Learning, as recognized by Ivan Pavlov, is basically a process which associates two events. After we see rain following dark cloud a number of times, the dark cloud will very likely trigger the memory of rain before it actually appears. The association between rain and dark cloud allows us to avoid unpleasant events well ahead. This ability is important for survival. When one learns to play chess, he should first memorize the patterns which will immediately lead to the capture of the king. Let us call these patterns {A}. After a few weeks of experience (learning), he may find that some patterns, called {B}, could lead to {A}. The association between {A} and {B} also allows the player to better plan his next move before too late. After years of experiences, he may discover that some patterns {D} are likely to result in patterns {C}, and {C} may lead to {B}. Therefore, a novice player can see only a couple of steps ahead, but an experienced player can see many steps in advance.

A major goal of physical sciences is to find the equations associating the state of a system at a given time t and its next moment, t + dt. This enables physicists to predict future time courses, given the present information (an initial condition). For instance, meteorologists have been able to develop a set of equations describing the time courses of atmospheric conditions (temperature, pressure, etc.). From the present information provided by satellites, they can predict rain several days before the dark cloud is formed.

In our brain, the hippocampus (Fig. 5.1) is the major area for learning. In 1950s, a few patients were treated for epilepsy by removing part of the hippocampus. After the surgery, the patients still remembered their names, the events of their childhood, and the language learned before the surgery. However, they could not form new memory. Although they could talk normally with their relatives and friends, but they forgot such events in the next day. The early stage of Alzheimer's disease is characterized by the same symptom, because the hippocampus is usually the first area to be damaged in Alzheimer's patients.

At the cellular and molecular level, there are two distinct learning mechanisms: the Pavlovian type and Hebbian type. In the Hebbian type of learning, both presynaptic and postsynaptic neurons are involved. The Pavlovian type does not involve postsynaptic neurons. Learning in the hippocampus is mainly the Hebbian type.

Figure 5.1. Schematic drawing of a human brain. The hippocampus is a major area for learning.