Stimulus-dependent oscillations in the cat visual cortex: differences between bar and grating stimuli

We have investigated the dependence of cortical oscillations on the type of visual stimulus. Single unit recordings were performed in areas 17 and 18 of the cat visual cortex. Among 217 cortical neurons oscillations in the frequency range of 22-102 Hz were found in 29 cells (13%). The proportion of oscillating cells was higher (16%) if both bar and grating stimuli were used to stimulate cortical neurons. It was found that gratings are more effective than bars in triggering oscillatory patterns in cortical cells. Among 21 oscillating cells which were stimulated with both bar and grating stimuli, oscillations evoked with gratings were found in 17 neurons (81%) while oscillations evoked with bar stimuli were triggered in 7 cells (33%). The distributions of oscillation frequencies were statistically different for oscillations evoked with bars and gratings. Frequencies of oscillations evoked with bars were in the lower and higher range than frequencies of oscillations evoked with gratings. In 3 cells (14%), rhythmic patterns could be evoked with both bar and grating stimuli. However, the oscillations were of different frequencies. No significant correlation was found between the strength of oscillations and firing rate of cortical neurons. Both simple and complex cells manifested the same dependence on stimulus type. However, complex cells mostly exhibited oscillations in the lower frequency range while simple cells did so when neurons were stimulated with bars. The results suggest that various classes of visual stimuli can be coded by a temporal pattern of cortical responses.