Encoding and maintenance of information in visual working memory in an S1-S2 task with a 1500？ms retention phase were investigated by means of event-related brain potentials (ERPs). Participants were asked to decide whether two visual stimuli were physically identical (identity comparison (IC) task) or belonged to the same set or category of equivalent patterns (category comparison (CC) task). The stimuli differ with regard to two features. (1) Each pattern can belong to a set of either four (ESS 4) or eight (ESS 8) equivalent patterns, mirroring differences in the complexity with regard to the representational structure of each pattern (i.e., equivalence set size (ESS)). (2) The set of patterns differ with regard to the rated complexity. Memory performance obtained the effects of the task instructions (IC versus CC) and the ESS (ESS 4 versus ESS 8) but not of the rated complexity. ERPs in the retention interval reveal that the stimulus-related factors (subjective complexity and ESS) affect the encoding of the stimuli as mirrored by the pronounced P3b amplitude in ESS 8 compared to ESS 4 patterns. Importantly, these effects are independent of task instructions. The pattern of results suggests an automatic processing of the ESS in the encoding phase. 1. Introduction Visual working memory (VWM) performance relies on the ability to maintain relevant information over a short period of time [1] in the service for other mental tasks [2]. This ability is highly restricted by individual differences in capacity [3]. It is assumed that VWM capacity in general is limited to an average of four objects [4, 5]. Bays and colleagues [6] demonstrated that two independent limiting factors may contribute to VWM capacity: encoding limits and storage limits. Hence it is important to decompose processes of memory encoding from processes of memory maintenance in order to investigate VWM processing and performance in more detail. One approach that allows for a separate investigation of encoding and storage function in VWM is the application of event-related brain potentials (ERPs). The human ERP is the averaged EEG activity time-locked to a relevant event (e.g., the presentation of a stimulus), and it composes of sequences of positive and negative components indicating different processing steps. With regard to the present study, the ERP provides two correlates of encoding into and maintenance in working memory: the P3b component reflecting early stages of stimulus processing in the context of memory performance (i.e., encoding, context updating, and categorization; see [7–9])
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