Electronic Thesis and Dissertation Repository


Doctor of Philosophy




Dr. William A. Roberts


Timing has been widely studied in humans and animals across a variety of different timescales. The concept of time as a stimulus dimension, and how it is processed relative to other stimulus dimensions, has only recently been scrutinized. In the current work I present a review of interval timing as it relates to stimulus control, and discuss the role of attention in timing in the context of three sets of studies in pigeons.

In the first set of studies, I analyzed whether the presence of a non-reinforced timed stimulus would disrupt timing of a stimulus reinforced on a fixed-interval schedule. In Experiment 1, half of the pigeons were trained on a 60-s fixed interval schedule of reinforcement signaled by onset of a sidekey; the other half of the birds had those same reinforced trials interspersed among trials in which the onset of a different sidekey signaled 60-s followed by non-reinforcement. Groups were reversed in the second phase of experimentation. Obtained peak-time curves showed flattened responding to the reinforced stimulus for birds which also received non-reinforced trials, suggesting that control by interval timing was overshadowed by the presence of a food/no food cue. Experiment 2 ruled out the possibility that this effect was caused by differences in reinforcement. Pigeons’ responding on this task was not controlled by timing because the visual discrimination based on food vs. no food was more salient than the temporal discrimination.

In the second set of studies, I examined the ability of pigeons to track the identity of multiple stimuli presented in order across a temporal interval terminating in reinforcement. In Experiment 1A, pigeons responded to the final stimulus in a three-item sequence regardless of the preceding order of stimuli, or even if previous stimuli had not been presented, suggesting that the birds attended only to the reinforced stimulus and not to the order of stimuli. In Experiment 1B, pigeons were presented with baseline non-reinforced trials in which the order of the first two stimuli was reversed, and results showed that they responded differently to the third stimulus based on the order of stimuli. Experiment 2 extended these results with a five-stimulus sequence. Though birds showed only a weak appreciation of order, they nonetheless responded differentially based on temporal order.

In the final study, I observed the tendency of pigeons to anticipate or perseverate after a mid-session reversal of response contingencies. The birds tended to make errors around the reversal point when the discrimination was a visually-based (red vs. green) task, and these errors were conclusively shown to be due to interval timing from the start of the session. However, when presented with a visual-spatial version of the same task, pigeons no longer made timing-induced errors and instead used a reinforcement-maximizing approach. The dimension of discrimination affected the strength of memory for the response and outcome of the previous trial, and in turn affected the tendency of birds to base their responding on an error-prone interval timing strategy.