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1 Department of Cognitive and Behavioral Sciences, Kyoto University Graduate School of Human and Environmental Studies, Kyoto, Kyoto, Japan
* To whom correspondence should be addressed. E-mail: h50400{at}sakura.kudpc.kyoto-u.ac.jp.
The thalamic mediodorsal nucleus (MD) has strong reciprocal connections with the dorsolateral prefrontal cortex (DLPFC), suggesting that the MD, like the DLPFC, participates in higher cognitive functions. To examine MD's participation in cognitive functions, we analyzed the characteristics of task-related activities sampled homogeneously from the MD while two monkeys performed a spatial working memory task using oculomotor responses. Of 141 task-related MD neurons, 26%, 53%, and 84% exhibited cue-, delay-, and response-period activity, respectively. Most of cue- and response-period activities showed phasic excitation and most of delay-period activity showed tonic sustained activation. Among neurons with response-period activity, 74% exhibited pre-saccadic activity. Most cue-period, delay-period, and pre-saccadic activities were directional, whereas most post-saccadic activity was omni-directional. A significant contralateral bias in the best directions was present in cue-period activity and pre-saccadic activity. However, such bias was not present in delay-period activity, although most neurons had a best direction toward the contralateral visual field. We compared these characteristics with those observed in DLPFC neurons (Funahashi et al. 1989-91). Response-period activity was more frequently observed in the MD (84%) than in the DLPFC (56%). The directional selectivity and bias of task-related activities, and the ratios of pre- and post-saccadic activities were different between MD and DLPFC. These results indicate that the MD participates in higher cognitive functions such as spatial working memory. However, the manner in which these two structures participate in these processes differs, in that the MD participates more in motor control aspects compared to the DLPFC.
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