The impact of the Hebbian learning rule on research in behavioural neuroscience.

[HEADNOTE]AbstractHebb's principal theoretical propositions, the cell assembly and the nature of synaptic change, were generated at a time when the focus of work in behavioural neuroscience was directed at understanding issues such as the principles governing the behaviour of animals in neuropsychological studies of learning and memory and the role of drives in the control of behaviours like sex and feeding and drinking. It was not until attention shifted to understanding the neural underpinnings of learning and memory that Hebb's propositions had an impact on behavioural neuroscience as they provided a simple, and testable, mechanism for synaptic plasticity observed both in learning and in other forms of experience-dependent neural change. But much of the field remains interested in other issues such as sensation and perception, motivation, attention, and so on, and to date, Hebb's propositions have had little impact.An understanding of the role of Hebb's postulates in research on brain and behaviour must be seen in historical context. In 1949, the field that we now call behavioural neuroscience was usually described as physiological psychology, or as the subtitle of Hebb's book suggested, neuropsychology. The focus of research in physiological psychology was different than the interests of today with the emphasis being on motivational systems and the mechanisms of learning and memory. The ascending activating systems of the brainstem had just been discovered and there was keen interest in the role of the hypothalamus in controlling both regulatory and nonregulatory behaviours. Psychology had a long tradition of study in learning (e.g., Harlow, 1949) and Lashley had just spent 40 years looking for the location of memory in the brain (e.g., Lashley, 1956). When Scoville and Milner (1956) described case H.M. this added even more fuel to the general interest in wherein the brain learning and memory might reside. But this interest was largely phenomenological in the sense that correlations between behavioural events and brain injury were being described with little direct study of the mechanisms that might account for the formation of memories.Hebb's own interests were different, however, as he was thinking about the development of perceptual systems and the manner in which the external world comes to be represented in the brain. Hebb's principal theoretical propositions, namely, the cell assembly and the nature of synaptic change, were directed at these perceptual questions and thus not directly relevant to the research topics that were more popular at the time. Thus, it is safe to say that Hebb's propositions had rather little direct impact on the field in general in the first decades after the publication of his book. I note parenthetically that Hebb did have considerable impact upon the development of the field of behavioural neuroscience, especially in Canada, as the leaders of the '50s, '60s, and '70s, such as Brenda and Peter Milner, Doreen Kimura, Gordon Mogenson, Graham Goddard, Mort Mishkin, and Dalbir Bindra, to name only a few, were all students or colleagues of Hebb, and this group trained the next wave of investigators who, in turn, crafted the field that we now recognize as behavioural neuroscience, at least in Canada.By the 1980s, there had been a significant shift in the nature of neuropsychological investigations and the field expanded beyond its beginnings as phsyiological psychology. As behavioural neuroscience emerged as a multidisciplinary discipline, researchers were no longer satisfied with descriptions of behavioural phenomena, which is where it obviously had to begin, but had become more focused on understanding the nature of the mechanisms that were postulated to underlie the observed behavioural phenomena. Thus, whereas studies might previously have been interested in the general role of cerebral structures in learning and memory, the spotlight shifted to physiological studies looking at cellular changes that might underlie learning and memory. …

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