DIRECTIONAL DEFECTS IN PURSUIT AND MOTION PERCEPTION IN HUMANS WITH UNILATERAL CEREBRAL-LESIONS

We tested motion perception and smooth pursuit in 26 patients with uni lateral cerebral hemispheric lesions. We used random dot cinematograms to test motion direction discrimination. We measured pursuit gain as they followed a predictable sinusoidal target moving horizontally at t hree different frequencies, and an unpredictable horizontal step-ramp target in the ipsilateral hemi-field. Six patients had defects in moti on perception when the targets were moving towards the side of the les ion ('ipsi-directional' defects) and two had bidirectional defects. Mo tion perception defects occurred with lesions of the junction of Brodm ann areas 19 and 37, a putative human homologue of the monkey V5 compl ex. Seven patients had ipsi-directional pursuit defects, Jive of whom had damage to the posterior limb of the internal capsule. Only two pat ients had ipsi-directional defects of both motion perception and sinus oidal smooth pursuit. Four patients had ipsi-directional defects of mo tion perception alone, and five patients had ipsi-directional pursuit defects alone. The two patients with bi-directional defects in motion perception had normal sinusoidal smooth pursuit. Patients with lesions at the 19/37 junction and defects of motion perception alone had norm al pursuit of unpredictable step-ramp targets in the ipsilateral hemi- field. In contrast, patients with ipsi-directional sinusoidal pursuit defects had decreased ipsi-directional and increased contra-directiona l velocities with step-ramp targets. No patient group had a motion-spe cific directional defect in saccadic accuracy. We conclude that neithe r predictable nor unpredictable pursuit is necessarily impaired by les ions of the 19/37 junction that cause ipsi-directional defects of moti on perception. These dissociations between smooth pursuit and motion p erception provide evidence that the pursuit system operates as an inte rconnected network with parallel pathways, rather than as a simple seq uential hierarchy of cortical areas.