Letter identification and spatial localization during visual working memory are enabled by unique sequences of stimulus-dependent neural operations

Working memory (WM) is often conceptualized as consisting of a supervisory central executive and the short-term storage of information over a brief period of time. In the present study, we examined the sequence of neural operations that are engaged for visual-verbal and visual-spatial information during demanding WM performance. Participants completed verbal and spatial 3-back tasks (visually presented stimuli), and event-related potentials (ERPs) were obtained for task performance. There was enhancement of anterior N1/posterior P1, P2, P3, and late anterior negative (400+ msec post-stimulus) component amplitude for the spatial compared to verbal 3-back task. We interpret these effects as reflecting spatial orienting (N1/P1, P2 effects) and updating of stimulus location during WM (anterior P3, late negativity effects). In contrast, the verbal compared to spatial 3-back task exhibited enhancement of an anterior P150/posterior N150 component, frontal N2 amplitude, a broader P3 component morphology with posterior localization, and a late anterior positivity/posterior negativity (550+ msec). We interpret these effects as reflecting identification of letter features (anterior P150/posterior N150), stimulus conflict monitoring (N2 effect), stimulus categorization (posterior P3 effect), and rehearsal/updating over the retention interval (late positivity/posterior negativity). These ERP effects likely reflect activity of the distinct ventral and dorsal visual processing streams associated with verbal/object and spatial information.