Gamma-band neuronal oscillations are largely absent in the lateral geniculate nucleus of the Awake behaving macaque monkey.

Shih-Cheng Yen and Charles M. Gray.

Center for Neuroscience, University of California, Davis, CA 95616.

Abstract

Several studies have shown that cells in the primary visual cortex display stimulus-evoked oscillatory firing patterns in the gamma frequency range (30-60 Hz). One possible source of these oscillations may be the afferent inputs from the lateral geniculate nucleus, where cells have also been shown to display oscillatory firing patterns. To test this hypothesis, we recorded spontaneous and stimulus-evoked unit activity in the LGN of an alert macaque monkey while the animal performed a visual fixation task. The stimuli included spots and drifting sine-wave gratings of varying spatial and temporal frequencies. We performed 38 single-unit and 48 multi-unit recordings in all layers of the LGN. The auto-correlation histogram and its power spectrum were computed for each spike train and compared to 500 pseudorandom spike trains with the same mean firing rate. A spectral peak was considered significant if it exceeded 99% of the peak values in the random spike trains. Eighteen of these recording sites (6 single-unit, 12 multi-unit) exhibited oscillations as a result of entrainment to the 80 Hz refresh rate of the computer monitor. Only eight (9.3%) recording sites (4 single-unit, 4 multi-unit) exhibited significant stimulus-evoked oscillatory activity. The frequency of these oscillations ranged from 30-100 Hz and their amplitudes were much smaller than those observed previously in the anesthetized cat. Oscillatory activity was either not significant or absent in the other 70% of the recording sites. Our results suggest that cortical gamma-band activity in the alert macaque monkey is unlikely to arise solely from afferent inputs from the LGN and is probably largely mediated by cortical mechanisms.

Supported by the McDonnell-Pew Program in Cognitive Neuroscience, and the National Eye Institute.