Academic journal article Canadian Psychology

Trains of Neural Thought

Academic journal article Canadian Psychology

Trains of Neural Thought

Article excerpt

Abstract

The neural implementation of concepts, as postulated by Hebb (1949), has become obsolete due to the advances in neuroscience during the last half-century, but the basic idea revolutionized psychology, and remains valuable. This paper charts changes that have been imposed on Hebb's cell assembly by our greater understanding of the anatomy and neurophysiology of the visual system and our better recognition of the influence of "top-down" attention on perception. Difficulties encountered in trying to find neural explanations for the association of ideas are also discussed.

The Cell Assembly

When this paper was requested, several possible topics came to mind. An obvious one was the neural basis of motivation, but for more than 20 years I have done no research in the area and I have little to add to what I have already written on the subject (Milner, 1991). Motivation is an important aspect of behaviour but the brain is a thinking machine; it is difficult to understand any aspect of behaviour without taking that into account. Hebb's pioneering model of the neural element of thought, his cell assembly, was developed more than half a century ago and has undergone many changes, but the machinery of thinking remains a challenge, so that was the topic I chose.

I have a soft spot for the cell assembly because it was Hebb's speculation about it in his book The Organization of Behavior (Hebb, 1949) that lured me into psychology. I had wanted to know how the brain works since I encountered science-fiction robots in the pulp magazines that kids read for entertainment in the days before television or computer games. When I read the chapter on the cell assembly in a manuscript of The Organization of Behavior, I thought Hebb might be on the way to an answer. If I studied with him I might even help to find it. I never did get to test Hebb's model in the lab but I did think about it, and whilst not many of Hebb's neurological speculations have survived the advance of neuroscience, I am still fascinated by the questions posed in The Organization of Behavior.

Hebb took up psychology in the late 1920s when behaviourism was at its most radical. Behaviourists, in their effort to create a scientific discipline, refused to acknowledge mental processes. Hebb, however, thought that psychology without thought was unthinkable, and to remedy the situation he set out to show how mental processes could be explained by brain activity, a legitimate subject for scientific study. Hebb's book was well received when it was published, and it undoubtedly helped to get cognition back into psychology.

Briefly, Hebb postulated that neurons in the visual cortex that fire in unison during a particular visual stimulus acquire connections with each other. He further postulated that on repeated presentation of the stimulus, connections between the cells become strong enough to enable impulses to circulate (or reverberate) among the neurons of the group in the absence of external input, constituting a mental image of the stimulus. Hebb called the group of connected neurons a cell assembly. In spite of some inherent difficulties that Hebb felt he had to shelve to avoid piling speculation on speculation, it proved an ingenious and valuable proposal.

One of the shelved problems might have led to the concept of working memory if it had been followed up. The cell assembly of an object was supposed to represent a concept of the object, not a specific instance. Thus, the cell assembly for a triangle, for example, would not distinguish between two different shaped triangles. Something extra, akin to a working memory, is obviously required to discriminate specific instances of a concept, but it was not clear how that could be incorporated into the cell assembly model. A related problem is that as the connections amongst a group of neurons become stronger, they dominate the pattern of firing of the group at the expense of the input from the eye. …

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