Entrainment of theta, not alpha, oscillations is predictive of the brightness enhancement of a flickering stimulus

Abstract

Frequency-dependent brightness enhancement, where a flickering light can appear twice as bright as an equiluminant constant light, has been reported to exist within the alpha (8–12 Hz) band. Could oscillatory neural activity be driving this perceptual effect? Here, in two experiments, human subjects reported which of two flickering stimuli were brighter. Strikingly, 4 Hz stimuli were reported as brighter more than 80% of the time when compared to all other tested frequencies, even though all stimuli were equiluminant and of equal temporal length. Electroencephalography recordings showed that inter-trial phase coherence (ITC) of theta (4 Hz) was: (1) Significantly greater than alpha, contralateral to the flickering stimulus; (2) Enhanced by the presence of a second ipsilateral 4 Hz flickering stimulus; and (3) Uniquely lateralized, unlike the alpha band. Importantly, on trials with two identical stimuli (i.e. 4 Hz vs 4 Hz), the brightness discrimination judgment could be predicted by the hemispheric balance in the amount of 4 Hz ITC. We speculate that the theta rhythm plays a distinct information transfer role, where its ability to share information between hemispheres via entrainment promotes a better processing of visual information to inform a discrimination decision.

Attention differentially modulates the amplitude of resonance frequencies in the visual cortex

“…These results can be partially explained by frequency-dependent enhancement of perceived stimulus brightness: For sinusoidal flicker, as the one we used, brightness enhancement peaks at ~16 Hz (Wu et al, 1996), for square-wave flicker the peak is at ~8 Hz (Bartley, 1938; but see Bertrand et al, 2018).…”

“…The luminance of the blue LEDs was also adjusted taking into account flicker frequency, with a frequency window for the QUEST procedure set to +-5 frequencies surrounding the frequency of interest. This was done to accommodate variability in perceived stimulus luminance as a function of the flicker frequency (Bertrand et al, 2018;Wu et al, 1996).…”

tACS phase-specifically biases brightness perception of flickering light

“…Perceptual competition between two concurrently presented flickering stimuli of equal luminance was biased in favor of the stimulus with greater inter-trial phase coherence of evoked SSRs [13,14]. Further, flicker evoked response amplitudes were shown to correlate not only with increases in physical luminance contrast [15][16][17][18], but also with the strength of illusory brightness percepts as early as in primary visual cortex [2,3,19,20].…”

tACS phase-specifically biases brightness perception of flickering light

“…These excitability modulations may influence the net synchrony in population firing to upcoming visual input, that generates SSR amplitude changes on a neural population level. Flicker-evoked response amplitudes have been repeatedly shown to correlate with the subjective experience of brightness [9,13,14]. As electrophysiological recordings during the application of tACS are contaminated by electrical artifacts, the direct measurement of SSR amplitude changes during tACS was not possible [51,52].…”

“…Noninvasive recordings in humans of flicker entrained activity, i.e., visually evoked steady-state responses (SSRs), converge with these findings. Perceptual competition between two concurrently presented flickering stimuli of equal luminance was biased in favor of the stimulus with greater inter-trial phase coherence of evoked SSRs [13,14]. Further, flicker evoked response amplitudes were shown to correlate not only with increases in physical luminance contrast [15][16][17][18], but also with the strength of illusory brightness percepts as early as in primary visual cortex [2,3,19,20].…”