Interhemispheric Connectivity Supports Load-Dependent Working Memory Maintenance for Complex Visual Stimuli

Abstract

A critical manipulation used to study the neural basis of working memory (WM) is to vary the information load at encoding followed by measurements of activity and connectivity during maintenance in the subsequent delay period. The hallmark finding is that delay period activity and connectivity increases between frontal and parietal brain regions as load is increased. Most WM studies, however, employ simple stimuli (e.g., simple shapes or letters) during encoding and utilize unfilled intervals (e.g., a blank screen or fixation cross) during the delays. In the present study, we asked how delay period activity and connectivity change during low and high load maintenance of complex stimuli. Twenty-two participants completed a modified Sternberg WM task with two or five naturalistic scenes as stimuli while scalp EEG was recorded. In each trial, the delay interval was filled with phase scrambled scenes to provide a visual perceptual control with color and spatial frequency similar to the non-scrambled scenes presented during encoding. The results showed that theta and alpha delay activity amplitude was reduced during high compared to low WM load across frontal, central, and parietal sources. Functional connectivity during the delay was assessed by phase-locking value (PLV) and revealed a network with higher connectivity during low WM load consisting of increased PLV between 1) left frontal and right posterior temporal sources in the theta and alpha bands, 2) right anterior temporal and left central sources in the alpha and lower beta bands, and 3) left anterior temporal and posterior temporal sources in the theta, alpha, and lower beta bands. These findings demonstrate a role for interhemispheric connectivity during WM maintenance of complex stimuli. We discuss significance with respect to allocation of limited attentional resources and the filtering of interference.