Absence of behavioural recovery because of absence of cortical reorganization? An fMRI investigation of a left hemianopic patient

Abstract

Following unilateral occipital damage of the primary visual cortex one of the most common visual field defects observed is Homonymous Hemianopia. Most studies have focused on either the visual deficit or the residual capacities in the contralesional visual field in hemianopes. However, visual processing in the central visual field, and its underlying cerebral network, are largely unknown in such patients.

Fourteen healthy males (mean age 55 years +/− 11.4) and a left hemianope following a right occipital lesion (male; age 71 years; delay from lesion: 20 months completed natural scene detection and categorization tasks. In the detection task participants had to press a button if a scene was present on the screen. In the categorization task, they had to press a button if the presented scene was a city and another button if it was a highway. Both tasks were performed in a 1.5T scanner to collect anatomical and functional data.

Regardless of task, behavioural data showed poorer performance in the hemianopic patient than in controls regarding either response accuracy (82% vs. 96%; F(1,42)=20.43; p[[lt]].0001) or response time ( 487.4 vs.391.7ms; F(1,42)=44.10; p[[lt]].0001). In controls, neuroimaging data revealed activation of the extra-striate occipital areas in both hemispheres during the detection task, but only in the left hemisphere during the categorization task. Quite surprisingly, the same networks were involved in the hemianopic patient despite the lesion.

Altogether, our data revealed that even though this patient demonstrates similar cerebral network activation as healthy controls, his poor performance on the tasks suggests: a) visual processing in the central visual field in hemianopic patients may not be as intact as often considered, and b) a lack of behavioural recovery seems to be associated with an absence of cortical reorganization.

Footnotes

 This research was supported by the Edmond and Benjamin de Rothschild Foundations (Geneva and New York).