Direction-selective motion blindness after unilateral posterior brain damage
Motion blindness (MB) is defined as the selective disturbance of visual motion perception despite intact perception of other features of the visual scene. MB is characterized by a pandirectional deficit of motion direction discrimination and is assumed to result from damage to the visual motion pathway, especially area MT/V5. However, the most characteristic feature of primate MT/V5 neurons is not their motion selectivity but their preference for one direction of motion (direction selectivity), which changes incrementally at neighbouring columns. In addition to this microscopic directional organization, studies in nonhuman and human primates suggest that single directions of motion are also coded at a more macroscopic level. We thus hypothesized that if MB in humans results from damage to direction-selective neurons in the visual motion pathway, posterior brain damage might cause MB which is direction selective, not pandirectional. The present study investigated motion direction discrimination in patients with posterior unilateral brain damage and determined separate psychophysical thresholds for the four cardinal directions. In addition, we analysed whether the direction of erroneous motion perception (i.e. the perception of right motion for upward motion) was random or showed a directional bias. We report three principal findings. First, motion direction discrimination was severely impaired in one or two directions while it was normal in the other directions. This constituted direction-selective MB. Second, MB was characterized not only by a quantitative direction-selective increase in psychophysical thresholds but also by a qualitative impairment of perceiving motion direction systematically in wrong directions. Both findings suggest that the cortical modules specialized for the perception of a single direction of motion might be larger than previously thought. Third, lesion analysis showed that unilateral damage, not only the human homologue of MT/V5 but also to parieto-occipital cortex, leads to MB.