THE RESPONSE OF CAT VISUAL-CORTEX TO FLICKER STIMULI OF VARIABLE FREQUENCY

We examined the possibility that neurons or groups of neurons along th e retino-cortical transmission chain have properties of tuned oscillat ors: To this end, we studied the resonance properties of the retino-th alamo-cortical system of anaesthetized cats by entraining responses wi th flicker stimuli of variable frequency (2-50 Hz). Responses were ass essed from multi-unit activity (MUA) and local field potentials (LFPs) with up to four spatially segregated electrodes placed in areas 17 an d 18. MUA and LFP responses were closely related, units discharging wi th high preference during LFP negativity. About 300 ms after flicker o nset, responses stabilized and exhibited a highly regular oscillatory patterning that was surprisingly similar at different recording sites due to precise stimulus locking. Fourier transforms of these steady st ate oscillations showed maximal power at the inducing frequency and co nsistently revealed additional peaks at harmonic frequencies. The freq uency-dependent amplitude changes of the fundamental and harmonic resp onse components suggest that the retinocortical system is entrainable into steady state oscillations over a broad frequency range and exhibi ts preferences for distinct frequencies in the theta- or slow alpha-ra nge, and in the beta- and gamma-band. Concomitant activation of the me sencephalic reticular formation increased the ability of cortical cell s to follow high frequency stimulation, and enhanced dramatically the amplitude of first-and second-order harmonics in the gamma-frequency r ange between 30 and 50 Hz. Cross-correlations computed between respons es recorded simultaneously from different sites revealed pronounced sy nchronicity due to precise stimulus locking. These results suggest tha t the retino-cortical system contains broadly tuned, strongly damped o scillators which altogether exhibit at least three ranges of preferred frequencies, the relative expression of the preferences depending on the central state. These properties agree with the characteristics of oscillatory responses evoked by non-temporally modulated stimuli, and they indicate that neuronal responses along the retino-cortical transm ission chain can become synchronized with precision in the millisecond range not only by intrinsic interactions, but also by temporally stru ctured stimuli.