Comparison of oculomotor neuronal activity in paralaminar and mediodorsal thalamus in the rhesus monkey.

We previously reported that neurons in the mediodorsal thalamic nucleus (MD) are topographically organized and express spatial and nonspatial coding properties similar to those of the prefrontal areas with which they are connected. In the course of mapping the dorsal thalamus, we also studied neurons in a subset of thalamic nuclei (the caudal part of the ventral lateral nucleus (VLc), the oral part of the ventral posterior lateral nucleus (VPLo), the parvocellular part of the ventral anterior nucleus (VApc)) lateral to the MD and just across the internal medullary lamina. We compared these "paralaminar" neurons to MD neurons by having monkeys perform the same spatial and nonspatial cognitive tasks as those used to investigate the MD; these included two saccadic tasks-one requiring delayed and the other immediate responses-and one picture fixation task. Of the paralaminar thalamic neurons modulated by the saccadic tasks, a majority had saccade-related activity, and this was nearly always spatially tuned. Also, for about half of these neurons, the saccade-related activity occurred exclusively during the delayed-response task. No neurons with event-related activity in the saccadic tasks were preferentially modulated by specific picture stimuli, although other neurons were. All of these results were similar to what we had found for MD neurons. However, in contrast to the high proportion of presaccadic responses observed in the MD, the majority of saccade-related neurons in paralaminar thalamus exhibited mid- or postsaccadic activity, i.e., that started during or after the saccade. Our findings suggest that neurons in the paralaminar thalamus may be possible conduits of oculomotor feedback signals, especially during memory-guided saccades.