Abstract
Visual search tasks with relation-based stimuli are more difficult for subjects than feature-based searches. Because the response time (RT) curves as a function of set size for target-present conditions have heretofore appeared to be linear in relational searches, it has been argued that subjects use a fully serial process during a search for a relational target (Logan, 1994). In three experiments, we show that search RT for relational stimuli follows a logarithmic function of set size, rather than a linear function, when a wider range of set sizes are included as conditions, which suggests that some parallel processing is present even during putatively relation-only searches. Using relation-only, difficult feature-only, and feature+relation search conditions, we showed that difficult feature-only search RTs follow a logarithmic curve, as expected, but that a relation-only search also follows a logarithmic curve, albeit steeper. When the relation must be used to find the search target, the feature+relation condition is more efficient than the feature-only condition, indicating the relation information is helping with parallel processing. We also show that when search stimuli are designed such that search can be reduced to a feature (color) only search, both feature-only and feature+relation conditions are more efficient, suggesting that the presence of relational information does not affect search slopes when the relations are irrelevant to the search, even if such information could be diagnostic. Finally, we found that spacing out search items increases the efficiency of relation-only search, meaning that nearby inter-item interactions that could produce texture fields or perceptual grouping cues are unlikely to be the cause of apparent parallel processing of relations, and in fact may inhibit parallel processing of relations.