The anatomy of blindsight

Abstract

Part of every neurological examination is an assessment of the visual field, obtained by asking patients to detect the presence of test targets on a defined background. Clinicians use either visual field testing to confrontation or more careful mapping and quantification of sensitivity of the visual field using automated perimetry. All of these approaches rely on the patient indicating whether they are aware of the presence of a test target at a particular location. Such testing can establish the presence of scotomas or blind regions of the visual field, where patients report that they do not perceive anything presented in such regions. Systematic correspondences between the location of visual field defects and the site of brain damage in the occipital lobe are sufficient to permit determination of a retinotopic map or projection of the retinal image onto what is now known as primary visual cortex (V1) on the medial surface of the occipital lobe (Inouye, 1909). Damage to V1 is thus associated with a lack of awareness for stimuli presented at corresponding points in the visual field, consistent with a role for V1 in visual awareness (Rees, 2007). However, during visual field testing, few if any clinicians would routinely press their patients to try and guess properties of stimuli that they resolutely deny being able to see when they are presented in a scotoma. But, when such a systematic investigation is performed using forced-choice procedures, remarkably, some patients show residual visual capacity in their blind field. These patients are able to perform certain discriminations and localizations better than chance in the acknowledged absence of awareness. This ability has become known as ‘blindsight’. We …