Table 1.

Properties of each stimulus type. There is no programmed consequence of lever pressing with stimulus type 1. Auditory stimulus remained constant for the duration of the stimulus presentation. Frequency of light flashing was randomly chosen from four or six different values in stimulus types 4 or 5, respectively. Duration of compound stimuli was also randomly chosen from 4 or 6 different values in stimulus types 4 or 5, respectively.

Stimulus Type	Audio Stimuli	Visual Stimuli	Frequencies (Hz)	Durations (sec)	N
1	No	None	None	None	8
2	No	1 second on	None	None	6
3	Yes	Flashing	1.25	2	9
4	Yes	Flashing	0.625, 1.25, 2.5, or 5	2, 4, 6, or 8	8
5	Yes	Flashing	0.313, 0.625, 1.25, 2.5, 5, 10	1, 2, 4, 6, 8 or 12	9