Fig. 2.

Results from Experiment 1. Freezing behavior during the first two conditioning trials is graphed as a function of time, separately for rats in the predictable-shock/short-tone group (n = 10) and rats in the unpredictable-shock/short-tone group (n = 10). Each data point during the tone presentations represents the mean percentage of time that groups of rats displayed freezing behavior during a 2-s period; each data point during the intervals after the tones represents the mean percentage of time that groups of rats displayed freezing behavior during a 10-s period. Error bars indicate ±1 SEM. The asterisk indicates a significant difference between the behavior of the two groups (*p < .05).