Table 1. Percentage of trials with persistence in Experiment 1.

Session of stable period	Pigeon
	361	362	363	364
Stimulus changes presenta
1	0	0	0	0
2	0	0	0	0
3	0	0	0	0
4	0	0	0	0
5	0	0	0	0
Stimulus changes absent
1	3	100	97	100
2	4	100	100	100
3	1	100	100	100
4	1	100	100	100
5	2	100	100	100

Note. Persistence in a trial is defined as completion of that trial (excluding FR 10 trials).

^a The data for the first five to eight sessions of this condition were lost because of a computer malfunction. The data shown here are from the first five sessions for which data are available. For this reason it is unknown for each pigeon whether “Session 1” is the first session of the stable period or is instead subsequent to the first session of the stable period.