Hierarchical Organization of Memory

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Hierarchy Long Term Memory

FIGURE 2 Hierarchical organization of memory. Short- and long-term memory is subject to being learned by either conscious or unconscious processes. Similarly, memory can be recalled either consciously or unconsciously. Many forms of simple learning such as motor learning, simple associative conditioning, and non-associative learning can be learned and recalled unconsciously. More complex forms of learning typically involve conscious processes. Short-term working memory is listed as a separate category because it is essentially entirely conscious and not stored for more than a few seconds.

FIGURE 2 Hierarchical organization of memory. Short- and long-term memory is subject to being learned by either conscious or unconscious processes. Similarly, memory can be recalled either consciously or unconsciously. Many forms of simple learning such as motor learning, simple associative conditioning, and non-associative learning can be learned and recalled unconsciously. More complex forms of learning typically involve conscious processes. Short-term working memory is listed as a separate category because it is essentially entirely conscious and not stored for more than a few seconds.

term (see Figure 2), and apply conscious and unconscious to it as well. Thus, any type of memory (with one exception, which is discussed later) falls into one of four categories: unconscious learning with unconscious recall, unconscious learning subject to conscious recall, conscious learning subject to unconscious recall, and conscious learning subject to conscious recall. Specific examples are listed in Figure 2 for illustrative purposes, and for the rest of this chapter and in Chapters 2 and 3 we will cover many specific examples in each category.

I like the nomenclature summarized in Figure 2 because it emphasizes that any given memory event comprises three components: learning, storage, and recall. An item or event is learned, stored for some period of time, and recalled. Highlighting these three components is necessary, in my opinion, because each corresponds to a distinct molecular and cellular set of events.

It also is important to note that the category for the learning, memory, and recall of a specific bit of information is not static over time, but instead is subject to change. Consider, for example, the learning and recollection of a phone number that becomes familiar with repetition. You first look up the number in the phone book and consciously store and recall the number. Over time, you repetitively punch in the number, and it is subject to being learned unconsciously as a motor pattern and recalled unconsciously in the same way. After a while, however, a thoroughly familiarized phone number can become difficult to recall in a direct, conscious fashion. Many times I have seen friends

"recall" a familiar phone number by pretending to punch it out on a touch-tone phone pad, and then consciously convert the motor pattern into a usable number. Thus over time the same bit of information has been subject to conscious learning, unconscious learning, conscious recall, and unconscious recall.

Finally, I should note that storage is unconscious in my model. This is not to say that all forms of memory are stored unconsciously—clearly several forms of short-term "working" memory are conscious. A good example of this is short-term storage of a phone number, where one can store information over time essentially by conscious repetition over a given time span. However, I place this form of memory in a separate category because I am approaching memory from a cellular and molecular perspective (see Figure 2). Working memory can be stored as a short-term change in firing pattern in cortical neurons, for example, in a reverberating circuit. As such, it does not require any persisting biochemical modification for its maintenance. Indeed, at the molecular level this seems likely to be the distinguishing characteristic of working memory. It is memory that cannot sustain itself in the absence of continuing neuronal firing.

My categories of learning and memory roughly correspond to the typically used "nondeclarative memory" and "declarative memory" nomenclature popularized by Squire and Kandel (Figure 3) and widely accepted and utilized. I prefer the conscious/unconscious terminology because it

Squire Milner Kandel

FIGURE 3 Subdivisions of human memory and associated brain regions. Human memory is typically divided into declarative and nondeclarative types, also known as explicit and implicit memory, respectively. In addition to various types of memory described in the text, priming is also listed. Priming is unconscious memory formation. For example, if one hears or reads a word, for a period of time afterward one is more likely to use that word in conversation or in a word completion task. This occurs even if no conscious memory for having heard the word is formed. Chart adapted from Milner, Squire, and Kandel (13).

This type of chart, which subdivides memory into several, separately identified components distills the modern concept of multiple memory systems. It is now clear that different anatomical structures in the brain are involved in different types of memory formation. Moreover, the different systems can operate as parallel processors that function independently. This allows multitasking, with conscious and unconscious memory systems operating simultaneously and increasing the overall "memory throughput" of the central nervous system. I highly recommend reading From Conditioning to Conscious Recollection by Eichenbaum and Cohen (4) for a more thorough treatment of the multiple memory systems concept.

FIGURE 3 Subdivisions of human memory and associated brain regions. Human memory is typically divided into declarative and nondeclarative types, also known as explicit and implicit memory, respectively. In addition to various types of memory described in the text, priming is also listed. Priming is unconscious memory formation. For example, if one hears or reads a word, for a period of time afterward one is more likely to use that word in conversation or in a word completion task. This occurs even if no conscious memory for having heard the word is formed. Chart adapted from Milner, Squire, and Kandel (13).

This type of chart, which subdivides memory into several, separately identified components distills the modern concept of multiple memory systems. It is now clear that different anatomical structures in the brain are involved in different types of memory formation. Moreover, the different systems can operate as parallel processors that function independently. This allows multitasking, with conscious and unconscious memory systems operating simultaneously and increasing the overall "memory throughput" of the central nervous system. I highly recommend reading From Conditioning to Conscious Recollection by Eichenbaum and Cohen (4) for a more thorough treatment of the multiple memory systems concept.

emphasizes the cognitive differences between the two forms more effectively, in my mind. In addition, I like these terms because they tend to highlight the intrinsic role of learning and memory in cognition in general. Finally and most importantly, I prefer this terminology because it seman-tically separates the learning from the memory from the recall - an important mindset to adapt as we seek to understand behavioral events in molecular terms.

B. Memory Exhibits Long-Term and Short-Term Forms

As has been emphasized by Eric Kandel, Jim McGaugh, and many others (see reference 4), almost all forms of memory can be either short-lasting or long-lasting. With only a few exceptions (see Box 2), the duration of the memory for a learned event depends on the number of times an animal experiences a behavior-modifying stimulus.

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