Two experiments were conducted demonstrating that under certain conditions pigeons may peck at a higher rate on a key that produces intermittent reinforcement following a delay than on one that always produces reinforcement following the same delay duration. In both experiments, concurrent chain schedules were employed. In Experiment I, a single peck on one key led to a white light and a delay of 15 sec, which always terminated with food. A peck on the other key led to its illumination by one of two colored lights and a delay period of 15 sec. The delay was followed by either food presentation or timeout, either one lasting 3 sec. In a control group, the lights on this key were not correlated with food or timeout. Under the correlated stimuli, birds more often pecked the key leading to intermittent reinforcement, whereas with uncorrelated stimuli they pecked the key leading to the white light and 100% reinforcement. In Experiment II, concurrent variable-interval schedules were employed in the first link. The results showed generally that the relative rate was higher on the key leading to intermittent reinforceimient when the stimuli were correlated with reinforcement and timeout than on the key leading to 100% reinforcement. There was some indication that this performance was affected by (1) the duration of the delay, (2) the percentage of reinforcement on the key yielding the higher percentage of reinforcement (the key with the white light), and (3) prior experimental conditions. Using a modified chain schedule procedure, Wilton and Clements (197 la) correlated with only one of the two stimuli in the terminal link; the other stimulus was followed by a short timeout. In another condition, food was delivered at the end of half of the terminal links, but neither stimulus was reliably correlated with its delivery. In the third condition, each terminal link, regardless of the stimulus, terminated with food delivery. The response rate in the initial component was highest in the first of these conditions, without much difference between the last two conditions. A study by Kendall (1972) replicated these findings with terminal links of two different durations and reinforcement in each. Response rates were higher in the initial component followed by one long and one short terminal component than in the component followed by two short terminal components. This effect disappeared, however, when the two stimuli for the terminal components were no longer reliably correlated with the duration of the terminal components. Wilton and Clements (1971a) and Wilton (1972) interpreted the findings of the Wilton and Clements experiments in terms of an information hypothesis.

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Two experiments studied the effects of brief response-dependent clock stimuli in fixedinterval schedules of reinforcement. In the first experiment, two pigeons were exposed to a fixed-interval schedule. Two conditions were compared. In both conditions each peck on the key produced a brief stimulus. In one condition, pecks produced a different stimulus in successive sixths of the interval. This was the clock condition. In the other condition, the same stimulus was produced throughout the interval. Response riates were lower and the pause after reinforcement was longer in the clock condition. In the second experiment, a two-key optional clock procedure was used. Responding on the clock key produced one of three stimuli correlated with the three successive minutes of a fixedinterval schedule. A response on the other key produced grain at the end of the 3 min. When the final stimulus was removed from the situation and pecking produced nothing during the third minute, responding to the clock key declined to a very low rate. When the first two stimuli were removed and the third one replaced, responding to the clock key was resumed.

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The phenomenon of sensory preconditioning was presented by Brogden (1939) with the implicit assumption that its quantitative variation and the independent variables to which it is functionally related are similar to those of conditioning. Brogden (1950) reported briefly about two experiments on sensory preconditioning in which variation in the number of preconditioning training trials appeared to have no differential effect upon the magnitude of the phenomenon. Because of the fundamental relation of amount of training to amount of conditioning, it was assumed that there were errors of some kind in the experiments, rather than that sensory preconditioning differs in respect to this relationship from standard conditioning. Hoffeld, Thompson, and Brogden (1958) studied the stimuli time relations during preconditioning training for conditions of 0, .5, 1, 2, and 4 sec, precedence of tone over light and found maximum sensory preconditioning to tone for 4 sec. precedence. Since these results are not compatible with the results on time relations