This thesis is concerned with the behaviour of animals on a simple irregular temporally defined schedule of reinforcement, the two-valued mixed fixed interval. Experiments I and II showed that the major determinant of the postreinforcement pause on an evenly probable two-valued mixed fixed interval was the duration of the short interval. It was also found that, if the difference between the two was great enough, the distribution of local rate of
responding around the end of the short interval took on an inverted U shape.
Experiments III, IV and V went on to investigate this pattern of responding in more detail. The results of these experiments showed that the local rate of responding up to the end of the short interval in an evenly probable
mixed fixed interval was very similar to the local rate of responding on an ordinary fixed interval. The inverted U-shaped distribution of local rate of responding developed when there was a sufficient difference between the two
intervals making up the fixed interval, the position of the peak of the distribution and the variation of the distribution being determined solely by the duration of the short interval.
It was argued that this distribution of local rate of responding constituted a gradient of temporal generalization. Experiments VI, VII and VIII investigated the
effect upon behaviour of varying the probability of reinforcement at the end of the short interval. It was found that when the probability of reinforcement at the
end of the short interval was 0.5 or above, the pattern of responding was unchanged. When the probability fell below 0.5, however, it was found that there was a systematic increase in the postreinforcement pause, as well as a decrease in the local rate of responding around the end of the short interval.