THE BRAIN ACTIVITY RELATED TO RESIDUAL MOTION VISION IN A PATIENT WITH BILATERAL LESIONS OF V5

We have used the technique of PET to chart the cortical areas activate d by visual motion in the brain of a patient with a severe impairment in the ability to recognize the motion of objects (akinetopsia), follo wing bilateral lesions which have so far been presumed to include area V5. High resolution MRI of her brain showed that the zone occupied by area V5 had indeed been destroyed bilaterally. Positron emission tomo graphy activation images, co-registered to the MRIs, showed three prin cipal regions of the cortex activated by motion. These were located (i ) bilaterally in the precuneus of superior parietal cortex (area 7 of Brodmann); (ii) bilaterally in the cuneus (a region considered to repr esent upper V3); (iii) in the left lingual and fusiform gyri (possibly lower V3 and adjacent areas). In contrast to normal subjects, there w as no significant activation of area V1 or V2. The stimuli used for sc anning were chosen by prior testing of the patient's visual capacities . The control stimulus was a static random distribution of light squar es on a dark background In the moving stimulus these squares moved coh erently, the direction of motion changing periodically between the car dinal directions (left, right, up and down). It tons activated by the stimuli used during scanning. ascertained that the patient could corre ctly identify these directions. We also found (i) that her occasional errors were always in the direction opposite to the motion presented, so that her identification of axis of motion (i,e. vertical or horizon tal) was 100% correct; (iii) that when a few static squares were added to the moving display her identification of direction fell to chance but her identification of the axis of motion remained 100%; (iii) that when a few squares moving opposite and orthogonal to the predominant direction of motion were incorporated, her performance on both directi on and axis fell to chance; (iv) that she was unable to identify motio n in oblique directions between the horizontal or vertical axes, alway s guessing one of the cardinal directions. In accounting for her resid ual vision in terms of cortex which remains active, we hypothesize: (i i) that the bilateral loss of V5 has affected direction sensitive mech anisms at other sites in the cortex which are interconnected with V5 a nd (ii) that in consequence her performance on our tests reflects the properties of dynamic orientation selective mechanisms that were also differentially activated by the stimuli used during scanning.