PRESERVED AND IMPAIRED DETECTION OF STRUCTURE-FROM-MOTION BY A MOTION-BLIND PATIENT

Following bilateral extrastriate damage to areas that include the susp ected human homologue of V5/MT, the patient LM has a specific deficit in processing moving stimuli. She has difficulty detecting the movemen t or coding the velocity of single moving dots. Nevertheless, we find that she can report human actions in Johansson ''biological motion'' d isplays. This requires the accurate coding of the direction and veloci ty of many moving dots. The implication is that structure can be extra cted from motion in regions of visual cortex other than those traditio nally associated with motion processing. However, she cannot report th e 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-centre d. Adding a small number of random stationary ''noise'' dots to the di splay prevents her from identifying the actions, suggesting that segre gation by motion is implemented within the traditional movement areas.