Previous research suggests that sets of similar items are represented using a rapid averaging mechanism that automatically extracts statistical properties within 50 ms. However, typically in these studies, displays are not masked, so it is possible that the sets are available for longer than this duration. In the present study, using masked displays, we (a) tested a newly proposed strategy for extracting the mean size of a set of circles, and (b) more precisely evaluated the time course of rapid averaging. The results indicate that when viewing conditions are poor, performance can be explained by assuming that observers rely on information from previous trials. In this study, observers required at least a 200-ms exposure time in order to derive the average size of a set of circles without relying on information from previously-viewed sets, suggesting that rapid averaging is not as fast as previously assumed and, therefore, that it may not be an automatic process.When shown a set of similar items, people can rapidly summarize the set according to statistical properties, such as the mean size. Ariely (2001) found that observers were able to determine the average size of a set of circles, but were unable to identify individual members of the set. Ariely interpreted this as evidence that the visual system can derive a statistical representation of the set without retaining specific information about the items within the set. Researchers have proposed that this is accomplished using a specialized averaging mechanism that evaluates all of the items in the set in parallel. Consistent with this proposal, Chong and Treisman (2005a) showed that averaging performance was better when attention was broadly distributed across a display than when attention was narrowly focused, suggesting that the specialized averaging mechanism operates preattentively, outside the focus of attention. Additional evidence for the automaticity of this process has been demonstrated through cuing and dual-task manipulations.
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