Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)




Professor H.M. Jenkins


The central question of this thesis is how the temporal distribution of stimulus-reinforcer pairings affects an animal's tendency to regard the stimulus as a signal. The effect of trial (stimulus-reinforcer) spacing was studied in a situation where a brief key light is paired with food presentation in the absence of any response contingency (autoshaping). As the intertrial interval (ITI) decreases in duration from 290 to 20 seconds, the number of trials required to initiate pecking in naive, hungry pigeons reliably increases.

Several theoretical accounts argue that acquisition is slower with short ITI's because each trial is subject to interference from the events immediately preceding or following that trial. The possibility of such local interference was examined by placing stimuli or reinforcers directly before or after each of a series of widely spaced trials. Although acquisition was reduced, it was not reduced to the level obtained when all trials were closely spaced. Further, the proximity of the added stimuli or reinforcers to the trial was found to have no effect on acquisition. These results indicate that the spacing effect cannot be entirely explained by interference from stimuli or reinforcers in close temporal proximity to trials.

Since the immediate context of trial presentation is inadequate to explain the entire spacing effect, subsequent analysis focused on the overall pattern of trial spacing. The separation of trials by long, event-free intervals or waiting periods, seems to be essential to rapid peck acquisition. On logical and theoretical grounds, three characteristics of waiting periods are expected to be important to effective waiting periods: (1) the number of waits and their position in relation to trials, (2) the boundary events which define waits, and (3) the stimulus context in which waits occur. Experimental manipulations of the number and position of waits showed that peck acquisition was equally rapid whether trials were clustered in closely spaced blocks or uniformly distributed across a long experimental session; however, a single long wait was not sufficient to produce rapid peck acquisition in a subsequent series of closely spaced trials. The boundary events which define effective waits were explored in an experiment where only one trial occurred in each autoshaping session. Peck acquisition was rapid when the single trial occurred in the middle of a long series of widely spaced reinforcer presentations, but slow when the trial occurred in the middle of closely spaced reinforcer presentations. Finally, the importance of stimulus context was experimentally tested by arranging for the presence or absence of long waits in the stimulus situation which constituted the background for trial presentation. Pecking to the key light developed rapidly only in animals which experienced long waits in the stimulus situation present during autoshaping trials.

The preceding results indicate that the development of a signalling stimulus is affected to a considerable extent by the conditions of waiting periods. Rapid peck acquisition requires an alternation between trials or reinforcer presentations and waiting periods in the stimulus situation. Although other accounts are not excluded, the present results are consistent with theoretical interpretations which argue that the spacing effect results from differential opportunity for the extinction of competing associations. Long intertrial or inter-reinforcer intervals allow considerable time for the extinction of stimulus - or response-reinforcer associations which would interfere with the development of pecking to the key-light stimulus. In contrast, short intertrial or inter-reinforcer intervals minimize the time available for extinction of competing associations, and consequently, response acquisition is slow. Alternatively, the present results may be explained by a theoretical account which holds that the development of a signalling stimulus is directly related to the duration of reinforcer-reinforcer intervals.

Included in

Psychology Commons