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.

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

1.2 Experiment Setup

1.3 Instructions

You need to follow these directions carefully for this experiment to work.

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 and 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.

1.4 Experiment

1.5 Results

1.6 Quiz