Retention performance of a learned delayed-alternation task after chemical lesions of the cats mediodorsal nucleus.

The mediodorsal nucleus of the thalamus was lesioned in cats, which had learned a spatial delayed-alternation task. Lesions were carried out with kainic acid or ibotenic acid. From altogether 29 cats, 9 cats with bilateral and 6 cats with unilateral lesions of the mediodorsal nucleus together with a control group of 4 cats, were included in the final data analysis. Lesions in the operated cats destroyed variable portions of the mediodorsal nucleus. Consistently, however, neither the midline nuclei, situated next to the damaged mediodorsal nucleus, nor fiber tracts traversing or by-passing the mediodorsal nucleus, were damaged. Furthermore, remote lesion effects were not detected either in the diazepam-pretreated cats with kainic acid-induced lesions or in the ibotenic acid-lesioned animals. Cats injected with ibotenic acid at a concentration 8-fold higher than the kainic acid solution, showed smaller thalamic lesions than kainic acid-injected cats. A direct correlation was found between the extent of neuronal damage within the mediodorsal nucleus and the degree of the behavioral impairment. Cats with complete or almost complete bilateral lesions of the mediodorsal nucleus manifested severe deficits in retention of the delayed-alternation task, while cats with small, bilateral lesions of the mediodorsal nucleus or with unilateral lesions were impaired less severely or even not at all.