International Journal of Comparative Psychology

Humans and rats can discriminate different fixed intervals (FIs) that are signaled by different stimuli. With only a few pairings of stimuli with intervals, temporal performance becomes a function of the stimuli, with responding increasing earlier for stimuli that signal shorter FIs compared to stimuli that signal longer FIs. As predicted by timing and conditioning models, the amount of training with the different stimuli and intervals determines the development of such stimulus control. This study reviews some earlier work from our group suggesting that the amount of training is necessary, but not sufficient, to account for the development of stimulus-controlled performance. Moreover, it describes an experiment in which participants were trained in a computerized shooting task with three FIs (target speeds) signaled by three stimuli (different background colors). In the first phase, the number of trials trained with each FI was held constant (60 trials each) across five experimental groups, but the order in which these trials were trained differed between groups, from a randomly determined FI in each trial (intermixed) to three consecutive blocks of 60 trials each (blocked). Intermediate groups had blocks of 10, 20, and 30 consecutive trials of each FI. Results showed that, although the amount of training was held constant across groups, the longer the training block the fewer the participants who demonstrated stimulus-controlled performance. In Phase 2, another 60 trials of each FI were trained, but intermixed for all groups. Results showed stimulus-controlled performance for all participants. These results represent another instance in which the amount of training is necessary, but not sufficient, for the development of stimulus control in temporal discriminations, and describe the effect of the number of consecutive trials within a block of training on temporal discriminations.