In a typical experiment by Robert Rescorla and Allan Wagner, one group of rats experienced a tone just before each of 20 shocks. A second group of rats experienced the same number of tone-shock pairings plus an additional 20 shocks with no tone. Rescorla found that the rats in the first group showed a much stronger conditioned fear response than the rats in the second group. How did Rescorla explain this finding?

When a neutral stimulus is coupled with a stimulus that elicits fear in an organism then such a process is termed as fear conditioning.
The responses in fear conditioning can be reflected in either the behavior or physiology of the organism.
After fear conditioning has taken place, the subjects start expressing the same responses to the neural stimuli that preceded the fear stimuli that they do for the fear stimuli.
Since in the second group there were 20 more shocks given without any tone, the first group of the rats were able to learn the predictability of an event more strongly and thus, the tone elicited a stronger fear response in the first group.