Involvement of the ventrolateral thalamic nucleus in rabbit classical eyeblink conditioning.

The involvement of the ventrolateral nucleus of the thalamus in relaying learning-related activity to higher brain structures during classical conditioning of the rabbit eyelid response was examined in two experiments. In the first study, multiple-unit ventrolateral thalamic nucleus activity was monitored before and after lesions of either the cerebellar interpositus nucleus or red nucleus were given. Before the lesions were given, conditioned response-related activity was observed in the ventrolateral thalamic nucleus. Lesions of the interpositus nucleus, but not the red nucleus, disrupted the conditioning-related activity in the ventrolateral thalamic nucleus, thus suggesting that an efferent copy of conditioned response-related activity is projected directly from the interpositus nucleus to higher brain areas by way of the ventrolateral thalamic nucleus. In the second study, multiple unit activity in the hippocampus was monitored before and after lesions were placed in the ventrolateral thalamic nucleus or red nucleus. Conditioning-related activity in the hippocampus was not affected by either lesion, thus suggesting that maintenance of training-related activity in the hippocampus is not critically dependent on cerebellar information relayed through the ventrolateral thalamic nucleus or red nucleus.