Reaction time to the second of two shortly spaced auditory signals both varying in intensity

Six subjects participated in a reaction time (RT) task in which the stimulus events consisted of two sinusoidal waves presented in close succession. The leading stimulus (S₁) had a duration of 25 msec while the binary reaction stimulus (S₂) was 250 msec. Presentation of the interstimulus interval (ISI) was irregular. S₁ as well as S₂ varied in intensity and were assigned randomly the values of 45, 68, 85 and 105 db (SPL). RT to S₂ was examined as a function of the intensity of both stimuli. Generally RT varied inversely with the intensity of S₁ and the downward gradient of RT with ISI became steepened at least for ISIs shorter than 200–250 msec. Concerning S₂, RT grew shorter as the intensity increased up to 85 db. Probably due to startle, RT tended to lengthen at S₂ = 105 db. When both S₁ and S₂ were 105 db the startle effect upon RT seemed most evident. Then the elevation in RT took place at especially those ISIs for which he fastest RTs might have been expected (about 200 msec). For the remaining intensities no interaction between S₁ and S₂ upon RT was found even for the shortest ISIs, implying that the effects of S₁ and S₂ intensities upon RT were independent.