EFFECTS OF FOREPERIOD DURATION ON REFLEXIVE AND VOLUNTARY RESPONSES TO INTENSE NOISE BURSTS

The question of whether a common mechanism underlies the facilitation of voluntary and reflexive reactions by a warning stimulus was investi gated in two experiments. In both studies, the foreperiod preceding an intense noise burst was manipulated within and between blocks of tria ls. Previous reaction time experiments have shown that individuals res pond fastest at the shortest foreperiod for between-block manipulation s and fastest at the longest foreperiod when foreperiod duration is va ried unpredictably from trial to trial. In the present research, this pattern was found for voluntary hand-grip responses, but acoustic star tle blinks were facilitated at long foreperiods for both within-and be tween-block manipulations. Invariance of the trisynaptic postauricular reflex across foreperiod conditions was evidence against any general activation of low-level motor pathways by warning stimuli. Analyses of nonreflexive lid movements subsequent to startle blink suggested that inhibition of spontaneous blinking during the foreperiod may have con tributed to the unexpected divergence between voluntary reactions and eyeblink reflexes.