A corollary discharge (CD) is a copy of a neuronal command for movement sent to other brain regions to inform them of the impending movement. In monkeys, a circuit from superior colliculus (SC) through medial-dorsal nucleus of the thalamus (MD) to frontal eye field (FEF) carries such a CD for saccadic eye movements. This circuit provides the clearest example of such internal monitoring reaching cerebral cortex. In this report we first investigated the functional organization of the critical MD relay by systematically recording neurons within a grid of penetrations. In two male rhesus macaque monkeys ( Macaca mulatta ), we found that lateral MD neurons carrying CD signals discharged before saccades to ipsilateral as well as contralateral visual fields instead of just contralateral fields, often had activity over large movement fields, and had activity from both central and peripheral visual fields. Each of these characteristics has been found in FEF, but these findings indicate that these characteristics ar...