Converging evidence for automatic perceptual processing in visual search.

Logan, Gordon D. · 1976 · Crossref

DOI: 10.1037/h0082061

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Summary

This paper investigates the theoretical debate regarding capacity limitations in visual search, specifically addressing whether perceptual processing is limited by central attentional resources or operates via unlimited-capacity channels. The "array size effect"—the increase in reaction time as the number of items in a visual display increases—has been interpreted by some as evidence of limited capacity, where active channels share processing resources. Conversely, unlimited-capacity models argue that perceptual channels are independent, and the array size effect arises from interactions between channels or decision processes rather than capacity constraints. To distinguish between these views, the author manipulated visual load (array size) and central processing load (via concurrent non-visual tasks) independently. The critical prediction was that if capacity limits drive the array size effect, adding a concurrent load should increase the slope of the reaction time function relative to array size (an interaction). If the effect is structural and automatic, the loads should have additive effects. Two experiments were conducted using tachistoscopic presentation of letter arrays containing 4, 8, or 12 items. In Experiment 1, subjects performed a visual search task (identifying an 'A' or 'V') either alone or while retaining seven digits in short-term memory. In Experiment 2, subjects performed the visual search while simultaneously maintaining or changing a stimulus-response mapping based on a signal within the array. Both tasks were designed to engage central processing capacity without involving visual structures. Reaction times were measured for correct responses, and the effects of array size and concurrent activity were analyzed using analysis of variance and linear regression slopes. The results from both experiments demonstrated that reaction times increased significantly with both array size and the presence of concurrent activity. However, the effects were additive; the concurrent tasks increased the intercept of the reaction time function but did not significantly alter the slope relative to array size. In Experiment 1, adding a memory load increased the intercept by 118 msec but reduced the slope by a non-significant 4 msec. In Experiment 2, changing the S-R mapping increased the intercept by 166 msec but reduced the slope by a non-significant 10 msec. Statistical analyses confirmed that the interaction between array size and concurrent load was not significant in either experiment. Furthermore, recall accuracy in Experiment 1 remained stable across array sizes, ruling out a speed-accuracy trade-off strategy. These findings support unlimited-capacity theories of visual search, indicating that the perceptual processing underlying the array size effect is automatic and not subject to on-line attentional control. The author concludes that while central capacity is required to prepare the target set (as seen in memory search paradigms), the activation of array items is automatic. The discrepancy between visual and memory search results is explained by the different sources of activation: array items are activated automatically by stimulus presentation, whereas target sets require internal activation that consumes capacity. This suggests that visual search involves an automatic summation of activation in feature detectors, distinct from the capacity-limited processes involved in maintaining mental sets.

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Topics

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• visual search
• inattentional change blindness
• useful field of view