Cerebral Cortex Advance Access published online on April 9, 2008
Cerebral Cortex, doi:10.1093/cercor/bhn043
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Medial Temporal Lobe Involvement in an Implicit Memory Task: Evidence of Collaborating Implicit and Explicit Memory Systems from fMRI and Alzheimer's Disease
1 Department of Neurology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA, 2 Department of Psychology, Columbia University, New York, NY 10027, USA
Address correspondence to Phyllis Koenig, PhD, Department of Neurology, 3 West Gates, University of Pennsylvania, 3400 Spruce Street, Philadelphia, PA 19104-4283, USA. Email: plkoenig{at}mail.med.upenn.edu.
We used a prototype extraction task to assess implicit learning of a meaningful novel visual category. Cortical activation was monitored in young adults with functional magnetic resonance imaging. We observed occipital deactivation at test consistent with perceptually based implicit learning, and lateral temporal cortex deactivation reflecting implicit acquisition of the category's semantic nature. Medial temporal lobe (MTL) activation during exposure and test suggested involvement of explicit memory as well. Behavioral performance of Alzheimer's disease (AD) patients and healthy seniors was also assessed, and AD performance was correlated with gray matter volume using voxel-based morphometry. AD patients showed learning, consistent with preserved implicit memory, and confirming that AD patients' implicit memory is not limited to abstract patterns. However, patients were somewhat impaired relative to healthy seniors. Occipital and lateral temporal cortical volume correlated with successful AD patient performance, and thus overlapped with young adults' areas of deactivation. Patients' severe MTL atrophy precluded involvement of this region. AD patients thus appear to engage a cortically based implicit memory mechanism, whereas their relative deficit on this task may reflect their MTL disease. These findings suggest that implicit and explicit memory systems collaborate in neurologically intact individuals performing an ostensibly implicit memory task.
Key Words: Alzheimer's • explicit memory • fMRI • implicit memory • medial temporal
Portions of this research were presented at the Society for Neuroscience Annual Meeting, Atlanta, 2006.
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