Preserved and impaired detection of structure from motion by a ''motion-blind'' patient

Following bilateral extrastriate damage to areas that include the suspected human homologue of V5/MT, the patient LM has a specific deficit in processing moving stimuli. She has difficulty detecting the movement or coding the velocity of single moving dots. Nevertheless, we find that she can report human actions in Johansson ''biological motion'' displays. This requires the accurate coding of the direction and velocity of many moving dots. The implication is that structure can be extracted from motion in regions of visual cortex other than those traditionally associated with motion processing. However, she cannot report the spatial disposition of the actors whose actions she has recognized, not their movement in depth relative to her. A possible interpretation is that coding in these additional regions is primarily object-centred. Adding a small number of random stationary ''noise'' dots to the display prevents her from identifying the actions, suggesting that segregation by motion is implemented within the traditional movement areas.