Imparied orientation discrimination and localisation following parietal damage: On the interplay between dorsal and ventral processes in visual perception

We report the case of a patient with left parietal damage (MH) who is selectively impaired at both detecting and localising targets defined in terms of their orientation. Performance was relatively good in other tasks where the target was defined either by a single feature (colour) or an orientation conjunction. The results are consistent with the idea that the dorsal stream supports some aspects of basic visual perception (i.e., the discrimination and localisation of orientation-defined targets). The effect of a parietal deficit may be to stress processing via the intact ventral stream, which groups information for object recognition, making differences in orientation less salient. Consistent with this, MH performed better when localising targets at a larger display size, where elements were less likely to group into a familiar shape and where local texture-based processes can come into play. In addition, MH's ability to localise a target was more impaired than his ability to detect a target, supporting the argument that orientation discrimination precedes (or operates independently of) feature localisation.