When attention meets working memory: Shifted visuo-spatial attention amplifies theta:gamma phase synchronisation in the posterior parietal cortex

Abstract

Introduction: When a visual target has to be detected one should hold a template of it in working memory (WM) in order to perform a memory match when a stimulus occurs. Here the question is addressed whether a shift of spatial attention amplifies this memory match, and how this is manifested in the human brain. It was shown that visual attention is associated with oscillatory brain activity at gamma frequency (around 40 Hz) and that theta activity (around 5 Hz) reflects WM processes. There is recent evidence that gamma and theta oscillations might be phase coupled during memory processes in animals as well as in the human EEG. Here, we investigate whether a shift of spatial attention leads to increased phase synchronisation of theta and gamma activity. Methods: A Posner task was run in which a central cue, directing attention to one visual hemifield, was presented. The cue could be valid or invalid. Subsequently, one of two peripheral targets was presented in the left or right visual field. Subjects had to indicate which target was shown. EEG was recorded during the task. In response to a target, induced gamma amplitude was increased at posterior parietal cortex contralateral to the stimulated hemifield. Using a beamformer algorithm, bilateral neural sources of posterior gamma activity were localised for each subject individually (Fig. 1). Ongoing scalp EEG was transformed into source space. Gabor expansion was used to obtain frequency information of source activity, and 40:n Hz phase synchronisation was calculated on a single trial basis. Results: Target presentation elicited higher phase synchronisation between 40 Hz and oscillatory activity between 1 and 10 Hz (with its maximum at 5 Hz) at the contralateral posterior source compared to ipsilateral between 100 and 200 ms poststimulus. This phenomenon was significantly more pronounced when spatial attention was shifted to the hemifield where the target was presented, i.e. in the valid cue condition than in the invalid condition (Fig. 2). Conclusion: Here, we show that a match between a visual target and its memory template is reflected by increased gamma:theta phase coupling in the posterior parietal cortex. We could further show that attention does not only amplify early visual processing as it was often discussed, but it even enhances the matching of visual information with WM contents.