Unconscious processing of orientation and color without primary visual cortex (original) (raw)
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Proceedings of the National Academy of Sciences, 2004
The proposed neural mechanisms supporting blindsight, the above-chance performance of cortically blind patients on forced-choice visual discrimination tasks, are controversial. In this article, we show that although subjects were unable to perceive foveally presented visual stimuli when transcranial magnetic stimulation over the visual cortex induced a scotoma, responses nonetheless were delayed significantly by these unconscious distractors in a directed saccade but not in an indirect manual response task. These results suggest that the superior colliculus, which is involved with sensory encoding as well as with the generation of saccadic eye movements, is mediating the unconscious processing of the transcranial magnetic stimulation-suppressed distractors and implicate a role of the retinotectal pathway in many blindsight phenomena.
Brain-Stimulation Induced Blindsight: Unconscious Vision or Response Bias?
PLoS ONE, 2013
A dissociation between visual awareness and visual discrimination is referred to as "blindsight". Blindsight results from loss of function of the primary visual cortex (V1) which can occur due to cerebrovascular accidents (i.e. stroke-related lesions). There are also numerous reports of similar, though reversible, effects on vision induced by transcranial Magnetic Stimulation (TMS) to early visual cortex. These effects point to V1 as the "gate" of visual awareness and have strong implications for understanding the neurological underpinnings of consciousness. It has been argued that evidence for the dissociation between awareness of, and responses to, visual stimuli can be a measurement artifact of the use of a high response criterion under yes-no measures of visual awareness when compared with the criterion free forced-choice responses. This difference between yes-no and forced-choice measures suggests that evidence for a dissociation may actually be normal near-threshold conscious vision. Here we describe three experiments that tested visual performance in normal subjects when their visual awareness was suppressed by applying TMS to the occipital pole. The nature of subjects' performance whilst undergoing occipital TMS was then verified by use of a psychophysical measure (d') that is independent of response criteria. This showed that there was no genuine dissociation in visual sensitivity measured by yes-no and forced-choice responses. These results highlight that evidence for visual sensitivity in the absence of awareness must be analysed using a bias-free psychophysical measure, such as d', In order to confirm whether or not visual performance is truly unconscious.
Consciousness and Cognition, 2011
In order to study whether there exist a period of activity in the human early visual cortex that contributes exclusively to visual awareness, we applied transcranial magnetic stimulation (TMS) over the early visual cortex and measured subjective visual awareness during visual forced-choice symbol or orientation discrimination tasks. TMS produced one dip in awareness 60-120 ms after stimulus onset, while forced-choice orientation discrimination was suppressed between 60 and 90 ms and symbol discrimination between 60 and 120 ms. Thus, a time window specific to visual awareness was found only in the orientation condition at 120 ms. The results imply that both conscious and unconscious perception depend on activity in early visual areas. On the basis of previous estimates of neural processing speed, we suggest that the late part of the activity period most likely involve local extrastriate-striate interactions which provide the contents for visual awareness but are not themselves sufficient for awareness to arise.
Action-blindsight in healthy subjects after transcranial magnetic stimulation
Proceedings of the National Academy of Sciences, 2008
Here, we show that blindsight can be induced in healthy subjects by using transcranial magnetic stimulation over the visual cortex. Transcranial magnetic stimulation blocked the conscious perception of a visual stimulus, but subjects still corrected an ongoing reaching movement in response to the stimulus. The data show that correction of reaching movements does not require conscious perception of a visual target stimulus, even in healthy people. Our results support previous results suggesting that an efference copy is involved in movement correction, and this mechanism seems to be consistent even for movement correction without perception.
The primary visual cortex, and feedback to it, are not necessary for conscious vision
Brain, 2011
A compelling single case report of visual awareness (visual qualia) without primary visual cortex would be sufficient to refute the hypothesis that the primary visual cortex and the back-projections to it are necessary for conscious visual experience. In a previous study, we emphasized the presence of crude visual awareness in Patient G.Y., with a lesion of the primary visual cortex, who is aware of, and able to discriminate, fast-moving visual stimuli presented to his blind field. The visual nature of Patient G.Y.'s blind field experience has since been questioned and it has been suggested that the special circumstances of repeated testing over decades may have altered Patient G.Y.'s visual pathways. We therefore sought new evidence of visual awareness without primary visual cortex in patients for whom such considerations do not apply. Three patients with hemianopic field defects (Patient G.N. and Patient F.B. with MRI confirmed primary visual cortex lesions, Patient C.G. with an inferred lesion) underwent detailed psychophysical testing in their blind fields. Visual stimuli were presented at different velocities and contrasts in two-and four-direction discrimination experiments and the direction of motion and awareness reported using a forced-choice paradigm. Detailed verbal reports were also obtained of the nature of the blind field experience with comparison of the drawings of the stimulus presented in the blind and intact fields, where possible. All three patients reported visual awareness in their blind fields. Visual awareness was significantly more likely when a moving stimulus was present compared to no stimulus catch trials (P 5 0.01 for each subject). Psychophysical performance in Patient F.B. and Patient G.N. was consistent with the Riddoch syndrome, with higher levels of visual awareness for moving compared to static stimuli (P 5 0.001) and intact direction discrimination (P 5 0.0001 for two-and four-direction experiments). Although the blind field experience of all three subjects was degraded, it was clearly visual in nature. We conclude that the primary visual cortex or back-projections to it are not necessary for visual awareness.
Neural Activity within Area V1 Reflects Unconscious Visual Performance in a Case of Blindsight
Journal of Cognitive Neuroscience, 2008
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 ar...
Visual stimuli as well as transcranial magnetic stimulation (TMS) can be used: (1) to suppress the visibility of a target and (2) to recover the visibility of a target that has been suppressed by another mask. Both types of stimulation thus provide useful methods for studying the microgenesis of object perception. We first review evidence of similarities between the processes by which a TMS mask and a visual mask can either suppress the visibility of targets or recover such suppressed visibility. However, we then also point out a significant difference that has important implications for the study of the time course of unconscious and conscious visual information processing and for theoretical accounts of the processes involved. We present evidence and arguments showing: (a) that visual masking techniques, by revealing more detailed aspects of target masking and target recovery, support a theoretical approach to visual masking and visual perception that must take into account activities in two separate neural channels or processing streams and, as a corollary, (b) that at the current stage of methodological sophistication visual masks, by acting in more (B.G. Breitmeyer).