Evoked responses to rhythmic visual stimulation vary across sources of intrinsic alpha activity in humans

Abstract

Rhythmic flickering visual stimulation produces steady-state visually evoked potentials (SSVEPs) in electroencephalogram (EEG) recordings. Based on electrode-level analyses, two dichotomous models of the underpinning mechanisms leading to SSVEP generation have been proposed: entrainment or superposition, i.e., phase-alignment or independence of endogenous brain oscillations from flicker-induced oscillations, respectively. Electrode-level analyses, however, represent an averaged view of underlying ‘source-level’ activity, at which variability in SSVEPs may lie, possibly suggesting the co-existence of multiple mechanisms. To probe this idea, we investigated the variability of SSVEPs derived from the sources underpinning scalp EEG responses during presentation of a flickering radial checkerboard. Flicker was presented between 6 and 12 Hz in 1 Hz steps, and at individual alpha frequency (IAF i.e., the dominant frequency of endogenous alpha oscillatory activity). We tested whether sources of endogenous alpha activity could be dissociated according to evoked responses to different flicker frequencies relative to IAF. Occipitoparietal sources were identified by temporal independent component analysis, maximal resting-state alpha power at IAF and source localisation. The pattern of SSVEPs to rhythmic flicker relative to IAF was estimated by correlation coefficients, describing the correlation between the peak-to-peak amplitude of the SSVEP and the absolute distance of the flicker frequency from IAF across flicker conditions. We observed extreme variability in correlation coefficients across sources, ranging from −0.84 to 0.93, with sources showing largely different coefficients co-existing within subjects. This result demonstrates variation in evoked responses to flicker across sources of endogenous alpha oscillatory activity. Data support the idea of multiple SSVEP mechanisms.

Spectral waveform analysis dissociates human cortical alpha rhythms

“…Furthermore, the lateral parietal region showed the most consistent relative harmonic phase, which was significantly non-uniform up to the sixth harmonic. Our finding of a distinct parietal alpha rhythm accords well with previous findings suggesting that parietal and occipital alpha oscillations may represent functionally distinct rhythms (Barzegaran et al, 2017;Haegens et al, 2014;Nuttall et al, 2022;Sokoliuk et al, 2019).…”

Electrophysiological model of human temporal contrast sensitivity based on SSVEP

“…Event-related alpha desynchronization during visual processes has been previously reported (Pfurtscheller et al, 1994 ; Suffczynski et al, 2001 ), which might have implications for the responses in that range. Variation in evoked responses to flicker across sources of endogenous alpha oscillatory activity has also been observed (Nuttall et al, 2022 ). In our previous research (Tsoneva et al, 2015 ), we also noted several distinct evoked patterns in delta (1–4 Hz), theta (4–8 Hz), alpha (8-12 Hz) and beta (15-30 Hz) bands, which could contribute to the SSVEP response measured at those frequencies.…”

Effect of frequency and rhythmicity on flicker light-induced hallucinatory phenomena

“…In this light, it should be considered that other mechanisms can also contribute to oscillatory activity, such as the superposition of event-related responses, which are evoked cortical responses to visual stimulation that add onto, but do not interact with, ongoing oscillations [48]. Moreover, a recent study using rhythmic flickering checkerboards found evidence for both frequency-specific neural responses, supporting the entrainment model, and frequency-independent resonance phenomena, supporting the superposition model [49]. As we found that intensity of FLS effects was affected by frequency, it is likely that underlying frequency-specific neural responses, such as entrainment, contribute to FLS effects.…”