Neural substrates of blindsight after hemispherectomy.

Blindsight is a visual phenomenon whereby hemianopic patients are able to process visual information in their blind visual field without awareness. Previous research demonstrating the existence of blindsight in hemianopic patients has been criticized for the nature of the paradigms used, for the presence of methodological artifacts, and for the possibility that spared islands of visual cortex may have sustained the phenomenon because the patients generally had small circumscribed lesions. To respond to these criticisms, the authors have been investigating for several years now residual visual abilities in the blind field of hemispherectomized patients in whom a whole cerebral hemisphere has been removed or disconnected from the rest of the brain. These patients have offered a unique opportunity to establish the existence of blindsight and to investigate its underlying neuronal mechanisms because in these cases, spared islands of visual cortex cannot be evoked to explain the presence of visual abilities in the blind field. In addition, the authors have been using precise behavioral paradigms, strict control for potential methodological artifacts such as light scatter, fixation, criterion effects, and macular sparing, and they have utilized new neuroimaging techniques such as diffusion tensor imaging tractography to enhance their understanding of the phenomenon. The following article is a review of their research on the involvement of the superior colliculi in blindsight in hemispherectomized patients. .     

Unlike in Clinical Blindsight Patients,Unconscious Processing of Chromatic Information Depends on Early Visual Cortex in Healthy Humans

BackgroundLarge amount of data concerning the neural mechanisms that mediate unconscious vision come from cortically blind patients who can process stimuli presented in their blind visual field. How well the findings generalize to neurologically healthy humans remains open.ObjectiveUsing transcranial magnetic stimulation (TMS), we studied whether chromatic processing depends on the early visual cortex in healthy participants.MethodWe employed a phenomenon called the redundant target effect (RTE): simple reaction times to two stimuli are faster than to one stimulus, even when one of the stimuli is presented outside consciousness.ResultsThe RTE produced by chromatically defined stimuli disappeared when the contralateral stimulus of two bilateral stimuli was suppressed from consciousness.ConclusionsIn contrast to studies on blindsight patients, the results imply that the early visual cortex is necessary for the processing of chromatic information in neurologically healthy humans.     

Effects of temporal cueing on residual visual discrimination in blindsight

We tested the ability of a blindsight patient, GY, to identify in which of two locations a target was presented in a spatial two-alternative forced choice paradigm (spatial 2AFC). On each trial the subject was asked to make a second manual response indicating whether he had had any awareness of an event occurring during the trial. A cue, presented at the fixation location, could signal the 0.4 s period over which the target appeared within the 10 s duration of each trial. Targets of three contrasts, 93, 43 and 22% were used. We found that GY's ability to discriminate the location of targets in his blind field remained significantly above chance, with and without cueing, for each contrast. Cueing, did, however, significantly improve his performance for low contrast targets. When he performed a similar task with near threshold contrast targets in his spared visual field his discrimination was at chance unless the presentation of targets was cued, despite his reporting more awareness for these stimuli than he did for low-contrast stimuli in his blind field. These results are compared with those previously reported in monkeys who received lesions to their visual cortices as infants or adults. We conclude that (1) GY's blindsight is qualitatively different from near-threshold normal vision. (2) In common with infant-lesioned monkeys his blindsight remains even in the absence of temporal cues. (3) Residual vision is subject to modulation by attentional processes, or arousal, associated with temporal cueing.     

Non-conscious recognition of affect in the absence of striate cortex

Functional neuroimaging experiments have shown that recognition of emotional expressions does not depend on awareness of visual stimuli and that unseen fear stimuli can activate the amygdala via a colliculopulvinar pathway. Perception of emotional expressions in the absence of awareness in normal subjects has some similarities with the unconscious recognition of visual stimuli which is well documented in patients with striate cortex lesions (blindsight). Presumably in these patients residual vision engages alternative extra-striate routes such as the superior colliculus and pulvinar. Against this background, we conjectured that a blindsight subject (GY) might recognize facial expressions presented in his blind field. The present study now provides direct evidence for this claim.     

Visual evoked potentials in the investigation of "blindsight"

We recorded long-latency visual evoked potentials in four patients with homonymous hemianopias, one of whom had clinical evidence of "blindsight." Stimuli consisted of different words that appeared randomly and at a constant angle to either side of the center of a TV screen, and subjects responded to one previously specified word (the "target") by finger extension. Target stimuli in the intact hemifield elicited a well-formed P3 response in all subjects, whereas stimuli in the blind field produced no such response except in the subject with blindsight. In addition, the earlier potentials in this subject were larger with stimulation of the blind hemifield than the intact field. By contrast, a P100 response was present only with stimulation of the intact field in this subject. These results indicate that cognitive processing of visual stimuli can occur even when subjective awareness of such stimuli is absent, and suggest that such processing occurs independently of the geniculostriate pathway.     

