The hippocampal formation, one of the most complex and vulnerable brain structures, is recognized as a crucial brain area subserving human long-term memory. Yet, its specific functions in memory are controversial. Recent experimental results suggest that the hippocampal contribution to human memory is limited to episodic memory, novelty detection, semantic (deep) processing of information, and spatial memory. We measured the regional cerebral blood f low by positron-emission tomography while healthy volunteers learned pairs of words with different learning strategies. These led to different forms of learning, allowing us to test the degree to which they challenge hippocampal function. Neither novelty detection nor depth of processing activated the hippocampal formation as much as semantically associating the primarily unrelated words in memory. This is compelling evidence for another function of the human hippocampal formation in memory: establishing semantic associations.The discovery that the mediotemporal brain regions, particularly the hippocampal formations, are essential for human memory (1, 2) set the ground for neuroscientific theories and experimental practice during the past 40 years. In the years following this discovery, research with amnesic patients led to the finding that memory is not a unitary system but is divided into subsystems, each supported by a different but partially overlapping neuronal network. The function of the human hippocampal formation was pinned down to declarative memory alone and became specified even further to declarative learning͞consolidation (3, 4), episodic memory (5), novelty detection (6-9), the retrieval of deeply encoded items (10), and spatial learning (11-13). At the same time, learning experiments with rats (14-17) indicated that the hippocampal formation is important for the establishment of associations between components of episodes in memory. The experience of an episode typically involves the simultaneous processing of diverse sensory inputs, bodily sensations, thoughts, and emotions in distributed cortical regions, creating patterns of coactivations in the cortex. The composition of these coactivations needs to be stored in memory for the later recovery of some or all aspects of that episode. The anatomy and physiology of the hippocampal formation (dentate gyrus, Ammon's horn͞hippocampus proper, presubiculum) lends itself to store such patterns of neuronal coactivations temporarily (3,4,(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24). The less complex a scene is, the fewer associations are required to memorize it, and, thus, the less hippocampal activation can be expected. We tested this hypothesis in the human, reducing the complexity of a natural scene to the sensorily restricted environment of a word-learning experiment in the positron-emission tomography (PET) scanner. Visually presented pairs of semantically unrelated nouns had to be learned by one of three strategies inducing a (i) deep or (ii) shallow encoding of the words in isolation of each ot...

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Studies of amnesia have demonstrated that the hippocampus is necessary for long-term memory, but its precise role in memory is unknown. We designed a positron emission tomography experiment with tailored encoding and retrieval tasks that permitted the isolation of different mnemonic functions theorized to be mediated by the hippocampus. These functions included encoding single items, establishing interitem associations, novelty detection, and retrieving recently formed associations. Of these, we found hippocampal and parahippocampal activation only during associative learning. Our results indicate that the hippocampal formation may be particularly involved in the establishment of associations among components of an episode in memory.

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The authors conclude that SelAH is a safe and effective surgical procedure for MTLE.