Expectation modulates identity-based selection, but not feature-based processing

Irrelevant visual information frequently interferes with the processing of information of interest. Previous research has shown that interference is reduced when the presence of irrelevant information is expected. In a distractor task, where expectation was modulated by manipulating the frequency with which distractors were presented, performance was specifically improved when presented flankers were expected, compared to when they were not expected. Yet, no improvement was observed when distractors were only expected, but not presented. This effect has been attributed to preparatory attention shifting in line with distractor suppression. Using a battery of experiments, we tested the robustness of this attentional modulation. By systematically varying target and flanker types and familiarity, as well as flanker number and distance, we found the effect to be robust to all but one manipulation – task-type. Identification performance in the presence of flankers was modulated by expectation. This effect was observed for familiar targets (numbers and letters), and unfamiliar targets (Thai and rotated Indo-Arabic numerals). The modulation was also present irrespective of target location (cardinal and oblique meridian), distractor number (4 and 22), distractor distance (outside and within the critical spacing for visual crowding) and distractor familiarity (letters, numbers and Japanese Katakana). Flanked orientation discrimination, however, did not benefit from expectation, despite the use of letter-like stimuli as targets (oriented Ts). Further, we similarly did not find an effect of expectation on flanked orientation discrimination tested in standard crowding setups with letter-like and clock-like stimuli, whether expectation was modulated based on frequency or explicit endogenous cues. Taken together, these findings suggest that expectation facilitates identity-based attentional selection while leaving lower-level feature processing largely unaffected. More generally, the results indicate that expectation-based distractor suppression operates at relatively late stages of visual processing.