The effects of thalamic, frontal cortical and hippocampal formation lesions on a delayed nonmatching to sample task

Two areas of the brain have been implicated in global amnesia: the medial temporal lobe and the midline diencephalon (Squire and Zola-Morgan; Victor, Adams, and Collins, 1989). A third area of the brain, the prefrontal cortex, has more recently been included as a memory related area (Fuster, 1989). The purpose of the study reported here was to further examine the neurological basis of the delayed nonmatching to sample place cue task (DNMTS-PC) deficit by comparing lesions in each of the three major areas attributed to memory processes. Patterns of impairments on the DNMTS-PC were further understood by manipulating retention and inter-trial intervals and by extensive pretreatment training. The effects of radio frequency lesions of the L-IML were compared to lesions in the MW and the dentate nucleus of the hippocampal formation (HP). DNMTS was measured with three retention intervals within session (0.4, 1.6, 6.4 s). Inter-trial interval was varied between sessions (0.8, 3.2, 12.8 s). L-IML, MW, and HP groups were significantly impaired on the initial 1008 trials. HP animals recovered their performance within the first 432 trials of post-surgical training. Following recovery of performance HP animals were not significantly impaired for the duration of post-surgical training. L-IML and MW animals continued to be impaired across different inter-trial interval manipulations. L-IML lesions result in substantial impairment on the DNMTS-PC task. MW lesions also produced substantial impairment on the DNMTS-PC task. Although the effects of the L-IML lesion on the DNMTS-PC task can be accounted for by the MW lesion, when compared across studies MW lesions produced more selective impairments on working memory tasks than did L-IML lesions