Can a motion-blind patient reach for moving objects?

It has been claimed that the visual brain is organized in two separate proc essing streams for spatial vision: one for perception and one for action. T o determine whether motion vision is also divided into vision for action an d 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 condu cted to examine the effects on performance of target speed (Expt 1), observ ation time (Expt 2) and visual feedback (Expt 3). As LM is only able to rea ch for objects which move at 0.5 m/s or less, her performance is inferior t o that of controls who can reach for objects moving at 1.0 m/s, but it is b etter than would be expected from her performance in psychophysical experim ents on her motion vision. Kinematic analysis of LM's reaching movements sh owed that she adapted the speed of her moving hand to the speed of the targ et but only when full Vision was available. In contrast to normal subjects, LM required long observation times and vision of her moving hand to produc e successful reaching responses. Thus, the impairment of both perception an d action in LM suggests that the motion area MT/V5 is located at an early s tage of the extrastriate hierarchy and provides input to both the perceptio n and the action processing streams.