Working Memory and Visual Perception Compete for Attention: An ERP/EEG Study Yigal Agam & Robert Sekuler Volen Center for Complex Systems, Brandeis University How do working memory, perception and attention interact with each other? Recent theories of working memory suggest that they are closely linked, and in fact share certain brain mechanisms. Here we use a sequential motion imitation task for a direct, online examination of memory load’s influence on the processing of visual stimuli. Based on event-related potentials (ERPs) and frequency analysis of the electroencephalogram (EEG), we show that the processing of successive incoming visual stimuli becomes less and less effective as additional items have to be held in working memory. Using a task in which subjects tried to reproduce random motion sequences from memory, we found a systematic decrease in ERP amplitude with each additional motion segment that was viewed and memorized for later imitation. High-frequency (>20 Hz) oscillatory activity exhibited a similar position-dependent decrease. When trials were sorted according to the accuracy of subsequent imitation, the amplitude of the ERPs correlated with behavioral performance: The larger the amplitude, the more accurate the imitation. As both ERP amplitude and high-frequency oscillations are known to reflect attention-driven visual processing, our results suggest that visual working memory is tightly linked to attention and interferes with visual perception in a load-dependent manner. Such a tradeoff between memory and perception may underlie fundamental properties of working memory, such as limited capacity and primacy effects. Introduction The interplay between working memory, visual perception, and attention, has been the subject of considerable debate. Even though working memory has received much interest from researchers in psychology and cognitive neuroscience, remarkably little is known about the mechanisms on which it depends, or about the factors that set its limits. At the theoretical level, two general classes of models posit different roles for attention in working memory. One approach conceptualizes working memory as a set of specialized buffers for the storage of information (Baddeley, 2003), which are controlled by an attention-based structure, the central executive. The multi-component, modular model emphasizes limited amounts of activation and temporal decay in the buffers as the sources of capacity limitations in working memory (Baddeley & Logie, 1999). Physiological demonstrations of activity in the prefrontal cortex (PFC) during the memory retention period have lead to the identification of the PFC as the neural substrate for the proposed storage buffers (Goldman-Rakic, 1987; Postle, 2006). An alternative view treats working memory not as a separate module, but as an emergent property, which harnesses existing neural mechanisms specialized for sensory perception and long-term memory representations (Cowan, 1999, 2000; Supported by NSF grant SBE-0354378 and NIH grant R01MH068404. E-mail: yigal@brandeis.edu Jonides, Lacey, & Nee, 2005; Pasternak & Greenlee, 2005; Postle, 2006). By this account, after the visual stimulus has disappeared from sight, visuospatial working memory is achieved by prolonged activations, via attention, of the same occipital and parietal regions that are thought to mediate visual perception (Druzgal & D’Esposito, 2001; Postle, Druzgal, & D’Esposito, 2003; Todd & Marois, 2004; Vogel & Machizawa, 2004). In the “embedded-process”, or “emergent property”, framework, the prefrontal cortex does not provide the actual substrate for memory storage, but rather mediates attentional control of the sensory reactivation process (Curtis & D’Esposito, 2003; Lebedev, Messinger, Kralik, & Wise, 2004; Postle, 2005). Consequently, this approach emphasizes attentional control as a limiting factor in working memory capacity (Cowan, 1999, 2000). Attempts to choose between these theoretical accounts have produced mixed results. The key issues that distinguish the two accounts are (i) the degree to which short-term storage of visual information overlaps with early stages of visual processing, and (ii) whether memory’s capacity limit is dictated by some limit on attentional selection of visual input. Several groups used a visual search paradigm in conjunction with a working memory task, measuring the degree to which the content of working memory affects search efficiency. Woodman, Vogel, and Luck (2001) found no detrimental effect on visual search when subjects had to concurrently remember a visual object. More recently, however, they found that when the information to be remembered was a location in space, the slope of the function relating reaction time to search array size increased, indicating reduced search efficiency (Woodman & Luck,