Increasing the variability of response sequences in pigeons by adjusting the frequency of switching between two keys.

Three experiments compared the amounts of behavioral variability generated with two reinforcement rules. In Experiments 1 and 2 pigeons received food whenever they generated a sequence of eight pecks, distributed over two keys, provided that the sequence contained a certain number of change-overs between the keys. Although no variability was required-the birds could obtain all reinforcers by repeating the same sequence-the pigeons emitted a large number of different sequences. In Experiment 3 pigeons received food whenever they generated a sequence that had not occurred during the last 25 trials. After prolonged training, the birds showed more sequence variability than in the first two experiments. The analysis of the internal structure of the response sequences revealed that, in general, (a) the location of the first peck was highly stereotyped; (b) as the trial advanced, the probability of switching to the initially preferred key decreased whereas the probability of switching to the other key increased; and (c) a first-order Markov chain model with transition probabilities given by a logistic function accounted well for the internal structure of the birds' response sequences. These findings suggest that, to a large extent, the variability of response sequences is an indirect effect of adjustments in changeover frequency.