Item

Abstract

The low frequency range (1-75 Hz) of the visual evoked potential (VEP) consists of a relatively robust signal consisting of an early negative deflection at approximately 75ms post-stimulus (N75) followed by a positive going deflection at approximately 100ms post-stimulus (P100). In the high frequency range of the VEP, studies have reported strong oscillatory activity in the 90-120 Hz range for humans (Heinrich et al. 2001), while in the primate it exists in the 80-180 Hz range (Yokoyama et al. 1999). To date, the relationship between the low and high frequency components of the scalp recorded VEP remains unclear. In this study, we measured the scalp VEP in the awake primate on two different days. The results from both days were similar. In the low frequency range, the results clearly showed the classical N75 P100 deflections. In the high frequency range, we observed a clear 100 Hz oscillation that coincided with the N75. However, this 100 Hz oscillation was significantly decreased in the early phase of the P100, and vanished completely at the peak P100 latency. Time-frequency analysis of the data revealed a significant increase in the 100 Hz oscillation at the peak N75 deflection, but not at the peak P100 deflection (p<0.001). Theses preliminary results suggest that the high frequency oscillations of the VEP coincide with the N75 component and not the P100. These findings agree with the notion that the generators of the N75 and P100 are physiologically distinct (Celesia et al. 1982). We are currently conducting intracortical recordings in order to directly compare the VEP frequency bands measured with scalp electrodes to those recorded with microelectrodes.