Object Unitization and Associative Memory Formation Are Supported by Distinct Brain Regions

While it has been established that the medial temporal lobe (MTL) is critical for successful memory formation, the precise contribution of one of the key MTL subregions, the perirhinal cortex (PrC), has been the subject of intense focus and debate. Although this region has been implicated in nonassociative item encoding, recent neuroimaging data have revealed that it also contributes to the binding of specific item-related associations (e.g., an object-color association). Based on neuropsychological evidence that associative memory for unitized word pairs does not require the hippocampus, it has been proposed that PrC contributes to associative memory formation by means of unitization. However, the role of PrC in unitization processes remains unclear. Here, we used fMRI to assess the involvement of PrC in object unitization as well as in successful episodic encoding of item-related details. Our results show that while PrC activation linearly tracks the amount of item-related information successfully encoded, it is not modulated by object unitization demands. Instead, the present data reveal that unitization of object fragments may be accomplished in visual/ventral temporal processing stages before PrC. Indeed, we observed a gradual transition from unitization to successful memory formation across the ventral visual pathway. This suggests that PrC may specifically serve to encode item-related event details, whereas their perceptual integration is established along preceding processing stages.