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Dongho Kim, Shigeaki Nishina, Takeo Watanabe; Decisions on “when” and “what” are biased by different temporal spans of past incidences. Journal of Vision 2009;9(8):827. doi: 10.1167/9.8.827.
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It has been proposed that decision making consists of two stages, integrating sensory evidence and choosing the alternative that is best supported by the evidence. The choice is thought to be made based not only on sensory evidence but also on statistical knowledge about past incidences (Gold and Shadlen, 2007). Here we show that decision regarding when an item is detected and decision regarding what item is identified are biased by different accumulation time spans of past incidences. Two experiments were conducted in which different groups of 5 subjects were asked to perform a “when” task and a “what” task, respectively. In the “when” task, subjects were asked to report in which one of two alternative events a stimulus with an orientation structure was presented. In the “what” task, subjects were asked to report which one of two alternative orientations was presented. The spatial and temporal configuration of the visual presentations in the experiments was identical. However, the task instructions differed. The incidence probability for each alternative was manipulated to examine how the observers' choice is influenced by the probability change. While observers' choices were biased toward the alternative with higher past incidence probability in both tasks, the time span of past incidences biasing a present choice is significantly narrower for the “when” than the “what” task. These differential results suggest that while a decision regarding when an event happens is partially determined by immediate past events, a decision regarding what feature is presented is determined by a longer accumulation of past experiences.
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