Date of Award

Fall 2004

Project Type

Dissertation

Program or Major

Psychology

Degree Name

Doctor of Philosophy

First Advisor

Robert G Mair

Abstract

Imaging, recording and lesioning studies implicate the basal ganglia and anatomically related regions of frontal cortex in visuomotor learning. Two experiments were conducted to elucidate the role of frontal cortex and striatum in visuomotor learning. Several tasks were used to characterize motor function including: a visuomotor reaction time (VSRT) task, measuring response speed and accuracy to luminance cues; simple stimulus-response (S-R) learning, measuring VSRT improvements when cues occurred in consistent locations over several trials; and a serial reaction time (SRT) task measuring motor sequence learning. SRT learning was characterized by incremental changes in reaction time (RT) when trained with the same sequence across daily sessions and by abrupt RT changes when switched to random sequence sessions.

In experiment 1, rats with excitotoxic lesions in primary (M1) or secondary (M2) motor cortex, primary and secondary (M1M2) motor cortices, medial prefrontal cortex (mPF) or sham surgery were tested on these tasks. Cortical lesions slowed RT in the VSRT task but did not impair short- or long-term simple S-R learning. Cortical lesions increased RTs for the initial response of a 5-response sequence in the SRT task that was exacerbated when performing repeated (learned) sequences. All groups demonstrated visuomotor sequence learning including incremental changes in RTs for later responses in learned sequences that reversed abruptly when switched to random sequences.

Rats in experiment 2 were given lesions in dorsolateral striatum, dorsomedial striatum, complete dorsal striatum, ventral striatum and sham surgery. Rats with ventral striatal lesions were unimpaired on any visuomotor task demonstrating shorter RTs than controls on most measures. Dorsomedial striatal lesions significantly impaired all VSRT performance measures. Striatal lesions had no effect on short or long-term simple S-R learning. Lesions involving dorsomedial striatum disrupted initiation of motor sequences in the SRT task. This impairment was exaggerated when performing well-learned sequences. Striatal lesions did not disrupt the incremental RT changes of later responses in the sequence indicative of motor learning. Results suggest that cortico-striatal circuits are involved in initiating learned motor sequences consistent with a role in motor planning. These circuits do not appear essential for acquisition or execution of learned visuomotor sequences.

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