Can a motion-blind patient reach for moving objects?


: Dr Thomas Schenk, as above.


It has been claimed that the visual brain is organized in two separate processing streams for spatial vision: one for perception and one for action. To determine whether motion vision is also divided into vision for action and for perception we examined the interceptive behaviour of the motion-blind patient LM. The task for LM and three age-matched control subjects was to reach-and-grasp for an object that moved away. Three experiments were conducted to examine the effects on perfomance of target speed (Expt 1), observation time (Expt 2) and visual feedback (Expt 3). As LM is only able to reach for objects which move at 0.5 m/s or less, her performance is inferior to that of controls who can reach for objects moving at 1.0 m/s, but it is better than would be expected from her performance in psychophysical experiments on her motion vision. Kinematic analysis of LM's reaching movements showed that she adapted the speed of her moving hand to the speed of the target but only when full vision was available. In contrast to normal subjects, LM required long observation times and vision of her moving hand to produce successful reaching responses. Thus, the impairment of both perception and action in LM suggests that the motion area MT/V5 is located at an early stage of the extrastriate hierarchy and provides input to both the perception and the action processing streams.