Hippocampus plays bigger memory role than previously thought

Psychologists and neuroscientists, in their research into how memory works, distinguish several different types of memory. At the highest level, there’s declarative memory (facts, knowledge, experiences) and procedural memory (skills). Declarative memory can be further divided into episodic memory (personal experiences) and semantic memory (facts about the world).

Two processes are considered to be aspects of episodic memory: recollection and familiarity. Recollection is the ability to recall specific details about an experience or event. Familiarity is the less specific sense that one has had an experience, but without being able to recall specific details. People who read a lot are well acquainted with this distinction, usually to their frustration and annoyance. One may be sure of having read somewhere about a certain piece of information or situation in a novel – but without any clue as to when or where, which makes it pretty difficult to go back to the source in order to check one’s memory and fill in the details.

Neuroscientists have considered several questions about this issue. Are the two processes basically the same, differing only in strength of recall? Are different parts of the brain associated with these two processes? Two papers just published deal with these issues and indicates that when the subjective feeling of confidence in the memory is high (“strength”) the hippocampus is significantly involved in both processes.

Hippocampus Plays Bigger Memory Role Than Previously Thought

Prevailing research posits that recollection and familiarity memories involve different regions in the brain’s medial temporal lobe: the hippocampus for recollection, the adjacent perirhinal cortex for familiarity.

“But given the connectivity in that part of the human brain, that separation seemed too clean, too neat,” said Squire, a leading expert on the neurological bases of memory. “The idea of distinct functions was unlikely.”

Recollection-based memories are typically associated with higher confidence and accuracy than familiarity-based decisions. Accordingly, in the past, comparisons between recollection and familiarity have also involved a comparison between strong memories and weak memories. So the question is how the brain accomplishes recollection and familiarity when the effect of memory strength is taken off the table.

Larry Squire, a prominent figure in memory research, has published a number of other studies on this subject (References). In one of the two new papers, Squire and his colleagues conclude

The Hippocampus Supports Both Recollection and Familiarity When Memories Are Strong

As in earlier studies, recollected items had higher accuracy and confidence than familiar items, and hippocampal activity was higher for recollected items than for familiar items. Furthermore, hippocampal activity was similar for familiar items, misses, and correct rejections. When the accuracy and confidence of recollected and familiar items were matched, the findings were dramatically different. Hippocampal activity was now similar for recollected and familiar items. Importantly, hippocampal activity was also greater for familiar items than for misses or correct rejections (as well as for recollected items vs misses or correct rejections). Our findings suggest that the hippocampus supports both recollection and familiarity when memories are strong.

Further reading:

Medial Temporal Lobe Function and Recognition Memory: A Novel Approach to Separating the Contribution of Recollection and Familiarity

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