Human electrophysiological correlates of visual working memory set-size effects at encoding

Abstract

The ability to encode, store, and retrieve visually perceived objects is referred to as visual working memory (VWM). Although crucial for many cognitive processes, previous research has found VWM to be strictly capacity limited. This capacity limitation is observable in the set size effect: the ability to successfully report items in VWM asymptotes at a small number of items. Research into the neural correlates of set size effects and the VWM capacity limitation in general has largely focused on the maintenance period of VWM. However, we previously reported that neural resources allocated to individual items during VWM encoding correspond to successful VWM performance. Here we expand upon those findings by investigating neural correlates of set size during VWM encoding. We hypothesized that neural signatures of encoding-related VWM capacity limitations should be differentiable as a function of set size. We tested our hypothesis using High Density Electroencephalography (HD-EEG) to analyze frequency components evoked by flickering target items in VWM displays of set size 2 or 4. Across frontal and occipital-parietal electrodes, set size modulated the amplitude of the 1st and 2nd harmonic frequencies corresponding to correctly recognized targets. Frontal sites exhibited the most robust effects for the 2nd harmonic (set size 2 > set size 4). These results are consistent with a capacity limited VWM resource at encoding that is distributed across the items in a VWM display. This neural set size effect supports the view that VWM capacity limitations begin with encoding related processes