What does Neural Plasticity Tell us about Role of Primary Visual Cortex (V1) in Visual Awareness?

Geraint Rees,Juha Silvanto

doi:10.3389/fpsyg.2011.00006

Abstract

The complete loss of visual awareness resulting from a lesion to the primary visual cortex (V1) suggests that this region is indispensable for conscious visual perception. There are however a number cases of conscious perception in the absence of V1 which appear to challenge this conclusion. These include reports of patients with bilateral V1 lesions sustained at an early age whose conscious vision has spontaneously recovered, as well as stroke patients who have recovered some conscious vision with the help of rehabilitation programs. In addition, the phenomenon of hemianopic completion and percepts induced by brain stimulation suggest that V1 may not be necessary for conscious perception in all circumstances. Furthermore, that the visual abilities in the cat are associated with the recovery of normal extrastriate tuning properties rather than emulation of V1 functions suggests that there is nothing unique about the functional properties of this region in visual awareness. Rather, the dramatic effect of a V1 lesion on visual awareness may be due to its role in providing the majority of extrastriate visual input, the loss of which abolishes normal neural responsiveness throughout the visual cortex.

Highlights

A lesion to the primary visual cortex (V1) abolishes all conscious visual perception in the corresponding part of the visual field
These include reports of children born without V1, stroke patients who have recovered some conscious vision with the help of rehabilitation programs, as well as the phenomenon of hemianopic completion and percepts induced by brain stimulation
Such cases are important to the study of neural correlates of consciousness (NCC): if conscious perception of a given attribute is initially abolished by a lesion but subsequently recovers, this indicates that the NCC of that attribute is not fixed to the affected brain region

Summary

Introduction

A lesion to the primary visual cortex (V1) abolishes all conscious visual perception in the corresponding part of the visual field. There are a number cases of conscious visual perception in the absence of V1 which appear to challenge the view that this region plays an indispensable role in visual awareness These include reports of children born without V1, stroke patients who have recovered some conscious vision with the help of rehabilitation programs, as well as the phenomenon of hemianopic completion and percepts induced by brain stimulation. At 6 years 8 months, visual recognition acuity was 10/10 in both eyes and neuro-ophthalmological examination was normal, except for persistence of the visual field defect in the upper hemifield and a selective impairment for complex visuospatial skills This case appears to suggest that conscious perception can almost fully recover in the absence of V1. Expansion of pathways that can bypass V1 and directly connect subcortical nuclei with extrastriate visual structures is believed to be critical in neural plasticity following the loss of V1 and may mediate blindsight (see, e.g., Cowey, 2004; Payne and Lomber, 2002)

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Conclusion