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Lisa A. Vandenbeld, Ronald A. Rensink; Visual search strategies in a change detection task. Journal of Vision 2002;2(7):525. doi: 10.1167/2.7.525.
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Visual Search Strategies in a Change Detection Task
Purpose: Serial visual search for a target amongst distractor items is widely believed to be a linear process (Treisman and Gelade, 1980). Visual search for change in simple orientation stimuli using a change detection paradigm also yields search slopes that reflect a linear process (Rensink, 2000). The present study investigates whether or not detecting change in more complex stimuli is also linear.
Method: 2 trained observers were shown an original and modified display presented in an alternating sequence, with each display shown for 1250ms separated by a 250ms blank screen. The stimuli were happy and sad schematic faces, with set sizes ranging from 2 to 10 items. Different ranges of set sizes were used in different experiments. A change in the facial expression of one of the faces occurred on half of the trials; reaction times to detect the change were measured.
Results: Change-absent search slopes increased by a factor of two at set size 4 or 6 (depending on the individual observer). In addition, reaction times for set sizes of 6 and 8 differed depended on the range of set sizes used in testing (eg. 2,4,6 versus 6,8,10), with reaction times for higher ranges of set sizes being longer.
Conclusion: These results suggest that observers use search strategies that depend on two factors: the context, or range in which the set size appears, and the capacity of visual short-term memory (vSTM). First, it appears that a display is searched more extensively when it is the smallest in a range of set sizes than when it is the largest. Secondly, the increase in slope at 4 or 6 items can be explained in terms of the capacity of vSTM, which is about 5 items (Rensink, 2000; Pashler, 1988): it may be that when this capacity is exceeded, search becomes less efficient.
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