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Lavanya Reddy, Rufin VanRullen, Christof Koch; Inter-stimulus distance effects in visual search. Journal of Vision 2003;3(9):332. doi: 10.1167/3.9.332.
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In a previous study, we showed that the attentional requirements of a task, as revealed by the dual-task paradigm, do not necessarily determine whether visual search will be parallel or serial. For example, natural scene categorization can be performed “preattentively” in a dual-task situation (i.e. a single scene containing animals can be discriminated from non-animal scenes even while attention is occupied elsewhere), and yet visual search for an animal scene among a number of non-animal scenes is a serial process. We interpreted these findings as follows: a task can be performed preattentively if there exist specific neuronal populations selective to the target and distractor categories, independent of the level of processing involved (from V1 to IT); when such selectivities exist, visual search is parallel only if the receptive fields of the relevant neurons do not significantly overlap. When receptive fields are too large, target and distractors compete within the same field and search is serial.
It follows that search performance should improve if target and distractors can be separated enough to prevent them from falling into the same receptive field. We tested this prediction and found that for preattentive tasks that usually result in serial visual search (eg.: color-orientation conjunction discrimination, upright vs. inverted face discrimination), search performance improved as inter-stimulus distance was increased. For preattentive parallel tasks (color discrimination, orientation discrimination), the effect of increasing inter-stimulus distance was negligible. These results support the idea that for preattentive tasks, competition within the relevant receptive fields can affect visual search performance.
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