Preserved motion processing and visuomotor control in a patient with large bilateral lesions of occipitotemporal cortex

According to Goodale and Milner, visual perception is mediated by the ventral stream and the visual control of action by the dorsal stream of cortical visual projections. The initial neuropsychological evidence for this idea was based strongly on a single case, DF, who has impaired object recognition but spared visual control of grasping, consistent with lesions to lateral occipital (LO) cortex but intact dorsal-stream processing. Here we present data from a new patient, MC, a 38-year-old woman who shows very similar behaviour to DF despite much more extensive bilateral occipitotemporal lesions that encompass not only LO but most of early visual cortex in the occipital lobe, except for a small tag of tissue in the rostral calcarine cortex (Culham et al., VSS 2008). MC shows some preserved motion perception, but is unable to identify line drawings (or real exemplars) of common objects or discriminate colours or visual textures. Not surprisingly, MC cannot discriminate between rectangular objects with different dimensions; nor can she indicate their width manually. Remarkably, however, when she reaches out to grasp such objects, her in-flight grasp scales to the object's size. Similarly, even though MC cannot discriminate between objects of varying shape, she chooses stable grasp points on those objects when she reaches out to pick them up. The case of MC not only reinforces the conclusions about separate visual processing for perception and action drawn from DF, but also suggests that visuomotor mechanisms in the dorsal stream are capable of mediating the processing of object features such as size, shape, and orientation for the control of visually guided grasping even with highly impoverished (or perhaps entirely absent) input from the ventral stream and early visual areas.