Evidence for feature-based suppression (FBS) in visual search literature has been controversial and mixed. Much of the mixed findings are due to differences between implicit FBS, the unconscious ignoring of a feature that appears as a distractor with high regularity, and explicit FBS, the active biasing of attention away from a feature. Given this discrepancy, the goal of this dissertation is to examine the benefits of implicit and explicit FBS in a visual search task. Across four experiments, participants searched for a Landolt C with a horizontal gap among eight distractors with vertical gaps, with three items appearing in each of three colors. In Experiment 1, one color (ignored color) appeared on a higher percentage of trials (80%) and always appear as a distractor. Participants were not explicitly aware of the suppression contingency with the goal that they would implicitly learn to ignore this color. In Experiment 2, we included an additional implicit feature-based gain (FBG) contingency - a color (attend color) appeared on a large percentage of trials (40%) and always as the target - to see if suppression of the ignore color persists if there is an implicit feature-based gain (FBG) component present. Experiment 3 replicated Experiment 2 but made the FBG component explicit by cuing participants about the color that was likely to be the target. Experiment 4 replicated Experiment 1 but made the FBS component explicit by cuing what color should be ignored. Experiment 1 found a strong implicit suppression benefit that required minimal training of the to-be-ignored feature. However, in Experiment 2 where there were implicit FBS and FBG contingencies, only the FBG impacted search – suggesting that implicit FBG interfered with the development or implantation of an implicit FBS mechanism. However, when the FBG component was made explicit (Exp. 3), the implicit FBS effect reappeared – suggesting the failure to find FBS in Experiment 2 was likely due to a competition for implicit resources between the FBG and FBS components. Finally, when FBS appeared alone but was made explicit (Experiment 4) there was week evidence for suppression. These findings suggest that implicit FBS can effectively allocate attention, but the circumstance under which they do so are limited – only when the contingencies are learned implicitly and in the absence of implicit feature-based gain contingencies. These restrictions may limit FBS’s utility as a real-world mechanism for guiding attention.
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