"Researchers have long posited that attention is controlled through two modes: a bottom-up mode in which attention is directed automatically to items based on their physical salience and a top-down mode in which the observer exerts volitional influence to prioritize information relevant to current task goals. Recently, however, others have begun to question the validity of this model and suggest that attention is also driven by prior experience. The current work investigates how this third mechanism, called selection history, is implemented in visual search. We conducted a series of experiments to investigate the conditions under which observers could learn to prioritize a frequently selected feature. Participants searched for a target horizontal or vertical line, which appeared in one of two possible colors, among diagonal distractors and made a judgement about the target (i.e. length or orientation). Each experiment began with a training phase, during which targets were presented more frequently in one of the two possible colors (called the "high probability color"), followed by a test phase in which targets appeared equally often in both colors. We predicted that participants would continue to respond more quickly to targets presented in the high probability color in the test phase, and that this learning would occur implicitly and thus be distinguishable from top-down control. In addition to this question, Experiments 1 and 2 examined whether color must be bound to other defining features of a stimulus (i.e. the lines must appear in colored font) for participants to implicitly develop a lingering bias toward the high probability color in the line length judgement task. The results of these experiments indicated that participants implicitly learned to prioritize the high probability color regardless of whether color was inherently bound to the stimulus. The remaining experiments tested whether implicit learning that leads to a lingering bias attributable to selection history occurs automatically when specific features are encountered more frequently and are diagnostic of outcomes. Specifically, we predicted that implicit learning would be modulated by the amount of attention devoted to the task and whether perceptual load permitted the passive processing of irrelevant but predictive color. Experiments 3 and 4 tested this prediction by manipulating the selection difficulty (the ease of distinguishing a target from distractors). In both tasks, participants prioritized the high probability color overall, but this effect was driven by participants who explicitly recognized the high probability color. Finally, Experiment 5 manipulated the perceptual difficulty of distinguishing the stimuli from the background by comparing the performance of participants who viewed the stimuli in a high contrast white font (Experiment 5a) to those who viewed the stimuli in a low contrast dark gray font against the black background (Experiment 5b). Although the participants in the low contrast condition responded more slowly overall, both groups showed evidence of implicit learning associated with a lingering bias for the high probability color. We conclude that implicit learning, beyond simple repetition priming, is not a process that occurs automatically when features are encountered repeatedly or diagnostic of outcomes, but may require an optimal level of task difficulty to ensure that attention is devoted to the task and free to process color."--Pages ii-iii.
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