Memory, Neural Basis

Memory and Neural Systems

Most modern discussions of memory and the brain begin with the case of Henry Molaison, who was known as patient H. M. to most psychologists and neuroscientists until his death in 2008. Molaison had suffered from serious epileptic seizures starting in childhood, and he underwent bilateral surgical removal of the medial temporal lobe in 1953 in order to remove the epileptic foci. While this surgery was effective in that the frequency of seizures decreased substantially, it resulted in a severe impairment in memory. Molaison was virtually unable to retain any new information, such as the names of new people he met. What was surprising at the time was that this profound memory impairment occurred with a background of normal intellectual functioning. H. M. was able to perceive, reason, and use language relatively normally. Thus, it appeared that memory could be selectively affected by a lesion, demonstrating that a distinct neural system supports memory processing. Although memory impairment had been reported before as the result of brain injury or neurological disease, such as Alzheimer's disease, these cases typically involved more widespread cognitive impairment. The careful study of patient Henry Molaison, and of other amnesic patients that followed, promoted the view that memory can be localized in the human brain. The brain structures that are often damaged in amnesia include the hippocampus and cortical regions in the medial temporal lobe that surround this structure. This medial temporal lobe system is organized in a hierarchical fashion. The hippocampus receives and sends projections to the entorhinal cortex, which is interconnected with the parahippocampal and perirhinal cortices (Figure 1).

Figure 1 Magnetic resonance images of the human brain, showing regions that are damaged in medial temporal lobe amnesia

[Page 485]These regions are, in turn, interconnected with a wide range of high-level association cortical areas, including frontal, temporal, and parietal regions. With this architecture, the hippocampus is positioned to receive highly processed input from all over the brain. This is consistent with the idea that memory representations are complex and multimodal. Amnesia can also result from damage to structures in the diencephalon that are connected to the hippocampus via the fornix.

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