Comparing Neural Correlates of Memory Encoding and Maintenance for Foveal and Peripheral Stimuli

Abstract

Visual working memory is believed to rely on top-down attentional mechanisms that sustain active sensory representations in early visual cortex. However, both bottom-up sensory input and top-down attentional modulations thereof have been shown to be biased towards the fovea, at the expense of the periphery, and initially peripheral percepts may even be assimilated by foveal processing. This raises the question whether and how visual working memory differs for central and peripheral input. To address this, we conducted a delayed orientation recall task in which an orientation stimulus was presented either at the center of the screen or at 15{degrees} eccentricity to the left or right. Response accuracy, electroencephalographic (EEG) activity and gaze position were recorded from 30 participants. Accuracy was slightly but significantly higher for foveal versus peripheral memories. Orientation decoding analyses of the EEG signals revealed a clear dissociation between early sensory and later maintenance signals. While sensory signals were clearly decodable for the central location, they were not for peripheral locations. In contrast, maintenance signals were decodable to an equal extent for both foveal and peripheral memories, suggesting comparable top-down components regardless of eccentricity. Moreover, while memory representations were initially spatially specific and reflected in voltage fluctuations, later in the maintenance period they generalized across locations, as emerged in alpha oscillations, thus revealing a dynamic transformation within memory. Moreover, these transformed representations remained accessible through sensory impulse-driven perturbations, unveiling the underlying memory state. The eccentricity-driven dissociation between disparate sensory and common maintenance representations indicates that storage activity patterns as measured by EEG reflect signals beyond primary visual cortex.