This rule of neuropsychology explains how the activation of neurons gives rise to memories.
The so-called Hebb’s law, proposed by the neuropsychologist Donald Hebb, states that synaptic connections are strengthened when two or more neurons are activated contiguously in time and space. When the firing of the presynaptic cell is associated with the activity of the postsynaptic, structural changes take place that favor the appearance of assemblies or neural networks.
In this article we will analyze the main approaches of this theory, which had a seminal influence on neuropsychology: among other aspects, it is considered that Hebb’s rule inspired the concept of long-term potentiation and the neural network models that explain learning and the memory.
Role of neural plasticity in learning
From a neuroscience point of view, the biological basis for learning lies in neural plasticity. This concept refers to the ability of the nervous system to modify the nature and strength of synapses, that is, the connections between neurons that allow the transmission of electrochemical impulses.
In recent decades, the hypothesis that our brain stores information in neural networks has gained great popularity and strong scientific support. The structure of the nervous system and the relationships between its elements constitute the information we process; memory, for its part, consists in activating these networks.
The origin of this type of approach can be traced directly to a specific hypothesis: Donald Hebb’s cell assembly theory. The study of neural networks, which constitutes a core framework in current cognitive neuroscience, has been developed around the basic principles proposed by this author.
Hebb’s Law (or Cell Assembly Theory)
In 1949 the psychologist Donald Hebb published the book “The organization of behavior”, where he developed a pioneering theory on the neural bases of learning. Although Hebb’s proposal is called “Cell Assembly Theory”, it is usually referred to through the term by which its basic principle is known: Hebb’s law.
Hebb’s rule states that if two neurons are active at approximately the same time, their connections are strengthened. Specifically, Hebb stated that if the axon of neuron A is close enough to cell B and repeatedly contributes to firing it, certain structural or metabolic changes will increase the efficiency of such a synapse.
Specifically, this would cause the appearance of terminal buttons, or the enlargement of existing ones, in the axon of the presynaptic neuron; these would be in direct contact with the soma of the postsynaptic cell. The physical and functional association between different neurons would give rise to engrams or cellular assemblies – nowadays “neural networks”.
Thus, the stronger the contingency between neuronal activation and a given type of stimulation, the greater the probability that the relevant neural networks will trigger impulses when the stimulus occurs again. This also explains why practice or review makes it difficult for synapses to weaken (as happens in forgetting).
For this to happen, Hebb proposed, the first neuron needs to fire immediately before the second; if the neural firing occurs at the same time in both cells, on the other hand, there is no causality in the synapse, so the connection would not be strengthened in the same way.
However, this law explains only the strengthening of associations, and not their formation. Thus, learning is based on the consolidation of pre-existing synapses, mainly determined by biological and genetic variables. According to Hebb, each neural circuit can be directly related to a learned activity.
Influence of this neuropsychological model
Hebb’s proposal had a strong impact on neuropsychology, becoming the nucleus of many approaches developed in subsequent decades, and continues to be a very important reference in this field today.
In the early 1970s, the existence of a very relevant mechanism for learning was discovered: long-term enhancement, which consists of the consolidation of memories through repeated experience. Thus, short-term memory is established by structural changes (gene expression, protein synthesis, and changes in synapses).
The validation of this model supported Hebb’s fundamental thesis, determining the concrete biological bases that explain his law. Today we also know with certainty that long-term potentiation is limited exclusively to neurons that are active at the same time, and that if several synapses converge on the same neuron, they become even stronger.
One of the most recent applications of Hebb’s rule is related to mirror neurons, which are activated both when we perform a behavior and when we see another living being doing the same and are understood as the basis of empathy and the theory of the mind. Relevant synapses have been found to be strengthened following Hebb’s law.