Abstract
It is often claimed that probabilistic expectations can affect visual perception directly, without any role for selective attention. However, these claims have recently been disputed, as effects of expectation and attention are notoriously hard to dissociate experimentally. Thus, despite voluminous amount of research on the topic, clear-cut demonstrations of direct expectation effects are still needed. In this study, we used a new approach to separate expectations from attention. In three experiments (N=45), participants searched for a digit or a letter defined by a low-level cue (color or shape) in a rapid serial visual presentation (RSVP) stream and had to report its identity. Expectations about the alphanumeric category of the target were probabilistically manipulated. Since the target was embedded among many distractors that shared its category and since category membership is a high-level feature, we predicted that targets from the expected category should not attract attention more than targets from the unexpected category. In all three experiments, expected targets were more likely to be identified relative to unexpected targets, indicative of a direct attention-independent expectation effect on perception. In Experiments 2 and 3, attention and expectation effects were measured separately using behavioral and electrophysiological measures. The results showed that category-based expectations had no modulatory effects on indices of attentional capture and of attentional engagement, confirming that the observed expectation effects were not mediated by attentional modulations. Expectation effects did, however, affect processing at later encoding-related stages. Alternative interpretation of the observed expectation effects in terms of repetition priming or response bias were also ruled out. Together, these observations provide new evidence for direct expectation effects on perception. We suggest that even when expectations partially overlap with attentional mechanisms, they also uniquely affect the speed with which expected target objects are encoded in working memory.