Trends in Neurosciences
Volume 20, Issue 10, 1 October 1997, Pages 451-459
Journal home page for Trends in Neurosciences

Network memory

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Abstract

Our thinking on the cortical organization of primate memory is undergoing a copernican change, from a neuropsychology that localizes different memories in different areas to one that views memory as a distributed property of cortical systems. We are shifting our focus from `systems of memory' to the memory of systems. The same cortical systems that serve us to perceive and move in the world serve us to remember it. Our memories are networks of interconnected cortical neurons, formed by association, that contain our experiences in their connectional structure. Perceptual and motor memory networks are hierarchically organized in post-rolandic and pre-rolandic neocortex, respectively. Recall, recognition and working memory consist largely in their reactivation, also by association.

Section snippets

Memory formation: from synapse to network

The acquisition of memory basically consists in the modulation of synaptic contacts between nerve cells[5], a notion first advanced by Ramón y Cajal in 1894 referring to motor memory. Memories are formed by facilitation—and selective elimination—of synaptic links between neuronal aggregates that represent discrete aspects of the environment or the inside of the body. Thus, memories are essentially associative; the information they contain is defined by neural relationships.

Hebb[3]proposed that

Phyletic memory

To understand the formation and topography of memory better, it is useful to think of the primary sensory and motor areas of the cortex as the repositories of a form of largely inborn memory that we may call phyletic memory or `memory of the species'. At birth, those areas already contain the essential `experience' of evolution in their synaptic structure, acquired through interacting with the world—that is, the neural representation of the essential features of sensation and movement. Thus,

Perceptual memory

Perceptual memory is memory acquired through the senses. It comprises all that is commonly understood as personal memory and knowledge—that is, the representation of events, objects, persons, animals, facts, names and concepts. There is a hierarchy of perceptual memories that ranges from the sensorially concrete to the conceptually general. At the bottom are memories of elementary sensations; at the top, the abstract concepts that, although originally acquired by sensory experience, have become

Motor memory

Motor memory is the representation of motor acts and behaviors. It includes much, if not all, of what has been termed procedural memory. In mammals, the lowest levels of the motor hierarchy are in the spinal cord, brain stem and cerebellum. These structures store the relatively simple forms of motor memory, for example, the repertoire of reflex acts that mediate many of the innate defensive reactions. Much of the motor memory in lower structures qualifies as phyletic, in that it is largely

Memory dynamics

At any time in our daily life, the bulk of our long-term memory is dormant and out of consciousness. Presumably, the neuronal aggregates of its networks are relatively inactive (Fig. 1B,E). A network is reactivated when the memory it represents is retrieved by the associative processes of recall or recognition. An internal or external stimulus, whose cortical representation is part of the network by prior association, will reactivate that representation and, again by association, the rest of

Concluding comments

In conclusion, the empirical evidence thus far indicates that, in humans and nonhuman primates, memory is stored in overlapping and widely distributed networks of interconnected cortical neurons. Because cortical connectivity can serve practically infinite potential associations, potential networks are practically infinite, and this fact confers uniqueness to the cognitive memory of a given individual.

According to the views expressed in this article, memory networks are made by simultaneous

Acknowledgements

I thank Eric Kandel, Larry Squire, Mortimer Mishkin, James McGaugh, Patricia Goldman-Rakic, Richard Thompson and Stephen Kosslyn for their valuable comments and suggestions.

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