Spatial generalization of peripherally encoded memories emerges before selection for report

Abstract

Visual working memory has been shown to transition from location-specific to spatially generalized representations. It remains unclear what the time course of this generalization is and to which information processing stages and signals it is linked - specifically whether it is tied to sensory or report stages. Here we used EEG to investigate spatial generalisation of peripherally encoded stimuli before and after an item was selected for report. Thirty healthy participants memorized two orientations presented serially at fifteen degrees eccentricity to the left and right of central fixation. One of the two items was subsequently cued as task relevant. During the delays, impulse signals were inserted to trace memoranda in potentially activity-reduced states. We observed that prior to the cue, patterns of activity transitioned from location-specific spatially generalized states from approximately 400-600 ms after onset in broadband voltage but not in oscillatory alpha signals. The picture was more mixed after the selection cue, with some evidence for locations-specific and spatially generalized codes in both alpha and broadband voltage. We conclude that the first stages of spatial generalization occur at sensory, pre-report levels of processing, while late stages are associated with selecting response-relevant spatial features.