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Eunsam Shin, Monica Fabiani, Gabriele Gratton; The interactive nature of multiple stimulus representations in visual-short term memory. Journal of Vision 2009;9(8):600. doi: 10.1167/9.8.600.
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Previously we found sequential deflections of encoding-related lateralization (ERL) waveforms in event-related brain potentials, suggesting that multiple levels of stimulus representation activate in different time windows (Shin, Fabiani, & Gratton, 2006). The ERLs reflect lateralized brain activities elicited by the degree of matching or mismatching between laterally-presented memory-sets and centrally-presented probes. Here we further investigated the nature of one of the ERLs (reported in Shin et al., 2006), showing a smaller ERL in the set-size 4 than in the set-size 2 condition, observed about 400 ms poststimulus at posterior electrode sites. Twenty participants performed a memory search task, in which a memory-set of homogeneous or heterogeneous letters with two letters in each hemifield and a two-letter probe in the center were presented in sequence. To test local suppression hypothesis, we varied distances of memory-set letters (close vs. far) but kept eccentricity of the letters from fixation constant. To test partial matching hypothesis, we presented probe letters completely or partially matching to the memory-set items. Data showed that the ERL was larger (a) in the complete-match than in the partial-match condition at all times; (b) in the far than in the close condition for the heterogeneous memory-set letters; (c) in the close than in the far condition for the homogeneous memory-set letters. These results support both local suppression and partial matching hypotheses, indicating that stimuli are represented in an interactive way and activation of the representations is maximized at a complete match of probe with memory representation. Also, perceptual grouping influences memory representations and their activations.
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