All identical objects reduce memory load at the late maintenance stage in working memory.

Visual working memory (VWM) is critical for temporarily storing and manipulating visual information but is limited in capacity. Previous research suggests that perceptual organization, such as grouping identical objects, may alleviate VWM resource constraints, yet the underlying mechanisms remain unclear. This study investigated whether identical orientation stimuli reduce VWM resource consumption and whether this reduction occurs during encoding or maintenance phase. Using the contralateral delay activity (CDA), an event-related potential (ERP) component that indexes the number of items stored in VWM, we examined three memory conditions: all-same (i.e., four identical orientations), partial-same (i.e., two pairs of identical orientations), and all-different (i.e., four different orientations) conditions. Behavioral results showed improved VWM performance as the number of identical orientations increased, with the highest performance in the all-same condition. ERP results revealed no significant differences in early CDA amplitudes across conditions, suggesting that identical stimuli do not reduce VWM resource consumption during early maintenance phase. However, late CDA amplitudes were significantly reduced in the all-same condition compared to the partial-same and all-different conditions, indicating that the benefits of identical objects emerge during the late maintenance phase. Notably, partially identical stimuli did not reduce VWM load, highlighting the complexity of integrating partially redundant information. These findings highlight the distinct contributions of early consolidation and maintenance phases in optimizing VWM through perceptual grouping and provide evidence that the facilitative effect of identical objects relies on their complete repetition and late-stage cognitive processes.