Stimulus rise time, intensity and the elicitation of unconditioned cardiac and electrodermal responses

In recent discussions on the differentiation of orienting, startle and defense responses, the importance of stimulus rise time for the elicitation of different cardiac response patterns was re-emphasized. Especially, it has been claimed that phasic accelerative heart rate (HR) responses-interpreted as indicators of startle-might not only be evoked by auditory stimuli with instantaneous rise times and high intensities, but also by low to moderate stimulus intensities with sudden onsets. The present study examined this question by manipulating rise time (instantaneous vs. 50 ms) and intensity (60 vs. 95 dB). Subjects (N = 120) were randomly assigned to one of the four independent groups. They performed a habituation experiment in which 12 tones of 1000 Hz with a constant interstimulus interval of 90 s were presented. On trial 13, a change in rise time was introduced by reversing the rise time condition in each group (i.e., from instantaneous to 50 ms and vice versa). Examination of HR changes across 4 poststimulus seconds, of maximal HR deceleration and acceleration, as well as examination of magnitude of skin conductance responses (SCRs) indicated clear intensity effects. Rise time, on the other hand, had no effects on HR and SC that could be interpreted as indicative of startle. Thus, it could neither be observed that instantaneous rise time led to stronger accelerative HR responses nor that the combination of instantaneous rise time and high intensity elicited anything but a dominant accelerative response pattern. These findings were also obtained when the first four trials were analyzed separately. As a consequence of stimulus change, larger SCRs as well as larger decelerative HR responses were observed without, however, being affected by the direction of the change in rise time. In sum, the present study suggests that the role attributed to rise time with respect to eliciting qualitatively different cardiac response patterns has been exaggerated. The consequences for the differentiation of different unconditioned responses are briefly addressed.