femme Neuroscience Notes

Some Topics

Medial Temporal Lobe

The identification of the medial temporal lobe, especially the hippocampus as a critical processor of memory began with studies of amnesia in humans following removal of the hippocampus. Without normal medial temporal lobe function, working memory cannot be translated into longer-term storage.

There are at least two aspects of episodic memory: 1. Content or recall of an event and 2. Source-associated information about when and how the episode occurred. The medial temporal region is critical for both source and content memory.

We know, for example, that the CA1 layer of the hippocampal cortex is a processor of incoming information, but we do not appreciate exactly how this works. John Lisman suggested: "In addition to its role as a decoder, CA1 may also have a role in making a match/mismatch computation in which sensory ''reality'' arriving directly from the entorhinal cortex is compared with predictions of reality made by dentate–CA3. This idea relates to the proposal that the brain forms a model of the world based on past events. This model requires stored sequences that allow prediction of expected events. The brain continuously compares these expectations with events. If the comparison shows a ''mismatch'' to expectations or something altogether novel, memory encoding and attentional processes are triggered. Evidence that the human hippocampus is involved in such processes comes from brain imaging. Related experiments show that hippocampal neurons become habituated to a repetitive stimulus but respond vigorously when the standard stimulus is replaced by an ''oddball'' stimulus. Other experiments in rat directly show activity related to match/mismatch conditions. The clearest demonstration of the existence of an internal model comes from experiments in which a repetitive stimulus was suddenly omitted. Cells in the mammillary body, one of the recipients of hippocampal output, fire in exact registration with the expected onset and duration of the absent stimulus. Because the CA1 region is the site of convergence of predictions from CA3 (via the Schaffer collaterals) and raw sensory information (via the perforant path input from the cortex), CA1 is well positioned to perform a match/mismatch computation."

One of the properties of the hippocampus is a cognitive-spatial map, a series of neuronal networks in which environmental features are encoded in terms of an animal's location and navigational cues. Aggleton and Brown suggested that the parahippocampal, perirhinal, entorhinal cortices and the hippocampus form a medial temporal memory system that acquires and holds new information, some of which may eventually be stored in the neocortex as longterm memory. They viewed the hippocampus as a mixer which combines different components of the memory. The area around the hippocampus can store representations of individual items while the hippocampus itself organizes memories according to relationships among items, including spatial relationships.

Eichenbaum et al proposed: “ that individual hippocampal cells encode regularities present in the animal's every experience, including spatial and nonspatial cues and behavioral actions… the coding of spatial locations by hippocampal place cells emerges from a fundamental representation of behavioral episodes. These representations involve a network of cells, each of which represents a temporally defined event… the hippocampus is central to episodic memory…episodic representations are tied to one another within a general memory organization, consistent with the common notion that networks of semantic knowledge are built from episodic experiences.”

Burgess et al suggested:” Finding one's way around an environment and remembering the events that occur within it are crucial cognitive abilities that have been linked to the hippocampus and medial temporal lobes. Our review concentrates on important concepts in this field: spatial frameworks, dimensionality, orientation and self-motion… While processing of spatial scenes involves the parahippocampus, the right hippocampus appears particularly involved in memory for locations within an environment, with the left hippocampus more involved in context-dependent episodic or autobiographical memory.”