(Copyright © 2000 Piero Scaruffi | Legal restrictions - Termini d'uso )
The American neuroscientist Howard Eichenbaum has written a comprehensive
and up-to-date survey of research on the neurological bases of memory.
The book is organized around four themes, which, in English, all begin with the letter "C": connection, cognition, compartmentalization, consolidation. These four aspects cover most of what one needs to know about how the brain "does" memory.
Connection is about the electrochemical mechanism that allows a brain to retain information about what happened. We know that this is due to the plasticity of the neural network, i.e. to the fact that the "strength" of each connection between two neurons can change in time, and does change in response to each new stimulus. The book offers a short introduction to the history of neurology through the main discoveries and the main contributors.
"Cognition" is about the external behavior of memory: what we see as psychologists, not as physicists.
"Compartmentalization" (a rather horrible term, and rather misleading in this case) is the most interesting part of the book, and has to do with the discovery of different "kinds" and processes of memory. The book describes how the brain accomodates several different memory systems, each of them involving the cortex but each characterized by different "pathways" leading from the cortex to other areas of the brain. Studies on amnesia (particularly by Neal Cohen in 1980) show that there are at least two different kinds of memory: "declarative memory" (the memory that one can consciously remember, which is forgotten in an amnesia) and "procedural memory" (the skills and procedures which are usually not forgotten, as people with amnesia can still perform most actions they have learned throughout their lives). Recent studies seem to prove that the hippocampus is the key to declarative memory, or at least the key to linking together declarative memories. Procedural memory is realized by circuits that involve the motor areas of the cortex and two loops that spread through the striatum and the cerebellum: acquiring skills is, indeed, a complex phenomenon. Emotional memory, on the other hand, seems to depend on the working of the amygdala. These three memory systems are physically connected to the cortex along different pathways, which means that they can work in parallel.
The book is also useful to learn more about the structure of the brain. There are four main areas (lobes) of the cortex: frontal, parietal, occipital, temporal lobes. Each one is a complex entity, with its own specialized sub-areas. So are the hippocampus and the amygdala: an understanding of their functions requires an understanding of their structure.
"Consolidation" is about the physical processes that allow for memories to become permanently encoded in the brain. Two main processes have been identified, one in which the strength of connections is fixed by a localized sequence of chemical events, and one in which the strength of connections is calculated through extensive processing that involves several areas of the brain (a "reorganization" of memory).
Finally, retrieval of a specific memory involves the ability to search the consolidated memories, which requires the existence of a "working memory", where we can store items for a few seconds. This function is most likely implemented by the prefrontal cortex.
The value of the book is not su much in speculating how many kinds of "memory" a brain has, but in offering detailed physical hypotheses on how each of these memory systems works inside the brain.