Stimulus repetition probability effects on repetition suppression are position invariant for faces

It has been shown that the probability of face repetitions influences the magnitude of repetition-related response reductions (Summerfield et al., 2008), implying that perceptual expectations affect adaptation and repetition suppression processes in the human central nervous system. An unresolved question is whether probability effects are specific for the retinal position of the stimuli or affect stimulus processing globally, throughout the visual field. To address this question we tested whether face repetition probability affects fMRI adaptation (fMRIa) when the repeated stimuli are presented on the same retinal position, overlapping each other or when they are presented in opposite hemifields. Subjects were exposed to either two identical (repeated trial, RT) or two different (alternating trial, AT) face stimuli. Both types of trials were presented either in blocks consisting of 75% (repeated block, RB) or 25% (alternating block, AB) of RTs. We found that repetition probability influences fMRIa equally for overlapping and nonoverlapping arrangements: the signal reduction after RT was more pronounced in RB than in AB for both spatial arrangements of stimulus-pairs. This effect was present in bilateral fusiform and occipital face areas, as well as in the lateral occipital cortex. Our results support the role of stimulus repetition probability in determining fMRIa and shows that the effect is invariant to the retinal position of stimuli.