Differentiating the Roles of the Hippocampus and Perirhinal Cortex in Processes beyond Long-Term Declarative Memory: A Double Dissociation in Dementia

There is increasing evidence to suggest that the hippocampus and perirhinal cortex may mediate processes beyond long-term declarative memory. We assessed patients with Alzheimer's disease (AD) or semantic dementia (SD) on a visual oddity judgment task that did not place an explicit demand on long-term memory and is known to be sensitive to hippocampal and perirhinal cortex lesions. Importantly, within the medial temporal lobe, AD is associated with predominant hippocampal atrophy, whereas SD patients have greater perirhinal cortex damage. The AD group was selectively impaired in oddity judgment for scenes, whereas the SD patients demonstrated a deficit in face oddity judgment only. This compelling double dissociation supports the idea that the hippocampus and perirhinal cortex may be critical for the processing of scenes and objects, respectively, in the domain of perception or very short-term working memory.