Contralesional neglect in monkeys with small unilateral parietal cortical ablations.

Transient contralesional spatial neglect, in addition to motor impairment in the contralesional arm, is sometimes seen in patients following cerebral infarction in the right hemisphere and is seen following experimental occlusion of the right middle cerebral artery in primates. To test whether contralesional visuospatial neglect arises from a disruption of the forward flow of information from the striate cortex through the dorsal territory of the middle cerebral artery, we made a small strip suction ablation in the right parietal cortex from the medial edge of the dorsal cortical surface to the posterior ventral edge of the superior temporal gyrus in marmoset monkeys. These monkeys did not exhibit a motor impairment, or misreaching, with the contralesional arm. When they were unrestrained and free to use either arm, they were impaired at finding rewards in their contralesional space and in choosing the nearer of two rewards hidden in ipsilesional space (i.e. they had an ultra-ipsilesional bias in ipsilesional space). Comparison of performance under four conditions in a task in which the monkeys were constrained to reach into each hemispace with each arm separately indicated that they were impaired at reaching into contralesional, but not ipsilesional, space with either arm but they did not exhibit any impairment confined to the contralesional arm. These impairments in contralesional space were transient suggesting that the monkeys were able to re-align their egocentric spatial coordinates to obviate these deficits.