Visual working memory (VWM) stores information from the visual world. Despite its importance in a variety of cognitive tasks, this process appears to be capacity limited to approximately 3 or 4 items. Previous studies examining the sources of this capacity limitation have largely focused on the maintenance phase of VWM. Here we investigate the possible role of the encoding phase of VWM as a potential source of capacity limitation. In the present study we addressed this question by examining the neural fate of an item through the measurement of activity during VWM encoding phase. We hypothesized that a greater amount of limited capacity neural resources at the time of encoding are needed to facilitate subsequent retrieval of the item. We tested this hypothesis using frequency tagging and EEG. For each trial, four novel shapes were presented. Each shape flickered at one of four distinct frequencies. After a blank delay period a single, static shape appeared at one of the previous locations. Participants were to respond whether the test item was "old" or "new". For each condition the amplitudes of the corresponding fundamental frequency and second harmonic (frequency tags) were extracted from the frequency spectrum. The amplitudes of the second harmonics corresponding to the probed item, measured at Parietal (P7/P8) and Central (C7/C8) electrode sites, were greater for correct than incorrect trials. The data support the hypothesis that neural resources allocated to individual items at the time of encoding play an important role in in the overall capacity limitations of VWM.
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