Two distinct deficits of visual tracking caused by unilateral lesions of cerebral cortex in humans.

We studied horizontal visual tracking in 20 patients with unilateral cerebral lesions and in 10 age-matched control subjects. Five patients, all with posterior lesions, showed impaired smooth pursuit of predictable targets moving toward the side of the cerebral lesion. Using nonpredictable step-ramp stimuli, we identified two distinct deficits of visual tracking. The first was a unidirectional deficit of smooth pursuit, for targets moving toward the side of the lesion, in response to stimuli presented into either visual hemifield. The second deficit, identified in a sixth patient who did not show pursuit asymmetry to predictable targets, was a bidirectional inability to estimate the speed of a moving target in the visual hemifield contralateral to the side of the lesion; this caused inaccurate saccades to moving (but not stationary) targets and impaired smooth pursuit initiation. These visual tracking deficits were independent of homonymous hemianopia or hemispatial neglect. These two tracking deficits are similar to those described in rhesus monkeys with lesions of the medial superior temporal and middle temporal visual areas.