The idea that learning is due to changes in the efficiency of synaptic transmission is quite old, having been suggested by Tansy in 1893. While many researchers in the area today would agree with the statement that details of the processes involved are not yet worked out after three-quarters of a century. The hypothetical modifications of neural connections induced by the learning process have been investigated primarily through two methods, which Bures terms static and dynamic. The static method Involves comparison of neural activity in a naive, untrained animal with that of a trained one. This method suffers from the disadvantage that it cannot differentiate neurons which change their activity during conditioning from those which are unaffected by learning. The neural basis of learning is better investigated by use of the dynamic method, in which the activity of single neurons is examined through the conditioning process.