Donald Hebb (1904-1985) is often considered the “father of neuropsychology” because of the way he was able to merge the psychological world with the world of neuroscience. This achievement was accomplished largely through his work The Organization of Behavior: A Neuropsychological Theory which was published in 1949.

The Organization of Behavior: Hebbian Theory

Hebb’s major contribution to the fields of both neuroscience and psychology was bringing the two together. Published in 1949, The Organization of Behavior: A Neuropsychological Theory is the book in which Hebb outlined his theory about how learning is accomplished within the brain. Perhaps the most well-known part of this work is what has become known as the Hebbian Theory or cell assembly theory.
The Hebbian theory aims to explain how neural pathways are developed based on experiences. As certain connections are used more frequently, they become stronger and faster. This hypothesis is perhaps best described by the following passage from The Organization of Behavior:

“When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A’s efficiency, as one of the cells firing B, is increased.”

So, cell A and cell B are located near each other. As cell A is repeatedly involved in the firing of cell B by exciting it, a change occurs in one of or both the cells. This change improves how effective cell A is at contributing to the firing of cell B. The two cells become associated with each other. This is often described as “cells that fire together, wire together.”
Hebbian theory provides a micro, physiological mechanism for the learning and memory processes. This theory has also been extended to computational machines that model biological processes and in artificial intelligence.

The Organization of Behavior: Bringing Two Groups Together

First and foremost, the purpose of The Organization of Behavior was to illustrate Hebb’s theory about behaviour. It also played a huge role in merging the fields of psychology and neuroscience. Hebb states this goal in the introduction to the book:

“Another [goal] is to seek a common ground with the anatomist, physiologist, and neurologist, to show them how psychological theory relates to their problems and at the same time to make it more possible for them to contribute to that theory.”

Hebb saw a need for psychology to work with neurology and physiology to be able to explain human behaviour in a more objective manner. In this way, the more abstract “mind” that psychology tended to focus on was merged with the physical, biological brain function. Hebb argued that this approach was necessary if psychology was going to be viewed as a scientific discipline.
The field of neuropsychology perseveres under the umbrella of both neuroscience and psychology, aiming to explore how behaviour correlates with brain function.

Three postulates of Hebb’s theory

Hebbian learning

Hebbian Learning: Connections between neurons increase in efficacy in proportion to the degree of correlation between pre- and post-synaptic activity.

In one of the most often quoted passages in Neuroscience Hebb boldly postulated: "When an axon of cell A is near enough to excite B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased" (p. 62). In Neuroscience, this proposal is referred to the "Hebb synapse", the first instances of which were later discovered in long term potentiation (Bliss & Lřmo, 1973) and kindling (Goddard, McIntyre & Leech, 1969). In Cognitive Science and Computational Neuroscience this proposal is referred to as the "Hebb rule" which provides a basic learning algorithm for adjusting connection weights in neural and artificial network models (Trappenberg, 2002).

Cell assemblies

Cell-Assembly: Group of neurons which tend to fire together.

The brain basis of mental representation (images, ideas) is groups or assemblies of neurons that tend to be active at the same time because of Hebbian learning. The firing of neurons in a cell-assembly can persist after the triggering event and this persistence is a form of memory. Some consider the cell-assembly proposal, to be Hebb's most important conceptual contribution (Milner, 1986).


Phase sequence

Phase Sequence: Thinking is the sequential activation of sets of cell-assemblies.

The following is an elaboration, from "The Organization of Behavior," of these postulates into a theory:

"Any frequently repeated, particular stimulation will lead to the slow development of a "cell-assembly," a diffuse structure comprising cells in the cortex and diencephalon (and also, perhaps, in the basal ganglia of the cerebrum), capable of acting briefly as a closed system, delivering facilitation to other such systems and usually having a specific motor facilitation. A series of such events constitutes a "phase sequence" - the thought process. Each assembly action may be aroused by a preceding assembly, by a sensory event, or—normally—by both. The central facilitation from one of these activities on the next is the prototype of ‘attention.’ ... The theory is evidently a form of connectionism... though it does not deal in direct connections between afferent and efferent pathways: not an S-R psychology, if R means muscular response... It does not, further, make any single nerve cell or pathway essential to any habit or perception." (p. xix)

Entire new fields of study on the role of early experience in perceptual development (Hunt, 1979), sensory deprivation (Zubek, 1969), self-stimulation (Olds & Milner, 1954), the stopped retinal image (Pritchard, Heron, & Hebb, 1960), synaptic modifiability (Goddard, 1980), and learning without awareness (McKelvie, 1987), were launched to test or validate Hebb’s neuropsychological theory and to explore its consequences.

Hebb knew that the details of his theory would be superseded (e.g. Milner, 1957). But he believed it was the right kind of theory, one that sought to explain what went on between stimulus and response using neuroscientific principles. The ideas at the core of Hebb’s Neuropsychological Theory – the Hebbian synapse, the Hebbian cell-assembly, and the proposal that thinking is the sequential activation of assemblies of neurons – seem to have captured fundamental truths about how the nervous system generates and organizes of behavior.

The Hebb’s principle or Hebb’s rule

Hebb says that “when the axon of a cell A is close enough to excite a B cell and takes part on its activation in a repetitive and persistent way, some type of growth process or metabolic change takes place in one or both cells, so that increases the efficiency of cell A in the activation of B “.

‘neurons that fire together wire together’

It is customary to be summarized as “neurons that fire together wire together”. That is, the simultaneous activation of nearby neurons leads to an increase in the strength of synaptic connection between them.

It is important to note that the neurons must be previously connected, sufficiently close to one another, so that the synapse can be reinforced.

At early 1970s, LTP (long term potentiation) was discovered, which confirmed Hebb’s theory. It was demonstrated that morphological changes take place, splitting into the receptor dendrites of the hippocampus, which reinforce the synaptic connection.

In short, Hebb’s principle is fundamental for the relationship between psychology and neuroscience, since it provides a general framework for relating learning and behaviour with neural networks.