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Biomedical subjects

L Weiskrantz

Publications and source records attributed to L Weiskrantz.

At least 19 recordsLinked to original sources
PubMed2004

Spatial attention speeds discrimination without awareness in blindsight.

An intimate relationship is often assumed between visual attention and visual awareness. Using a subject, patient GY, with the neurological condition of "blindsight" we show that although attention may be a necessary precursor to visual awareness it is not a sufficient one. Using a Posner endogenous spatial cueing paradigm we showed that the time our subject needed to discriminate the orientation of a stimulus was reduced if he was cued to the location of the stimulus. This reaction-time advantage was obtained without any decrease in discrimination accuracy and cannot therefore be attributed to speed-error trade-off or differences in bias between cued and uncued locations. As a result of his condition GY was not aware of the stimuli to which processing was attentionally facilitated. Attention cannot, therefore be a sufficient condition for awareness.

Adult↗
PubMed2004

Roots of blindsight.

The chapter reviews the historical background to demonstrations that there is residual visual function in the total absence of striate cortex (V1) in monkey and humans. The late 19th century evidence by Munk and others, as reviewed by William James, was that this was not possible in humans, and doubtful at best in monkeys. It has gradually become realized, starting in the middle of the 20th century, that even total bilateral removal of striate cortex in monkeys does not abolish all visual capacity, including spatial and pattern vision. The situation regarding unilateral or incomplete bilateral lesions in the monkey did not become clarified until Cowey's doctoral work in the 1960s, demonstrating that field defects were not absolute, that sensitivity continued to improve over several months of postoperative testing, that the size of the field defect gradually shrank, that the sensitivity was poorest at the center of the field defect, and that recovery was not spontaneous but depended on sustained practice. In human subjects with unilateral lesions from the 1970s onwards, using forced-choice methodology parallel to animal studies, a wide range of visual discriminations was demonstrated but with alterations or complete absence of acknowledged awareness by subjects (blindsight). Various varieties of skepticism are discussed and rebutted. The gap between humans and animals was diminished by the demonstration by Cowey and Stoerig that monkeys, like humans, classify responses to blind-field stimuli as being 'unseen'. Further recent degrees of closure and developments in human blindsight research are discussed.

Animals↗
PubMed2003

Brain potentials associated with conscious aftereffects induced by unseen stimuli in a blindsight subject.

The study is of brain activity in a blindsight subject (D.B.), who reports conscious visual afterimages of stimuli of which he is unaware when they are presented. This contrast offered a unique opportunity to study event-related potential recordings of conscious versus unconscious visual phenomena generated by the very same stimulus in the identical locus of the visual field. The behavioral results confirmed the reliability of the difference in the subject's report for inducing stimuli versus their aftereffects. The rationale of the event-related potential analysis was to subtract "on" signals from "off" signals, the latter associated with the onset of conscious events and the former for events that remained unconscious. Because there are inherent differences in on and off potentials, the subtractive resultants for the blind hemifield were compared with the same subtractions for the good hemifield when the subject was aware both of the stimuli and their afterimages. A differential pattern in subtractive resultants emerged with a strong anterior left frontal focus for the blind field and a posterior focus for the intact field. The results are compared with other studies suggesting an anterior focus for conscious visual events.

Afterimage↗
PubMed2002

Psychophysical and pupillometric study of spatial channels of visual processing in blindsight.

To date no systematic method has been used for characterising the residual capacity of blindsight subjects that would allow comparison and generalisation across all subjects. The detection of isoluminant gratings of varying spatial and temporal frequencies commends itself for detailed between-subject comparison, and for mapping results onto physiological properties in relation to neuronal circuitry. We report the ability of a blindsight subject (CS) to detect suprathreshold sine-wave gratings over a range of spatial and temporal frequencies using psychophysical techniques. A band-pass spatial channel with an upper cutoff below 3.5 cycles/deg is specified. The data also have been analysed to compare differences between two types of blindsight performances, type I and type II. Spatial gratings were also used to elicit a pupillary grating response, offering an objective method that is free of verbal nuances and response bias, and the resulting band-pass channel can be used both for clinical screening and for prediction and comparisons with psychophysical profiles. Finally, we have compared our results with those reported in studies of a well-known subject, GY, which demonstrate remarkable similarities. Implications are discussed in relation to blindsight research.

Adult↗
PubMed2002

Prime-sight and blindsight.

