Influence of seamlessness between pre- and poststimulus alpha rhythms on visual evoked potential

Abstract

The influence of seamlessness between the prestimulus alpha rhythm and poststimulus alpha ringing on the visual evoked potentials (VEPs) was investigated. Subjects passively viewed a series of 1000 flash stimuli with their eyelids closed, and their VEPs were recorded. The instantaneous phase angle of the alpha rhythm in each subject was calculated by using a two-cycle complex exponential sequence. VEPs were classified into four subsets according to seamlessness: how well the phase angle of the prestimulus alpha rhythm and the backward-extrapolated phase angle from poststimulus alpha ringing were synchronized. VEPs of each subset were averaged. A one-way repeated measures analysis of variance revealed that seamlessness significantly affected the amplitude of P100. Moreover, the level of seamlessness significantly affected the phase locking index. Two models for the generating evoked potentials have been proposed; one is the phase resetting model (Makeig, S., Westerfield, M., Jung, T.P., Enghoff, S., Townsend, J., Courchesne, E., Sejnowski, T.J., 2002. Dynamic brain sources of visual evoked responses. Science 295, 690–694) and the other is the evoked model (Mäkinen, V., Tiitinen, H., May, P., 2005. Auditory event-related responses are generated independently of ongoing brain activity. Neuroimage 24, 961–968). These results suggest that alpha ringing is possibly generated by the phase-resetting alpha rhythm and support the phase resetting model.

Introduction

Over the years, many studies have investigated the relationship between ongoing oscillations, say, alpha, beta, gamma rhythms, and evoked potentials (EPs), which are recorded with electroencephalography (EEG). Many studies indicated that the amplitudes of the ongoing oscillations influenced the amplitudes of EPs (Başar and Stampfer, 1985, Brandt et al., 1991, Brandt and Jansen, 1991, Jansen and Brandt, 1991, Rahn and Ba°ar, 1993a, Rahn and Ba°ar, 1993b, Brandt, 1997, Barry et al., 2000, Makeig et al., 2002, Klimesch et al., 2004). On the other hand, several studies reported that the phase of the alpha rhythm at the stimulus onset influenced the amplitudes of EPs (Dustman and Beck, 1965, Rémond and Lesèvre, 1967, Jansen and Brandt, 1991, Brandt, 1997, Haig and Gordon, 1998, Barry et al., 2003, Barry et al., 2004). However, the previous studies regarding the phase relationship have an essential problem. The problem is how to derive the adjustment angle by which the absolute phase angle of the alpha rhythm at the stimulus onset is determined. The alpha frequency and latency necessary for transmitting information on a stimulus to cortex are different among subjects. Therefore, the apparent phase angles at the stimulus onset have different meanings among subjects in the light of processing in cortex (Barry et al., 2004). The adjustment angles of all subjects were, nevertheless, treated as a common angle in the previous studies (Dustman and Beck, 1965, Rémond and Lesèvre, 1967, Jansen and Brandt, 1991, Brandt, 1997, Barry et al., 2003, Barry et al., 2004). Such a treatment does not allow for generalization.

In this study, we determined the adjustment angles for individual subjects by alpha ringing. Alpha ringing is a sequence of visual evoked potentials (VEPs) in the alpha frequency range (8–13 Hz) around 500 ms in latency (Jung et al., 2001, Makeig et al., 2002). The prestimulus alpha rhythm whose phase at the stimulus onset is the adjustment angle can seamlessly connect to poststimulus alpha ringing. Then, we investigated the relationship between the phase of the alpha rhythm at the stimulus onset based on the adjustment angle and the amplitude of P100: i.e., the relationship between seamlessness from the prestimulus alpha rhythm to poststimulus alpha ringing and the amplitude of P100.