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Jonathan M. Keefe, Igor S. Utochkin, Jonas S.H. Lau, Timothy F. Brady, Viola S. Stoermer; Information about all items is actively held in mind when computing ensemble statistics about a set. Journal of Vision 2020;20(11):1523. doi: https://doi.org/10.1167/jov.20.11.1523.
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© ARVO (1962-2015); The Authors (2016-present)
A large body of research on ensemble perception has shown that humans can effectively represent statistical summary information about features in the environment. However, it is unclear whether ensemble representations are the result of an effortless perceptual process or whether information must be actively held in mind when performing ensemble tasks. In order to distinguish between these possibilities, we had subjects perform three different tasks in a blocked design while recording EEG activity. Participants (N=20) were presented with bilateral displays of oriented triangles and asked to either remember 1, remember 4, or compute the average orientation of 4 triangles over a 900 ms delay. In order to test whether subjects were actively holding individual orientations in mind while calculating the mean orientation, and if so, how many, we compared the amplitude of the Contralateral Delay Activity (CDA) in each task: a slow-wave ERP that increases in magnitude as more information is held in mind (Vogel & Machizawa, 2004). Critically, we found a main effect of task upon CDA amplitude (p < 0.001), which was driven by a significantly smaller CDA amplitude when subjects were asked to remember 1 triangle vs. remember 4 or report the mean of 4 triangles (ps < 0.001). However, there was no difference between the CDA amplitude when subjects were asked to remember 4 or report the average of 4 orientations (p = 0.91, BF01 = 4.3), indicating that subjects were holding a similar amount of information in mind while performing each task. This result demonstrates that when computing summary statistics, rather than perceptually extracting a single mean during encoding and maintaining only that mean representation, information about all items is held in mind. Overall, this suggests that ensemble representations may be the result of actively maintaining information in neural populations of relevant sensory cortices.
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