Listening to subject's commentaries can be a useful spur to novel scientific departures, as in studies of blindsight. Recently further testing (after a gap of 17 years) has been possible with subject DB, who was a blindsight patient (the first) tested intensively over a period of 10 years and who was the subject of the book, (Blindsight OUP, 1986, 2nd edition, 1998). Essentially his original capacity is the same or somewhat more sensitive. Some further types of discriminations have now been tested that were not possible in the original study. But a new feature was discovered, namely that he describes conscious after-images of a wide range of inducing stimuli of which he is unconscious. The properties of the after-images are briefly described, such as their occurrence following unseen colour patches, luminance changes, shape, spatial frequency, their long durations, showing inter-ocular transfer, as well as their conforming to Emmert's Law. Various approaches are discussed towards finding the source of their generation, such as the use of after-images of colour mixes of separate inputs to the two eyes ("cortical yellow"). They are also discussed in terms of after-images in normal subjects that are generated by imagination. The discovery offers a rare potential opportunity to compare the brain activity associated with both the conscious and unconscious attributes of the precisely the same stimuli in the same locus in the same subject.

Adult↗
PubMed2001

Unseen stimuli modulate conscious visual experience: evidence from inter-hemispheric summation.

Emotional facial expression can be discriminated despite extensive lesions of striate cortex. Here we report differential performance with recognition of facial stimuli in the intact visual field depending on simultaneous presentation of congruent or incongruent stimuli in the blind field. Three experiments were based on inter-hemispheric summation. Redundant stimulation in the blind field led to shorter latencies for stimulus detection in the intact field. Recognition of the expression of a half-face expression in the intact field was faster when the other half of the face presented to the blind field had a congruent expression. Finally, responses to the expression of whole faces to the intact field were delayed for incongruent facial expressions presented in the blind field. These results indicate that the neuro-anatomical pathways (extra-striate cortical and sub-cortical) sustaining inter-hemispheric summation can operate in the absence of striate cortex.

Blindness↗
PubMed2001

Differential extrageniculostriate and amygdala responses to presentation of emotional faces in a cortically blind field.

Patient G.Y. is able to discriminate emotional facial expressions presented in his blind (right) hemifield despite an extensive lesion of the corresponding (left) striate cortex. One proposal is that this residual ability (affective "blindsight") depends on a subcortical visual pathway comprising the superior colliculus, posterior (extrageniculate) thalamus and amygdala. Here we report differential amygdala responses in G.Y. to presentation of fearful and fear-conditioned faces in his blind (right) hemifield. These amygdala responses exhibited condition-dependent covariation with neural activity in the posterior thalamus and superior colliculus. Our results provide further evidence that an extrageniculostriate (colliculo-thalamo-amygdala) neural pathway can process fear-related stimuli independently of both the striate cortex and normal phenomenal visual awareness.

Adult↗
PubMed2000

Early extrastriate activity without primary visual cortex in humans.

Damage to the primary visual cortex (V1) destroys the major source of anatomical input to extrastriate cortical areas (V2, V3, V4 and V5) and produces cortical blindness--an absence of any sensation of light and colour--in the visual field contralateral to the side of the lesion. Neuroimaging studies, nevertheless, have recently demonstrated dorsal and ventral extrastriate activation for stationary stimuli presented to the blind visual field in the absence of V1 activity in human subjects. To clarify the moment in time that visual information reaches extrastriate areas, by means of event-related potentials (ERPs) we tracked the temporal course of responses to complex visual stimuli (faces) presented in the blind field of a hemianopic patient. Stimulation of the normal visual field elicited a positive occipital deflection (P1) at 140 ms. A P1 response was also observed with stimulation of the blind field, although slightly delayed (20 ms) and reduced. Its topography and timing demonstrate that early neural activity for stationary stimuli takes place within extrastriate regions despite V1 denervation.

Adult↗
PubMed1999

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.

Adult↗
PubMed1999

The unseen color aftereffect of an unseen stimulus: insight from blindsight into mechanisms of color afterimages.

We show here that, in the absence of a direct geniculostriate input in human subjects, causing loss of sight in the visual half-field contralateral to the damage, the pupil responds selectively to chromatic modulation toward the long-wavelength (red) region of the spectrum locus even when the stimulus is isoluminant for both rods and cones and entirely restricted to the subjects' "blind" hemifields. We also show that other colors are less or wholly ineffective. Nevertheless, red afterimages, generated by chromatic modulation toward the green region of the spectrum locus, also cause constrictions of the pupil even when green stimuli are themselves completely ineffective in the blind hemifield. Moreover, human subjects with damage to or loss of V1 are typically completely unaware of the stimulus that generates the aftereffect or of the aftereffect itself, both of which can be seen clearly in normal vision. The results show that pupillary responses can reveal the processing of color afterimages in the absence of primary visual cortex and in the absence of acknowledged awareness. This phenomenon is therefore a striking example of "blindsight" and makes possible the formulation of a model that predicts well the observed properties of color afterimages.

