Why is “blindsight” blind? A new perspective on primary visual cortex, recurrent activity and visual awareness

The neuropsychological phenomenon of blindsight has been taken to suggest that the primary visual cortex (V1) plays a unique role in visual awareness, and that extrastriate activation needs to be fed back to V1 in order for the content of that activation to be consciously perceived. The aim of this review is to evaluate this theoretical framework and to revisit its key tenets. Firstly, is blindsight truly a dissociation of awareness and visual detection? Secondly, is there sufficient evidence to rule out the possibility that the loss of awareness resulting from a V1 lesion simply reflects reduced extrastriate responsiveness, rather than a unique role of V1 in conscious experience? Evaluation of these arguments and the empirical evidence leads to the conclusion that the loss of phenomenal awareness in blindsight may not be due to feedback activity in V1 being the hallmark awareness. On the basis of existing literature, an alternative explanation of blindsight is proposed. In this view, visual awareness is a “global” cognitive function as its hallmark is the availability of information to a large number of perceptual and cognitive systems; this requires inter-areal long-range synchronous oscillatory activity. For these oscillations to arise, a specific temporal profile of neuronal activity is required, which is established through recurrent feedback activity involving V1 and the extrastriate cortex. When V1 is lesioned, the loss of recurrent activity prevents inter-areal networks on the basis of oscillatory activity. However, as limited amount of input can reach extrastriate cortex and some extrastriate neuronal selectivity is preserved, computations involving comparison of neural firing rates within a cortical area remain possible. This enables “local” read-out from specific brain regions, allowing for the detection and discrimination of basic visual attributes. Thus blindsight is blind due to lack of “global” long-range synchrony, and it functions via “local” neural readout from extrastriate areas

Inducing conscious perception of colour in blindsight

The phenomenon of blindsight the manifestation by forced-choice guessing of visual processing despite the total loss of visual awareness caused by damage to striate cortex (V1) [1] has influenced many current theories of visual awareness [2,3]. We have investigated whether hemianopic subject GY is able consciously to experience colour in his blind field by using a TMS-chromatic adaptation paradigm, in which TMS applied after visual adaptation to a coloured background preferentially facilitates the perception of the adapted attribute [4]. Our results show that GY can perceive colour in his blind field, and that the identity of the colour is determined by the contralesional hemisphere

Effects of reduced contrast on the perception and control of speed when driving

Misperception of speed under low-contrast conditions has been identified as a possible contributor to motor vehicle crashes in fog. To test this hypothesis, we investigated the effects of reduced contrast on drivers’ perception and control of speed while driving under real-world conditions. Fourteen participants drove around a 2.85 km closed road course under three visual conditions: clear view and with two levels of reduced contrast created by diffusing filters on the windscreen and side windows. Three dependent measures were obtained, without view of the speedometer, on separate laps around the road course: verbal estimates of speed; adjustment of speed to instructed levels (25 to 70 km h-1); and estimation of minimum stopping distance. The results showed that drivers traveled more slowly under low-contrast conditions. Reduced contrast had little or no effect on either verbal judgments of speed or estimates of minimum stopping distance. Speed adjustments were significantly slower under low-contrast than clear conditions, indicating that, contrary to studies of object motion, drivers perceived themselves to be traveling faster under conditions of reduced contrast. Under real-world driving conditions, drivers’ ability to perceive and control their speed was not adversely affected by large variations in the contrast of their surroundings. These findings suggest that perceptions of self-motion and object motion involve neural processes that are differentially affected by variations in stimulus contrast as encountered in fog

Methodological considerations in studying awareness during learning: Part 1: Implicit learning

Methodological problems of how awareness during learning should be measured have been extensively discussed and investigated in cognitive psychology. This review considers; 1)whether amnesics can perform implicit learning tasks at a similar level to normal controls, 2) whether differences in instructional orientations create dissociations in performance in tests of implicit and explicit knowledge, and 3) whether participants can retrospectively verbalise the learning outcomes. The paper concludes that; (1) amnesics’ implicit learning abilities differ from the normal controls, (2) instructions on implicit learning do not guarantee the occurrence of implicit learning, and (3) objective and subjective awareness measures used in the literature face inherent problems and so the awareness controversy remains unsettled