Spontaneous alpha-band oscillations modulate stimulus-specific features representation

Abstract

Perception varies between observers, as it does within the same observer at different moments in time, even if the stimulus is held constant. Numerous studies have demonstrated that moment-to-moment fluctuations in neuronal excitability, indicated by spontaneous neuronal oscillations in the alpha-band range, are related to this variability in perceptual decision making. Specifically, strong alpha power has been shown to induce a more conservative criterion in signal detection tasks, thereby reducing the number of correctly detected stimuli. However, the exact mechanism of this effect is largely unknown: do alpha oscillations modulate observers' stimulus-independent decision strategy or the subjective evidence for the specific to-be-detected stimulus? To tackle this question, we recorded EEG from observers (N=30) who performed a detection task with target gratings of a specific orientation and spatial frequency embedded in gaussian noise. We then used a reverse-correlation analysis to test how random, noise-induced variations in target-specific and unspecific features affected observers’ decisions. Specifically, we convolved all presented stimuli with a set of gabor filters of different orientations and spatial frequencies and tested for which stimulus features prestimulus alpha power had the strongest effect on perceptual decisions. Results confirmed that strong prestimulus alpha power reduced the number of correctly reported stimuli. Importantly, this effect was restricted to the features of the to-be-detected stimulus. This finding suggests that the state of neuronal excitability at the moment of stimulus presentation shapes the subjective perception of task-relevant stimulus features.