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The Journal of Neuroscience, November 2, 2005, 25(44):10239-10246; doi:10.1523/JNEUROSCI.2704-05.2005

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Behavioral/Systems/Cognitive
Functional Specialization in the Human Medial Temporal Lobe

Morgan D. Barense,¹ Timothy J. Bussey,² Andy C. H. Lee,¹ Timothy T. Rogers,¹ R. Rhys Davies,³ Lisa M. Saksida,² Elisabeth A. Murray,⁴ and Kim S. Graham¹

¹Medical Research Council Cognition and Brain Sciences Unit, Cambridge CB2 2EF, United Kingdom, ²Department of Experimental Psychology, University of Cambridge, Cambridge CB2 3EB, United Kingdom, ³Department of Clinical Neurosciences, University of Cambridge, Addenbrooke's Hospital, Cambridge CB2 2QQ, United Kingdom, and ⁴Laboratory of Neuropsychology, National Institute of Mental Health, Bethesda, Maryland 20892

Investigations of memory in rats and nonhuman primates have demonstrated functional specialization within the medial temporal lobe (MTL), a set of heavily interconnected structures including the hippocampal formation and underlying entorhinal, perirhinal, and parahippocampal cortices. Most studies in humans, however, especially in patients with brain damage, suggest that the human MTL is a unitary memory system supporting all types of declarative memory, our conscious memory for facts and events. To resolve this discrepancy, amnesic patients with either selective hippocampal damage or more extensive MTL damage were tested on variations of an object discrimination task adapted from the nonhuman primate literature. Although both groups were equally impaired on standard recall-based memory tasks, they exhibited different profiles of performance on the object discrimination test, arguing against a unitary view of MTL function. Cases with selective hippocampal damage performed normally, whereas individuals with broader MTL lesions were impaired. Furthermore, deficits in this latter group were related not to the number of discriminations to be learned and remembered, but to the degree of "feature ambiguity," a property of visual discriminations that can emerge when features are part of both rewarded and unrewarded stimuli. These findings resolve contradictions between published studies in humans and animals and introduce a new way of characterizing the impairments that arise after damage to the MTL.

Key words: amnesia; hippocampus; medial temporal lobe; perirhinal cortex; object discrimination; declarative memory; perception; feature conjunctions

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Received Feb 11, 2005; revised September 2, 2005; accepted September 13, 2005.

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