Chapter 1. Basic Hebbian Learning

1.1 Introduction

It is a basic truism to say that learning takes place in the brain. But learning implies change. If you know something now that you did not know before, something in the brain must have changed. The question then becomes: What changes in the brain? Psychologist Donald Hebb proposed a possible simple solution. He focused on the synapse, the connection between two neurons, highlighted in the figure below. This connection is where one neuron can make the following neuron more or less likely to fire. Moreover, the connection is not of a fixed strength; some connections are stronger than others. Without specifying how, Hebb (1949) proposed that when the presynaptic (before the synapse) and postsynaptic (after the synapse) neurons fire together, then the connection strengthens.

Experiment Setup

1.2 Instructions

In this experiment, you will be examining the connection between two neurons. The first neuron is connected to the second neuron through a synapse. The first neuron is called the presynaptic neuron; the second neuron is called the postsynaptic neuron. You can stimulate the presynaptic neuron to generate action potentials that will travel down its axon and across the synapse to stimulate the postsynaptic neuron. The stimulation strength that crosses the synapse is indicated by the number just above the synapse. A graph of what happens in the postsynaptic neuron is shown above the postsynaptic neuron. When the voltage reaches the labeled threshold line, the postsynaptic neuron will fire an action potential. You can also stimulate the postsynaptic neuron by changing its voltage with the postsynaptic stimulation slider, which shows how long your postsynaptic stimulation is. It is always strong enough to trigger an action potential.

Your job is to try to determine what is necessary to increase the connection strength between the two neurons and to discover the basics of Hebb’s Law. This law describes one basic way that scientists believe connections strengthen between neurons. We will proceed in guided steps to discover this law.

1.3 Experiment

1.4 Results

1.5 Debriefing

Hebb’s Law, the name for this rule about how synaptic connections can grow stronger, was the first rule proposed for how learning can effect individual connections in the brain. This rule specifies that for a connection between two neurons to become stronger, the activity of the presynaptic neuron must lead to the firing of action potentials by the postsynaptic neuron. One or the other is not enough. Here is a model of synaptic plasticity at its most basic. One of the ways this rule seems to work in the brain is in what is called long-term potentiation which is observed most clearly in the hippocampus. The firing of presynaptic neurons that lead to the firing of postsynaptic neurons, even for a brief period of time, can lead the postsynaptic neurons to cause a far stronger response in the output neuron that lasts for hours. The synaptic connection is strengthened.

One other place that Hebb’s Law has been used is in the development of computer models of neural networks. In most cases, the development of a computer neural network is an attempt to develop a learning neural network. One of the features that researchers examine is the rule used to change the connection strength between the modeled neurons. Researchers often use some variation on Hebb’s basic rule for their starting place.

Further Reading:

Hebb, D. O. (1949). The Organization of Behavior. New York: Wiley.

1.6 Quiz