Cross-frequency coupling of neural signals within and between early visual areas V1 and V2

Recently, it was shown that the power of high frequency bands like gamma (>40 Hz) or spiking rate of cortical neurons is modulated by the phase of low frequency bands (especially delta 1-4Hz) - referred to as cross-frequency coupling (CFC). CFC has been shown in wakefulness and under anesthesia during either spontaneous activity or sensory stimulation; and might play an important role in sensory processing and influencing information flow in brain. Here we studied CFC within and between early visual areas V1 and V2 in three anesthetized monkeys (Macaca mulatta). We simultaneously recorded spontaneous activities from 2+2 electrodes localized in V1 and V2 respectively. The signals were filtered into delta (1-4 Hz), low-gamma (20-60 Hz), gamma (65-120 Hz), high-gamma (125-300 Hz) and multiple unit activity (MUA) (900-3200 Hz). The amplitude envelop for low-gamma, gamma, high-gamma and MUA, along with the instantaneous phase [-π, π] for the delta band were extracted using the Hilbert transform. CFC between delta-phase and the amplitude of low-gamma, gamma, high-gamma and MUA was first calculated separately for V1 and V2, and then between V1 and V2 (e.g. comparing the phase of V1 with the power of V2). We found significant CFC in V1 and V2, as well as between V1 and V2. While there were no differences in CFC within the same areas, we observed differences in CFC between the various frequency bands studied and between different areas. Generally the peak for the low-gamma amplitude fell onto the rising phase of the delta oscillation, while the amplitude of the gamma band peaked during the negative slope of the delta phase. The amplitude for high-gamma and MUA peaked when delta was in its negative trough (0 phase). Interestingly, while the preferred phase for intra-area CFC was always around zero, combining the delta phase from either V1 or V2 with the amplitude of the high frequency signals from either V2 or V1 revealed a different picture. When taking the delta phase from V1, the preferred phase of the amplitude of high frequency signals from V2 fell onto the rising phase of the delta oscillation; however, when the delta phase came from V2, the amplitudes of the high frequency bands in V1 peaked during the negative slope of the delta oscillation. Our data suggest that CFC is a general property of the early visual areas, and that different profiles of inter-area CFC might reflect feed forward and feedback processing between cortical areas