Impaired peripheral reaching and on-line corrections in patient DF: optic ataxia in visual form agnosia?

An influential model of vision suggests there are two visual streams within the brain: a dorsal occipito-parietal stream which mediates action and ventral occipito-temporal stream which mediates perception. One of the cornerstones of this model comes from DF, a patient with visual form agnosia after bilateral ventral stream lesions. Despite her inability to recognize visual objects it has been shown that she can execute visually-guided actions towards them with a high-level of skill. These observations have been widely interpreted as demonstrating a double dissociation from optic ataxia, a condition observed after bilateral dorsal stream damage in which patients are unable to act towards objects that they can recognize. One major criticism against this view is that, while the visuomotor deficits in optic ataxia are typically manifest only with peripheral visual guidance, the integrity of action toward peripheral targets has not been closely examined in DF. Here we addressed this question, by asking DF to reach to stationary visual targets and also to perform fast on-line corrections in response to jumping targets, both in free and peripheral vision. Surprisingly, DF was remarkably inaccurate when reaching to peripheral stationary targets, but not when she was allowed to foveate the same targets. Moreover, she was also significantly impaired at correcting her reaches to target jumps, even in free vision. Our data suggest that DF presents visuomotor deficits surprisingly similar to those observed in optic ataxia, when tested under appropriately matched conditions