Impaired inter-hemispheric connectivity is a predictor of the failure to retrieve meaning from shape in visual agnosia
Radek Ptak1,2, François Lazeyras3; 1Division of Neurorehabilitation, Department of Clinical Neurosciences, Geneva University Hospitals, Geneva, Switzerland, 2Faculty of Psychology and Educational Sciences, University of Geneva, Geneva, Switzerland, 3Department of Radiology and Medical Informatics, University of Geneva, Geneva, Switzerland
The neural mechanisms underlying the access to object knowledge from a 2D- or 3D-representation of shape are uncertain. On the one hand functional imaging studies support the view that representations of visual properties are distributed across occipito-temporal cortex of both cerebral hemispheres. Brain lesion studies on the other hand show that focal damage to the left or right lateral occipital cortex may lead to visual agnosia - a generalized impairment of object recognition. Using functional MRI we studied functional connectivity (FC) in AL, a patient with visual agnosia following left lateral occipital damage. Despite intact global and local processing of 2D and 3D object structure, the patient made consistent object identification errors. Six different experiments testing naming, visual matching or object priming showed that his errors mainly reflected the global visual similarity between objects. We compared AL’s functional connectivity while watching similar or dissimilar and scrambled objects with age-matched healthy controls. Compared to controls AL exhibited strongly reduced FC between the damaged left and the intact right medial/lateral occipital cortex. In addition, controls showed stronger connectivity between the intact right occipital cortex and the left and right occipito-temporal and prefrontal cortices when participants viewed visually dissimilar as compared to similar objects. These findings support the view that bilaterally distributed coding is necessary for the retrieval of assocative knowledge from shape, and that focal damage to the lateral occipital cortex may have global effects on representations of objects in bilateral occipito-temporal cortex.
Topic Area: PERCEPTION & ACTION: Vision