Visual restoration in cortical blindness: insights from natural and TMS-induced blindsight

Unilateral damage to visual cortex of the parietal or occipital lobe can cause the patient to be unaware of contralesional visual information due to either hemispatial neglect or hemianopia. It is now known that both neglect and hemianopia result from the disruption of a dynamic interaction between cortical visual pathways and more phylogenetically primitive visual pathways to the midbrain. We consider the therapeutic implications of these cortical-subcortical interactions in the rehabilitation of hemianopia. We start with the pheonmenon of "blindsight", in which patients with hemianopia can be shown, by implicit measures of visual detection or discrimination, to process visual information without conscious awareness. Some variants of blindsight have been postulated to recruit subcortical processes, while others may reflect compensatory optimisation of processing of spared visual cortex. Both mechanisms may offer opportunities for innovative strategies for rehabilitation of visual field defects. We relate the neural mechanisms that have been proposed to underlie blindsight to those that have been suggested to underlie the recovery of visual function after rehabilitation. It is suggested that the similarity and overlap of the neural processes supporting blindsight and recovery of visual function might provide insights for effective rehabilitation strategies for restoring visual functions.     

Subjective Perceptual Distortions and Visual Dysfunction in Children with Autism

Case reports and sensory inventories suggest that autism involves sensory processing anomalies. Behavioral tests indicate impaired motion and normal form perception in autism. The present study used first-person accounts to investigate perceptual anomalies and related subjective to psychophysical measures. Nine high-functioning children with autism and nine typically-developing children were given a questionnaire to assess the frequency of sensory anomalies, as well as psychophysical tests of visual perception. Results indicated that children with autism experience increased perceptual anomalies, deficits in trajectory discrimination consistent with dysfunction in the cortical dorsal pathway or in cerebellar midsagittal vermis, and high spatial frequency contrast impairments consistent with dysfunctional parvocellular processing. Subjective visual hypersensitivity was significantly related to greater deficits across vision tests.     

Absence of S-cone input in human blindsight following hemispherectomy

Destruction of the occipital cortex presumably leads to permanent blindness in the contralateral visual field. Residual abilities to respond to visual stimuli in the blind field without consciously experiencing them have, however, been described in cortically blind patients and are termed 'blindsight'. Although the neuronal basis of blindsight remains unknown, possible neuronal correlates have been proposed based on the nature of the residual vision observed. The most prominent but still controversial hypothesis postulates the involvement of the superior colliculi in blindsight. Here we demonstrate, using a computer-based reaction time test in a group of hemispherectomized subjects, that human 'attention-blindsight' can be measured for achromatic stimuli but disappears for stimuli that solely activate S-cones. Given that primate data have shown that the superior colliculi lacks input from S-cones, our results lend strong support to the hypothesis that 'attention-blindsight' is mediated through a collicular pathway. The contribution of a direct geniculo-extrastriate-koniocellular projection was ruled out by testing hemispherectomized subjects in whom a whole hemisphere has been removed or disconnected for the treatment of epilepsy. A direct retino-pulvinar-cortical connection is also unlikely as the pulvinar nucleus is known to receive input from S-cones as well as from L/M-cone-driven colour-opponent ganglion cells.     

Visual attention in blindsight: sensitivity in the blind field increased by targets in the sighted field

Damage to primary visual cortex results in a scotoma, or region of blindness, which can nevertheless be accompanied by residual visual abilities in the affected field. This dissociation of visual processing and visual experience is often described as blindsight. We investigated the role of visual attention in blindsight and the application of limited visual resources between the blind and sighted fields in a well-studied case. Contrary to expectations based on normal capacity limits, processing a target in the sighted field improved rather than interfered with processing of targets in the blind field. The benefit from the sighted field was greater when blind and sighted targets were in symmetric locations about the vertical meridian.     

Anomalous perception of coherence and transparency in moving plaid patterns.

While the low-level processes mediating the detection of primary visual attributes are well understood, much less is known about the way in which these attributes are assigned to objects in the visual world. For example, when a region of the retinal image contains multiple motion signals at a range of spatial scales, how do we know whether these signals come from a single object or multiple objects? Here, we present data from four neurological patients on a psychophysical task requiring them to report whether the two components of a plaid pattern appear to move coherently or transparently. The spatial frequency of one component of the plaid is held constant while that of the other is manipulated. While some of the patients perceive coherent motion over a much smaller range of spatial frequencies than normal controls, others report coherence over almost the entire range tested. We discuss the implications of these findings for computational theories of motion perception and higher-level visual processing.     

Localization of unseen visual stimuli by humans with normal vision

Thirty university students were shown tachistoscopically presented stimuli (half contained an open circle at one of six locations, half were blanks) at four durations. Short durations and a pattern mask reduced subjects' ability to detect the stimuli. For trials on which the subjects reported that the circle was absent, they nevertheless guessed the exact locations of the targets significantly better than chance. A signal detection theory analysis found that subjects were more sensitive for the location decision than for the detection decision. These findings show that the blindsight phenomenon of localization of “unseen” stimuli, previously reported for cortically blind patients, also occurs for humans with normal vision.