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WIREs Cogn Sci
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The amnesias

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Abstract Throughout history, memory and amnesia have been central to philosophical thought and empirical investigation. There has been particular interest in whether there are multiple forms of memory, how they are represented in the brain, how they are parsed following neurological compromise, and how their breakdown is best characterized. Differential abilities among amnestic individuals have advanced the distinction between various forms of memory, such as implicit and explicit memory in the case of H.M., and episodic and semantic memory in the case of K.C. This overview brings together a variety of perspectives on memory and the amnesias, from Clinical Neuropsychology, Neurology, and Cognitive Science, to Animal and Human Neuroscience. We begin with a brief history of the study of memory organization in the healthy brain and dissociations in memory as uncovered in extensively studied cases of amnesia. Memory impairment resulting from damage to the medial temporal lobe (MTL), diencephalon, basal forebrain, and the frontal lobes is then elaborated, with a focus on MTL amnesia, which has received the most attention among memory researchers. Alternative theories of MTL amnesia are considered through a selective review of the current amnesia and neuroimaging literatures on the seemingly disparate roles of the hippocampus in episodic and spatial memory, future imagining, perception, implicit memory, and working memory. If amnesia is a disorder that is not restricted to explicit (consciously accessible) memory, this will force us to rethink the way in which memory is represented in the human brain. WIREs Cogn Sci 2012, 3:47–63. doi: 10.1002/wcs.155 This article is categorized under: Neuroscience > Cognition

The hippocampal‐diencepahalic systems showing connections between medial temporal lobe (MTL) structures, diencepahlic (thalamic) nuclei, and the frontal lobes. Solid lines show the extended hippocampal system, presumed to mediate recollection, and dotted lines show the extended perirhinal system, presumed to mediate familiarity. (Reprinted with permission from Ref 27; modified from Ref 46. Copyright 1999 John Wiley & Sons, Ltd.)

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Magnetic resonance imaging (MRI) displaying extensive damage to patient K.C.'s MTL/hippocampus (arrows) in axial (left images) and coronal (right images) as a result of a severe closed‐head injury. (Reprinted with permission from Ref 43. Copyright 2006 John Wiley & Sons, Ltd.)

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Medial temporal lobe (MTL) structures viewed from the underside of the brain. A, amygdala; E, entorhinal cortex; H, hippocampus; PH, parahippocampal cortex; PR, perirhinal cortex. (Reprinted with permission from Ref 27. Copyright 2005 John Wiley & Sons, Ltd.)

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H.M.'s medial temporal lobe (MTL) surgical resection, depicted in scans conducted in 2002–2004 (ages 74–77), presented in coronal (Aa, Dd), sagittal (Bb, Ee), and axial (Cc, Ff) views in the left (Aa–Cc) and right (dD–Ff) hemispheres on his T1‐weighted magnetic resonance imaging (MRI) (MPRAGE) scan. (Reprinted with permission from Ref 21. Copyright 2006 John Wiley & Sons, Ltd.)

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Anterograde and retrograde amnesia are defined by the time of lesion onset (dotted line). Following damage to the medial temporal lobe (MTL) and related brain regions, anterograde amnesia, in which patients are unable to form new explicit memories of events and facts since their brain damage, is often more severe overall. Retrograde amnesia, in which patients are unable to retrieve memories from before the time of their brain damage, appears to be ungraded (i.e., equal impairment for recent and remote memories) for personal episodes, but temporally graded (i.e., greater impairment for recent than remote memories) for semantic information. Patients with less extensive damage may show temporal gradients in both episodic and semantic memory loss.

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Sample object (facial) identity (a, c) and scene (b, d) stimuli from a typical oddity judgment task. Participants select the ‘odd man out’ from displays in which the stimuli are presented in the same (a, b) or different (c, d) views. Performance on the different view conditions is associated with perirhinal and parahippocampal activation for faces and scenes, respectively, in fMRI studies and is impaired in patients with lesions to respective regions. (Reprinted with permission from Ref 77. Copyright 2005 John Wiley & Sons, Ltd.)

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Region of the left posterior hippocampus showing increased activation as participants elaborated on events that were high in detail, whether the events were from the past or future. (Reprinted with permission from Ref 74. Copyright 2008 John Wiley & Sons, Ltd.)

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