Adult↗
PubMed1999

Attention without awareness in blindsight.

The act of attending has frequently been equated with visual awareness. We examined this relationship in 'blindsight'--a condition in which the latter is absent or diminished as a result of damage to the primary visual cortex. Spatially selective visual attention is demonstrated when information that stimuli are likely to appear at a specific location enhances the speed or accuracy of detection of stimuli subsequently presented at that location. In a blindsight subject, we showed that attention can confer an advantage in processing stimuli presented at an attended location, without those stimuli entering consciousness. Attention could be directed both by symbolic cues in the subject's spared field of vision or cues presented in his blind field. Cues in his blind field were even effective in directing his attention to a second location remote from that at which the cue was presented. These indirect cues were effective whether or not they themselves elicited non-visual awareness. We concluded that the spatial selection of information by an attentional mechanism and its entry into conscious experience cannot be one and the same process.

Accidents, Traffic↗
PubMed1999

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.

Adult↗
PubMed1999

Differential pupillary constriction and awareness in the absence of striate cortex.

The fact that the pupil constricts differentially to visual stimuli in the absence of changes in light energy makes it a valuable tool for studying normal function as well as residual capacity in hemianopic subjects. When pupillometrically effective stimuli such as equiluminant gratings or coloured patches with an abrupt onset and offset are presented to the 'blind' hemifield, a hemianopic subject with damage largely restricted to striate cortex (V1) sometimes reports being 'aware' of the transient onset/offset, although without 'seeing' as such. The question addressed here is whether the pupil still responds in the condition of blindsight in its strict sense--i.e. discriminative capacity in the absence of acknowledged awareness--when stimuli are deliberately designed to eliminate awareness. This was accomplished by making stimulus onset and offset slow and gradual. The results with a well-studied hemianope, G.Y., demonstrate that there is still a pupillary constriction to isoluminant achromatic gratings and red-coloured stimuli, although reduced in size, in the absence of acknowledged awareness.

Accidents, Traffic↗
PubMed1998

Awareness and confidence ratings in motion perception without geniculo-striate projection.

We have previously reported that under certain experimental conditions, a subject with damaged primary visual cortex performed significantly above chance in discriminating motion direction and orientation either with or without awareness of the stimulus presentation in his blind hemifield. The subject's performance varied as a function of stimulus speed, excursion length, and background luminance. Present observations confirm the previous findings of above chance discrimination scores both with and without awareness, but nevertheless indicate an overall increase in sensitivity over the past 2 years. In addition to discriminating the direction of motion, the subject was asked on every trial to report either awareness or confidence or both, on a six-point scale. The results show that the introduction of a six-point scale for the reported level of awareness yielded similar results to those when the subject was given a binary choice to indicate the presence or absence of conscious awareness. The area under the receiver operating characteristic (ROC) curves obtained from the rating data was used to compare results across different experimental conditions. It was found that although the subject's reported confidence and awareness varied monotonically as a function of stimulus speed, they were not equivalent measures.

Adult↗
PubMed1998

Learning from the pupil: a spatial visual channel in the absence of V1 in monkey and human.

The pupil of the normal human subject constricts in response not only to average increases in light energy, but also selectively to the spatial structure of a visual stimulus even when there are no energy changes. This enables one to measure visual acuity and sensitivity as a function of spatial frequency. It is known that pupillometric measures of acuity correlate well with those determined psychophysically for normal human observers. The purpose of the present study was to measure pupillary changes with stimuli delivered to the 'blind' hemifields of monkeys with unilateral V1 removal, and also with stimulation of a human subject (G.Y.) with putative V1 destruction. The results show that there are small but reliable pupillary changes to flux-equated gratings in the blind fields both in monkeys and human. The response profile in both species is very similar: it is narrowly tuned, with a peak at approximately 1 cycle/degree and a cut-off acuity of approximately 7 or 8 cycles/degree, a significant reduction compared with the intact hemifield. The result also maps well onto the psychophysically determined spatiotemporal response profile to gratings in the blind field, as determined independently for G.Y. Thus, there is a narrowly tuned spatial visual channel that does not require the integrity of V1 in monkey or human. There was no evidence under these particular conditions in either monkeys or subject G.Y. of a pupillary response to an equiluminant change from white to green or red in the hemianopic field. The pupil holds promise as a non-verbal, indirect method for determining the spatial profile, sensitivity and other properties of residual visual capacity, both in animals and humans.

Adult↗