8/30/2006

Dynamic Gating in Long-Term Memory?

Some theories of working memory function posit that midbrain dopamine projections will trigger the updating of representations in prefrontal cortex via the thalamo-cortical loop. In other words, representations enter working memory by a phasic increase in dopamine, which "opens a gate" in the basal ganglia/prefrontal cortex circuit. Might similar gating processes exist in the hippocampus, thought to be responsible for many aspects of long-term memory?

In a new issue of TICS, Fernandex & Tendolkar argue that ento- and peri-rhinal cortex might subserve exactly this function. The authors suggest that a gating function is necessary to prioritize the encoding of more important or unfamiliar events as they occur, and that rhinal cortex fits the bill in more ways than one: it appears to function on the basis of semantic or conceptual associations, it is known to be important in encoding, and is situated right next to the hippocampus.

Experimental evidence also supports the claim that rhinal cortex may "gate" information into long-term memory. As the authors note, fMRI studies have shown that rhinal cortex activity decreases with familiarity. ERP studies have shown that an electrical wave called the "anterior medial temporal lobe N400" is generated by rhinal cortex and decreases with familiarity. Conversely, successful memory encoding is associated with increases in rhinal cortex activity as measured by electrophysiological, fMRI, and ERP techniques. [Incidentally, these results have been interpreted as the reason that spaced learning is better than blocked or massed learning.]

The authors conclude that familiarity detection in rhinal cortex may be an organizing principle of long-term memory encoding and retrieval. During encoding, a lack of familiarity may provide a signal that additional associative encoding should take place; during retrieval, lack of familiarity may signal the need for additional retrieval cues. The authors suggest that this dual role of familiarity detection may provide insight into the role of rhinal cortex and the way in which novelty can be used to optimally allocate encoding resources as well as provide cues for fast retrieval.

Related Posts:
Familiarity vs. Recollection
Implicit vs Explicit Memory: Two Distinct Systems?
EEG Signatures of Successful Memory Encoding

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