Visual discrimination performance following partial ablations of the temporal lobe: II. Ventral surface vs. hippocampus.

Abstract

Results of an earlier study showed that marked impairment on visual discrimination followed ablation of the ventral, but not the lateral, portions of the temporal lobes of monkeys. This study attempted (1) to delimit further the locus of damage necessary to produce a gross decrement in performance and (2) to determine whether or not such decrement increased with an increase in the difficulty of the task. After preoperative training on a series of discriminations of varying difficulty ventral temporal cortex was removed bilaterally in 3 monkeys, the hippocampal formation was removed in 3 others, and lateral and polar control operations were performed on 2 monkeys. The lesions were verified post-mortem. All animals were retrained postoperatively with the discriminations presented in 2 different sequences. Markedly greater impairment was produced by the ventral surface lesions than by either the hippocampal or control ablations. The number of postoperative failures to learn and the amt. of retardation in relearning varied directly with the difficulty of the tasks, irrespective of the order in which they were presented. Deficit on the discrimination was not correlated with visual field defects, losses in acuity, or impairment in visual delayed response.