Memory deficits for implicit contextual information in amnesic subjects with hippocampal damage

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Abstract

The role of the hippocampus and adjacent medial temporal lobe structures in memory systems has long been debated. Here we show in humans that these neural structures are important for encoding implicit contextual information from the environment. We used a contextual cuing task in which repeated visual context facilitates visual search for embedded target objects. An important feature of our task is that memory traces for contextual information were not accessible to conscious awareness, and hence could be classified as implicit. Amnesic patients with medial temporal system damage showed normal implicit perceptual/skill learning but were impaired on implicit contextual learning. Our results demonstrate that the human medial temporal memory system is important for learning contextual information, which requires the binding of multiple cues.

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Figure 1: Two sample displays from our contextual cuing visual search protocol.
Figure 2: Mean correct response time to localize and discriminate targets in old versus new configurations as a function of epoch (groups of five blocks), plotted separately for the control group (n = 10) and the amnesic group (n = 4).
Figure 3: Mean correct response time for a second group of controls (n = 5) on a more difficult version of the search task ( Fig. 1b ).

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Acknowledgements

We thank Kevin O'Connor for assistance in running the experiments, Sarah Raskin and Meike Verfaellie for patient referrals and Larry Squire, Mark Packard, John Gabrieli, John Kihlstrom and Jeansok Kim for discussions. This study was supported by National Institute of Mental Health grant MH50812 to E.A.P. and in part by National Science Foundation grant BCS-9817349 to M.M.C.

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Correspondence to Marvin M. Chun.

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Chun, M., Phelps, E. Memory deficits for implicit contextual information in amnesic subjects with hippocampal damage. Nat Neurosci 2, 844–847 (1999) doi:10.1038/12222

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