Lateral neglect in a head movement task: more impairment with unilateral than bilateral lesions of the superior colliculus in the rat.

Unilateral lesions of the superior colliculus produce contralateral neglect. This study determined if bilateral lesions would impair head orientation in a way expected from the summation of two contralateral neglects. Rats were trained to make various head movements to get water at a 3 x 3 array of holes, each with a recessed water-baited dipper. On each of 20 trials in a session, a water-deprived subject approached and sampled the array by orienting its head and inserting its snout in the holes in a self-determined sequence. The measures recorded for each position and for each trial were: the order in which a baited position was sampled (selection order), the number of times it was not sampled (misses), and the number of times it was sampled when dry (perseverations). Radio-frequency lesions were made and testing was resumed after 3 days. Large unilateral lesions of the superior colliculus produced a neglect of contralateral positions and increased perseverations to ipsilateral positions. Center positions were also neglected but to a lesser extent. Bilateral lesions of the superior colliculus produced only a moderate increase in lateral misses and no increase in lateral perseverations. Bilateral lesions did produce a neglect of upper positions on both sides. The results indicate that a major contributor to contralateral neglect produced by a unilateral lesion is an antagonistic process that promotes orientation to the side ipsilateral to the lesion.