The memory system that we usually refer to is the "explicit memory". Implicit memory operates outside our awareness. Semantic memory handles conceptual knowledge. Procedural memory is used to learn skills and acquire habits. Then there is Tulving's episodic memory (that is also responsible for the "autobiographical" feeling).

Rephrasing Tulving's ideas on encoding and decoding, Schacter makes the important distinction between field memory (you are in it) and observer memory (you are not in it). We tend to recall older events as field memories, and more recent ones as observer memories. But this is not a given: it really depends on what we focus on. If asked to focus on the details of an event, we recall them as observer memories. If asked to focus on our feelings during the event, we tend to switch to a field memory. Thus the "perspective" is not part of the stored information: the perspective is manufactured at the time of recall.

Under the influence of his mentor Tulving, Schacter makes the point that memory does not store events as they happened (a memory of a scene is not a snapshot of that scene), nor does it store the events per se: it stores our experience of them. That experience is more than just a recording of what happened. It contains information on how to operate in a similar situation. Memory of the past determines how we act in the future.

Subjective experience is crucial to the process of remembering: it literally shapes what we remember.

Memories are somehow encoded, so that they can be decoded whenever needed. But the decoding does not seem to be deterministic. And the encoding is likely to be very subjective (two people encode the same event in two different ways, and the very same person may encode the same event in two different ways if it occurs twice). Schacter thinks that we remember what we analyzed, and that the process of encoding an event is simply a by-product of our analysis of that event.

The retrieval is no less complicated than the original encoding. Schacter (following his fellow psychologist Morris Moscovitch) distinguishes at least two kinds. "Associative retrieval" is the kind of remembering that we can't help: we are reminded of something, whether we want to or not. "Strategic retrieval" is the kind of remembering that we do when we try very hard to remember something (a name, for example). Moscovitch believes that the former is mainly carried out by the hippocampus, while the latter is probably performed in the cortex: basically, these are different processes that take place in different places.

Schacter believes that the cue is also part of the "remembered": the cue combines with the engram (the memory trace) to yield a subjective recollection. Basically, depending on which cue triggers the "remembering" of an event, we remember that event in a slightly different light.

This is also why we forget: new experiences blur previous engrams. Cues that used to be very effective become less and less effective.

At the same time, we consolidate memories, and they become more fault-tolerant. This happens mainly through "reuse" of them: as we use them, we rehearse the connection between the medial temporal region (the "index" of features that are stored in the cortex, what Antonio Damasio calls "convergence zone") and the storage areas in the cortex (that contain those features). Basically, when we use and reuse a memory, the medial temporal region rehearses how to reconstruct that memory. The hippocampus also contributes to consolidating memories, as per Jonathan Winson's theory of dreams.

Thus different aspects of a memory are stored in different regions of the brain, and then connected by a special memory system, which is therefore responsible for the "autobiography" that each of us has of her/himself.

Schacter believes that there are three levels of autobiographical knowledge: lifetime periods (e.g., childhood), general events (e.g., a holiday) and events (e.g., the first kiss). Each remembered episode is actually a mixture of these three levels.

Schacter makes the point that, because the encoding process is subjective, there is a limit to how accurate the decoding can be. Just like rehearsal can increase the likelihood of retrieving a memory, it can also increase its inaccuracy. We distort our memories of ourselves. We construct our own autobiography, which is only loosely based on what truly happened.

Implicit memory contributes to the confusion of the self. One form of implicit memory that is particularly intriguing is "priming". Schacter analyzes a number of neurological cases and concludes that priming likely depends on the "perceptual representation system" that allow us to recognize and identify objects. Priming is active all the time, even though we are unaware of it.

But the book is, perhaps, mainly an annotated encyclopedia of important neurological and psychological cases.

TM, ®, Copyright © 2005 Piero Scaruffi