Repetition suppression in occipitotemporal cortex despite negligible visual similarity: evidence for postperceptual processing?

The reduced neural response in certain brain regions when a task-relevant stimulus is repeated ("repetition suppression", RS) is often attributed to facilitation of the cognitive processes performed in those regions. Repetition of visual objects is associated with RS in the ventral and lateral occipital/temporal regions, and is typically attributed to facilitation of visual processes, ranging from the extraction of shape to the perceptual identification of objects. In two fMRI experiments using a semantic classification task, we found RS in a left lateral occipital/inferior temporal region to a picture of an object when the name of that object had previously been presented in a separate session. In other words, we found RS despite negligible visual similarity between the initial and repeated occurrences of an object identity. There was no evidence that this RS was driven by the learning of task-specific responses to an object identity ("S-R learning"). We consider several explanations of this occipitotemporal RS, such as phonological retrieval, semantic retrieval, and visual imagery. Although no explanation if fully satisfactory, it is proposed that such effects most plausibly relate to the extraction of task-relevant information relating to object size, either through the extraction of sensory-specific semantic information or through visual imagery processes. Our findings serve to emphasize the potential complexity of processing within traditionally visual regions, at least as measured by fMRI.