Date of Award

Spring 1995

Project Type


Program or Major


Degree Name

Doctor of Philosophy

First Advisor

Robert G Mair


Twenty-four male rats were pretrained on a two-choice, auditory match to position (AMTP) task and randomly assigned to one of three treatment groups: radio-frequency lesions of either the medial wall (MW) of prefrontal cortex; lesions of the lateral internal medullary lamina site (LIML) of thalamus; or sham surgical control (CON).

Both lesion groups exhibited significant impairment on initial post-surgical testing of discrimination trials in the AMTP task. However, the MW group showed partial recovery with extensive retraining. In contrast, the LIML lesion group remained impaired throughout post-surgical discrimination testing. Neither the MW or the LIML group showed signs of more rapid forgetting on retention interval (RI) trials. Rather, both groups were impaired on trials when no delay was present. This deficit was consistent across all phases of AMTP testing for the LIML group. The MW group was also impaired across all conditions but comparisons to the CON group did not reach statistical significance. Manipulations of the temporal parameters of the task failed to produce differential effects on wither of the lesion groups.

Subsequent training on a go/no-go auditory discrimination demonstrated that the MW lesion group was able to discriminate the auditory stimuli on par with the CON group. This suggested that visuo-spatial factors present in the AMTP task, but not in the go/no-go task may have been responsible for the moderate impairment of the MW lesion group observed on the AMTP task. The LIML lesion group showed varying levels of discrimination in the initial condition of the go/no-go task, and all but one of the LIML lesioned rats were able to demonstrate discrimination when the response requirements of the go/no-go task were further simplified.

The present findings demonstrate that both LIML and MW lesions appear to cause delay independent deficits, rather than impairments that result from imposed delays. The pattern of results from the AMTP task is consistent with previous findings and provides further evidence that medial thalamic lesions produce a polysensory deficit related to task complexity rather than working memory.