Effects of neuronic shutter observed in the EEG alpha rhythm

Abstract

The posterior alpha rhythm, seen in human electroencephalogram (EEG), is posited to originate from cycling inhibitory/excitatory states of visual relay cells in the thalamus. These cycling states are thought to lead to oscillating visual sensitivity levels termed the “neuronic shutter effect.” If true, perceptual performance should be predictable by observed alpha phase (of cycling inhibitory/excitatory states) relative to the timeline of afferentiation onto the visual cortex. Here, we tested this hypothesis by presenting contrast changes at near perceptual threshold intensity through closed eyelids to 20 participants (balanced for gender) during times of spontaneous alpha oscillations. To more accurately and rigorously test the shutter hypothesis than ever before, alpha rhythm phase and amplitude were calculated relative to each individual’s retina-to-V1 conduction delay, estimated from the individual’s C1 visual-evoked potential latency. Our results show that stimulus observation rates are greater at a trough than a peak of the posterior alpha rhythm when phase is measured at the individual’s conduction delay relative to stimulus onset. Specifically, the optimal phase for stimulus observation was found to be 272.41°, where observation rates are 20.96% greater than the opposing phase of 92.41°. The perception-phase relationship is modulated by alpha rhythm amplitude and is not observed at lower amplitude oscillations. Collectively, these results provide support to the “neuronic shutter” hypothesis and demonstrate a phase and timing relationship consistent with the theory that cycling excitability in the thalamic relay cells underly posterior alpha oscillations.

Significance Statement After accounting for neural conduction delays, we found that threshold intensity stimuli are observed at higher rates when the alpha wave is at a trough phase than at a peak phase, but only when alpha amplitude is high. Our results were derived using methods consistent with a specific hypothesis about a mechanism of visual perception, considering the structure, physiology, and transmission delays in the visual system. The results of this rigorous study design add support to the neuronic shutter hypothesis and are consistent with the theory that posterior alpha reflects cycling excitability in thalamic relay cells, thereby gating the flow of visual information.