Principal Investigator: Diane Swick
Contributions of Prefrontal Cortex and Hippocampus to Episodic Memory: ERP and Lesion Studies
The ability to consciously recall some event or recognize a particular stimulus presented in the past is known as episodic memory. Although neuropsychological data have traditionally supported a primary role for the medial temporal lobes in episodic memory, the advent of functional neuroimaging has highlighted the importance of the frontal lobes. The primary goal of this proposal is to provide further insight into the brain mechnisms responsible for episodic memory in the verbal domain. The project will utilize a combined neuropsychological and event-related brain potential (ERP) approach to illuminate controversies surrounding the functional neuroanatomy of human memory and the time course of encoding and retrieval. Previous studies suggested that left prefrontal cortex contributes to novelty detection and strategic processing during continuous recognition (Swick & Knight, submitted). Conversely, left frontal damage did not alter the ERP old/new effect, a positive-going ERP elicited by correct recognition of previously studied words. Other results demonstrated that lesions of right prefrontal cortex had no effect on memory in a cued recall task (Swick & Knight, 1996), contrary to what might be predicted by the neuroimaging literature. In the current proposal, three groups of patients with defined deficits in memory abilities will be studied. These patients have focal, MRI-confirmed lesions in subregions of (I) prefrontal cortex, (II) medial temporal areas including posterior hippocampus and parahippocampal cortex, and (III) bilateral hippocampal damage primarily restricted to the CA1 field. Two experiments are proposed to examine the neuroanatomical and ERP correlates of the episodic memory processes observed during recognition and cued recall. This project will provide a wealth of data on the neuroanatomical and electrophysiological substrates of memory, which are essential for a greater understanding of how these operations are affected by aging, neurodegenerative diseases, and neurological insults. The pattern of spared cognitive abilities in these patients is of great interest, with future applications in the development of improved rehabilitation strategies. The excellent temporal resolution of ERPs, in conjunction with the high spatial resolution of MRI-defined lesions in neurological patients, will yield insights into the time-course and functional neuroanatomy of episodic memory.
The major questions posed by this project are as follows:
Exp. #1: What are the neural substrates underlying recognition judgments accompanied by conscious recollection of the prior episode? Do they differ from those underlying familiarity-based recognition? Damage to hippocampal structures will likely lead to impairments in both. The neuroimaging literature predicts that prefrontal cortex (particularly right anterior regions) is critical for recollective experience but not for familiarity. What is the time-course of these two types of remembering? Familiarity effects are likely to have an earlier onset and a shorter duration.
Exp. #2: Patients with left frontal lesions showed a reduction in cued
recall accuracy (as observed in one of two prior behavioral tests). Is
this impairment due to a deficit in encoding or a deficit in retrieval?
Is the intact performance of right frontal patients due to their use of
familiarity-based mechanisms? These issues will be addressed by obtaining
sensitive ERP measures of memory encoding and memory retrieval during
cued recall. The results from frontal lesioned patients will be compared
to those from patients with unilateral and bilateral hippocampal damage.
The hippocampal groups are expected to have moderate to severe impairments
in encoding.
