On oscillating neuronal responses in the visual cortex of the monkey.

1. Recent studies of visual processing in the cat have shown stimulus-related oscillations in the 30- to 70-Hz range. We sought to replicate these findings in the monkey. 2. We recorded multiunit activity (MUA) and local field potentials (LFP) in areas V1 and middle-temporal area (MT), and MUA from the inferotemporal cortex (IT) of monkeys (Macaca fuscata). Recordings in all areas were made under conditions of anesthesia as close as possible to those in previous studies of oscillating responses in the cat. In addition, we recorded MUA in the IT of behaving monkeys while the monkeys performed a face discrimination task. 3. In areas V1 and MT, LFP power spectra showed broadband increases (1-100 Hz) in amplitude on stimulation by swept optimally oriented light bars, and not a shift in power from low to midfrequency, as has been reported in the cat. 4. MUA autocorrelograms (ACGs) classified by fitting Gabor functions, showed oscillations at approximately 10% of recording sites in V1 and MT, but these oscillations were in the alpha range (12-13 Hz). 5. MUA ACGs from IT in the anesthetized monkey showed no oscillations. 6. For MUA ACGs from IT in the behaving monkey, only two recording sites (out of 50) showed an oscillating response, with frequencies of 44 and 48 Hz. One oscillating response was associated with stimulation, and the other was associated with the absence of stimulation. 7. The very low incidence in the monkey of oscillating responses in the 30- to 70-Hz range (2 in 424 recordings made at 142 recording sites) and the absence of stimulus dependence suggest that such oscillations are unlikely to serve a function in the monkey, and that there may be a species difference between monkey and cat in the dynamics of neural activity in the visual cortex. 8. We found that methods of classifying responses as oscillating used in some of the studies of the cat may have led to overestimation of both the number of sites showing oscillation and the number of pairs of sites showing phase coherence. These problems arise from the failure to take account of badness of fit between Gabor functions and their corresponding ACGs, and from Gabor functions "ringing" in response to short phasic phenomena that could be consistent with nonoscillatory activity.