Evidence for spatiotemporally distinct effects of image repetition and perceptual expectations as measured by event-related potentials.

Repeated stimulus presentation leads to reductions in responses of cortical neurons, known as repetition suppression or stimulus-specific adaptation. Circuit-based models of repetition suppression provide a framework for investigating patterns of repetition effects that propagate through cortical hierarchies. To further develop such models it is critical to determine whether (and if so, when) repetition effects are modulated by factors such as expectation and attention. We investigated whether repetition effects are influenced by perceptual expectations, and whether the time courses of each effect are similar or distinct, by presenting pairs of repeated and alternating face images and orthogonally manipulating expectations regarding the likelihood of stimulus repetition. Event-related potentials (ERPs) were recorded from n = 39 healthy adults, to map the spatiotemporal progression of stimulus repetition and stimulus expectation effects, and interactions between these, using mass univariate analyses. We also tested for another expectation effect that may contribute to repetition effects in many previous experiments: that repeated stimulus identities are predictable after seeing the first stimulus in a trial, but unrepeated stimulus identities cannot be predicted. Separate blocks were presented with predictable and unpredictable alternating face identities. Multiple repetition and expectation effects were identified between 99 and 800ms from stimulus onset, which did not statistically interact at any point and exhibited distinct spatiotemporal patterns of effects. Repetition effects in blocks with predictable alternating faces were smaller than in unpredictable alternating face blocks between 117-179 ms and 506-652ms, and larger between 246 and 428ms. The distinct spatiotemporal patterns of repetition and expectation effects support separable mechanisms underlying these phenomena. However, previous studies of repetition effects, in which the repeated (but not unrepeated) stimulus was predictable, are likely to have conflated repetition and stimulus predictability effects.