The auditory evoked response was measured between the vertex and left ear in 9 Ss. In an extension of the idea of the recovery function, series of tones, including sets of 5 tone pips with ½, 1, or 2 sec between them, and 11 sec between sets, were presented, while Ss were instructed to press a button whenever there was a change in tone length. Because of the slow recovery of this response, no subtraction procedure was necessary. There were 6 runs representing 6 experimental conditions for each S.
Habituation within runs was not observed, but there was significant habituation between rims in spite of a constant vigilance level. The evoked responses to the second and subsequent stimuli of each set were much smaller than to the first, depending on the interstimulus interval within sets. Evoked responses to 65 db stimuli were less than to 85 db, but showed proportionally the same changes with successive stimuli.