PERCEPTION AND ACTION IN ‘VISUAL FORM AGNOSIA’

TLDR

A single case study of a patient with visual form agnosia is presented. The authors propose that the perceptual disorders arise from a selective loss of cortical magnocellular processing and a disconnection from a central occipital processor of visual boundaries and shape primitives. The patient exhibits a severe visual recognition deficit affecting shape, reflectance, and orientation discrimination, yet retains visual acuity, colour vision, tactile recognition, and intelligence, can use visual patterns to guide hand movements and read whole words, and lacks awareness of shape primitives via Gestalt grouping.

Abstract

A single case study of a patient with 'visual form agnosia' is presented. A severe visual recognition deficit was accompanied by impairments in discriminating shape, reflectance, and orientation, although visual acuity and colour vision, along with tactile recognition and intelligence, were largely preserved. Neuropsychological and behavioural investigations have indicated that the patient is able to utilize visual pattern information surprisingly well for the control of hand movements during reaching, and can even read many whole words, despite being unable to make simple discriminative judgements of shape or orientation. She seems to have no awareness of shape primitives through Gestalt grouping by similarity, continuity or symmetry. It is proposed that many of these perceptual disorders might be the combined result of (1) a selective loss of the cortical elaboration of the magnocellular visual processing stream, and (2) a selective output disconnection from a central processor of visual boundaries and shape primitives in the occipital cortex.