Table 1.

Extinction ITI parameters.

	ITI average (s)	Min ITI (s)	Max ITI (s)	SD	SD: mean ITI
Experiment 1
Not variable (fixed)	60	60	60	0	0
Variable	60	20	100	30	0.5
Experiment 2
Not variable (fixed)	120	120	120	0	0
“Small” variability	120	80	150	15	0.125
“Large” variability	120	20	240	60	0.5

In Experiment 1, the average inter-trial interval (ITI) was 60 s. In one extinction condition, the ITI did not vary, thus the minimum (min) ITI and maximum (max) ITI were 60 s, and the standard deviation (SD) and SD: average ITI ratio was 0. In the other extinction condition, the ITIs were randomized and thus had a non-zero SD and SD: mean ITI ratio. In Experiment 2, the average ITI was 120 s. One extinction condition had a non-variable ITI, similar to Experiment 1. The range of ITIs in the “small” variability extinction condition was similar to that of the variable condition in Experiment 1. The SD: mean ITI ratio for “large” variability condition was the same as the variable condition from Experiment 1.