The present studies investigated the relationship between prepulse effects on the modification of the brainstem startle reflex and magnitude estimates of startle-eliciting stimuli. In Experiment 1,startle eyeblink responses were elicited in 24 students, half of whom were instructed to estimate the loudness of the startle stimulus (actual intensities of 80,90, and 100dB) and half of whom were instructed to estimate the magnitude of their eyeblink. When weak acoustic prepulses preceded the startle-eliciting stimulus, eyeblink amplitude was inhibited, and estimates of response magnitude decreased, but estimates of startle stimulus magnitude decreased only when 100-dB startle stimuli were presented. In Experiment 2, the same startle stimuli were preceded on some trials by a vibrotactile prepulse to the hand. In conditions in which startle amplitude was inhibited, startle stimulus magnitude estimates were not affected. This suggests that the effect of acoustic prepulses on 100-dB startle stimuli in Experiment 1 may have been due to loudness assimilation, an effect independent of the prepulse inhibition of startle responding.The purpose of the present studies was to evaluate the effects of a prepulse on both the startle blink reflex and the estimation of the magnitude of either the startle-eliciting stimulus or the blink response itself. The startle response is a brainstem reflex to a sudden stimulus, such as a sudden sound, a flash of light, a tap to the forehead, a puff of air to the side of the face, or an electrical pulse to the forehead (Anthony, 1985). This response is very sensitive to small variations in the eliciting stimulus, and startle provides an excellent measure of sensory processing and motor excitation (Britt & Blumenthal, 1993;Graham, 1975). The startle response can be inhibited by the presentation of a low-intensity stimulus, called a prepulse, between 30 and 500 msec before startle stimulus onset (Graham, 1975). This startle response inhibition can be used to assess the early stages of prepulse processing. Graham (1979) has stated that inhibition of the startle response may be due to the activation of a mechanism that protects the preattentive processing of the prepulse and partially prevents this processing from being interrupted by the startle stimulus. Evidence for this protection has recently been reported (Norris & Blumenthal, 1995). Yamada (1983) showed that both the amplitude of the eyeblink (electromyography [EMG] of orbicularis oculi) and estimations of the magnitude of the blink-eliciting sounds increased as stimulus intensity increased from 80The authors would like to thank E. Krauter for his assistance in the planning of these studies. Portions of this research were presented at the Psychonomic Society meeting, St. Louis, Missouri, November 1994. Correspondence should be addressed to T. D. Blumenthal, Department of Psychology, Wake Forest University, Box 7778, Reynolda Station, Winston-Salem, NC 27109 (e-mail: blumen@wfu.edu).73 to 120 dB. Blumenthal (1993) presented startle-eliciting s...

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