Spontaneous alpha-band amplitude predicts subjective visibility but not discrimination accuracy during high-level perception

Abstract

Near-threshold perception is a paradigm case of perceptual reports diverging from reality - perception of an unchanging stimulus can vacillate from undetected to clearly perceived. Among the many factors that predict whether a stimulus will reach awareness, the amplitude of low-frequency brain oscillations - particularly in the alpha frequency band (8-13 Hz) - has emerged as a reliable predictor of trial-to-trial variability in perceptual decisions. Analysis grounded in signal detection theory suggest that strong prestimulus alpha oscillations diminish subjective perception without affecting the accuracy or sensitivity (d) of perceptual decisions. These results, coupled with recent studies on sensory responses, point to an inhibitory influence of alpha-band amplitude on early visuocortical activity. The findings to date have been based on simple, low-level visual stimuli, which warrant a focus on early visual processing. However, the physiology of alpha in higher-level visual areas is known to be distinct from early visual cortex, with evidence indicating that alpha amplitude in the inferior temporal (IT) cortex is excitatory (rather than inhibitory, as in early visual cortex). Here, we addressed the question of how spontaneous oscillatory amplitude impacts subjective and objective aspects of perception using a high-level perceptual decision task. Human observers completed a near-threshold face/house discrimination task with subjective visibility ratings while electroencephalograms (EEG) were recorded. Using a single-trial multiple regression analysis, we found that spontaneous fluctuations in pre-stimulus alpha-band amplitude were negatively related to visibility ratings but did not predict trial-by-trial accuracy. These results suggest that the inhibitory influence of prestimulus alpha activity in early visual cortex, rather than the excitatory influence of alpha in IT, comes to bias high-level perceptual reports. Our findings provide further evidence that ongoing alpha amplitude dissociates subjective and objective measures of visual perception.