Eight rhesus monkeys (Macaca mulatta) were trained to detect an instantaneous lateral displacement of a small spot of light. The smallest movement for 79% correct performance was then determined. Severing the splenium of the corpus callosum, which has to be done to reveal the superior colliculi, had no effect on the movement threshold. But when the superior colliculi were damaged in addition, there was an initial substantial impairment in the detection of movement and a smaller but permanent elevation in the threshold. Only if the rostral superior colliculi and pretectum were spared, was there no change in threshold, and there was evidence that the pretectal rather than collicular damage may be more important in relation to movement discrimination. Tests with human observers performing the same task showed that the threshold can be elevated by imprecise fixation and accomodation, suggesting that the consequences of mid-brain damage on movement detection may be wholly or in large part attributed to oculomotor disorders.