The pulvinar nucleus is perhaps the most enigmatic structure in the diencephalon. In man and monkey it occupies the posterior pole of the thalamus and is larger than any other thalamic nucleus. Presumably, its importance is somewhat proportional to its size; and yet, its function remains obscure since damage to the pulvinar rarely produces a discernable behavioral effect. Since neurons in the pulvinar have visual receptive fields 1,22, it is likely that the pulvinar mediates a visual function. However, the results of numerous behavioral studies have shown that monkeys with pulvinar lesions perform normally on a variety of visual discrimination tasks12,24, a7 (for exceptions, see refs. 10 and 34). This is particularly puzzling since there are extensive reciprocal connections between the pulvinar and cortical areas involved in higher levels of visual processing, namely striate 5-9,2°,~a,27,2s,aS, prestriate6-a,11,15,~6,1s,2a,as, 40 and inferior temporal cortex 11, ~6,18,19,aa,as,s8. But, the pulvinar also receives a strong projection from the superficial layers of the superior colliculus4,2~,~4, 36, and the role of the pulvinar in vision may be more closely related to its interaction with the colliculus than to its interaction with the cortex. For example, pulvinar lesions, like collicular lesions, may produce oculomotor impairmentslT,~5,a~,41, a2. To investigate this possibility, we have examined the eye movements of monkeys with pulvinar lesions and compared them with those of normal monkeys during acquisition of a visual pattern discrimination problem. The experimental procedure was as follows. At least 4 months prior to testing, 4 rhesus monkeys (Macaca mulatta) received bilateral pulvinar lesions. The lesions were produced by passing radio frequency current through stereotaxicaUy placed electrodes (see ref. 37). The histological findings are presented in Fig. 1. In all 4 monkeys the lesions were extensive, but subtotal, in all cases sparing the anterior portion of the nucleus. However, in two of the 4 monkeys (P-G17 and P-G19) the caudal extent of pulvinar lateralis and pulvinar inferior was completely destroyed; this portion of the