Abstract
In human subjects, damage of posterior parietal cortex in the right hemisphere often produces a neglect of visual stimuli in the left (contralateral) visual field. Unilateral removal or reversible cooling deactivation of the equivalent region in cats also results in an equally severe contralateral neglect, with the cats unable to report the position of visual stimuli in the contralateral hemifield as assessed by visual orienting. In order to compare the effects of cooling posterior parietal cortex or the superior colliculus in the left hemisphere while posterior parietal cortex of the left hemisphere was deactivated, we implanted cooling loops bilaterally in the posterior middle suprasylvian (pMS) sulcus and over the dorsal surface of the left superior colliculus. As expected, cooling of right pMS sulcal cortex resulted in a profound neglect in the left hemifield with no performance decrease in the right hemifield. Additional deactivation of left pMS sulcal cortex resulted in a restoration of visual orienting performance in the neglected hemifield. Qualitatively, the responses in the restored hemifield were strong, accurate, and indistinguishable from normal. Across the entire visual field, the only positions that maintained decreased performance levels were in the far periphery, bilaterally. Deactivation of the contralateral superior colliculus also resulted in a restoration of visual orienting responses into the neglect hemifield. However, these responses were much weaker and had longer latencies. Furthermore, across also positions in the left hemifield, mean performance levels were only about 50% correct. Therefore, deactivation of the contralateral superior colliculus results in a restoration of visual orienting responses that is both qualitatively and quantitatively inferior to that identified during cooling of the contralateral posterior parietal cortex.
Supported by NSF and NINDS.