EEG Cross-Frequency Phase Synchronization as an Index of Memory Matching in Visual Search

Abstract

Visual perception is influenced by our expectancies about incoming sensory information. It is assumed that mental templates of expected sensory input are created and compared to actual input, which can be matching or not. When such mental templates are held in working memory, cross-frequency phase synchronization (CFS) between theta and gamma band activity has been proposed to serve matching processes between prediction and sensation. We investigated how this is affected by the number of activated templates that could be matched by comparing conditions where participants had to keep either one or multiple templates in mind for successful visual search. We found that memory matching appeared as transient CFS between EEG theta and gamma activity in an early time window around 150ms after search display presentation, in right hemispheric parietal cortex. Our results suggest that for single template conditions, stronger transient theta-gamma CFS at posterior sites contralateral to target presentation can be observed than for multiple templates. This lends evidence to the idea of sequential attentional templates and is understood in line with previous theoretical accounts strongly arguing for transient synchronization between posterior theta and gamma phase as a neuronal correlate of matching incoming sensory information with contents from working memory.