Pattern evoked potentials in awake rhesus monkeys.

The pattern evoked potential (EP) in man to checkerboard stimulation has been shown to consist of various components originating in different regions of the visual cortex. Surface recordings, however, cannot unambiguously localize the sources of these components; for precision, depth recordings seem to be indicated. Considering the close correspondence in cortical architecture to man, rhesus monkeys (Macaca mulatta) seem to be a suitable animal for such experiments. However, Padmos et al. demonstrated that in monkeys anesthetized by pentobarbital (Nembutal), no pattern EP as found in man could be recorded. The present experiments were carried out to investigate whether the lack of contrast-specific EPs in monkey can be attributed to the effects of anesthesia. Four rhesus monkeys were trained to fixate at a television screen on which checkerboard and bar patterns of various sizes could be presented. The results from this study demonstrate that in monkey as in man, pattern EPs can be obtained that can be distinguished from those evoked by luminance variations. Therefore the awake rhesus monkey seems to be a suitable experimental model in the search for the origin of the pattern EP.