Visuospatial discrimination deficit in rats after ibotenate lesions in anteromedial visual cortex.

To assess the role of the rat anteromedial extrastriate cortex (AM) in a visuospatial discrimination task, restricted bilateral ibotenic acid lesions were placed stereotaxically in this region. Gray rats with lesions in AM were trained in a task requiring them to discriminate the location of a light stimulus placed vertically at different elevations. Correct responses required pressing right or left levers to obtain rewards. In contrast to unoperated controls, lesioned rats failed in learning the visuospatial discrimination task. A correlation was found between the bilateral extent of the lesion in area AM and the behavioral deficit. Another group of lesioned rats was trained to discriminate brightness differences of the light stimulus but requiring the same egocentric right/left motor response. The performance of these rats was similar to that of controls. From these results we conclude that extrastriate area AM in the rat is necessary for visuospatial discrimination, but not for correct egocentric motor responses. The visuospatial functions of area AM in the rat are reminiscent of visuospatial functions ascribed to the parietal streams of extrastriate visual areas in the monkey.