BRIEF REPORT Impact of stimulus signal-to-noise ratio on prepulse inhibition of acoustic startle JOSEPH C. FRANKLIN, NICOLE A. MORETTI, and TERRY D. BLUMENTHAL Department of Psychology, Wake Forest University, Winston-Salem, North Carolina, USA Abstract Prepulse inhibition (PPI) of the human acoustic startle response is reduced in the presence of background noise of a sufficient intensity, possibly due to a reduction in signal-to-noise ratio (prepulse intensity relative to background noise). We examined this hypothesis by varying prepulse intensity and background noise intensity in order to hold three different signal-to-noise ratios constant (5, 15, and 25 dB(A) above background noise intensity). The results showed that signal-to-noise ratio proved to be a more important factor than absolute stimulus intensity in determining the degree of PPI of startle eyeblink response magnitude. Therefore, the effectiveness of a prepulse is determined by prepulse salience, not intensity, and this effectiveness is equivalent across a range of physical intensities. Descriptors: Startle, Eyeblink, Prepulse, PPI, Stimulus intensity, Signal-to-noise ratio The startle response is a defensive reflex that occurs in reaction to a sufficiently sudden and intense stimulus. In humans, the startle response is most often quantified with electromyography (EMG) of an eyeblink response to an acoustic stimulus (Blumenthal et al., 2005; Yeomans, Li, Scott, & Frankland, 2002). Prepulse inhibition (PPI) of the human acoustic startle response occurs when a stimulus, or prepulse, is presented 15–500 ms before a startle-eliciting stimulus (Blumenthal, 1999; Graham, 1975). PPI may function to protect the processing of the prepulse from interruption by the startle-eliciting stimulus (Graham, 1975). More intense prepulses generally result in greater PPI (Blumenthal, 1995; Graham & Murray, 1977), and higher back- ground noise intensities result in less PPI (Blumenthal, Noto, Fox, & Franklin, 2006; Flaten, Nordmark, & Elden, 2005; Hsieh, Swerdlow, & Braff, 2006). Background noise is noise that is constant throughout a testing session, and studies have used background noise levels as low as 28 dB(A) (ambient), although the majority of PPI studies use background noise levels between 60 and 75 dB(A) (Blumenthal et al., 2006). The primary reason for a decrease in PPI as background noise increases or prepulse intensity decreases may be due to a decreased signal-to-noise ratio between prepulse intensity and background noise intensity. Presumably, a decreased signal-to-noise ratio interferes with the processing of the prepulse, and, therefore, activation of the in- hibitory mechanism is reduced. This is caused by an increase in the peripheral masking of the prepulse by the background noise. Previous studies have investigated the effect of signal-to-noise ratio by varying either background noise intensity or prepulse in- tensity. However, the three parameters of background noise inten- sity, prepulse intensity, and signal-to-noise ratio have only two degrees of freedom, such that changing any two parameters deter- mines the third. Although previous literature has attributed effects of background noise intensity to a change in signal-to-noise ratio, it is possible that the same signal-to-noise ratio may have varying effects at different points on the physical intensity spectrum. To address this possibility in the present study, we held signal-to-noise ratio constant across a range of prepulse and background noise intensities. We used background noise intensities of 50, 60, and 70 dB(A) as the between- groups variable and prepulse intensities that were 5, 15, and 25 dB(A) above each background intensity as the within-participants variable. If PPI is determined by signal-to-noise ratio, then varying prepulse intensity and background noise level together to achieve a range of signal-to-noise ratios should result in variations in PPI. If PPI is determined by stimulus intensity, then variations in prepulse intensity or background noise while holding signal-to-noise ratio constant should affect PPI, independent of signal-to-noise ratio. The findings of this study will allow a more complete under- standing of the impact of stimulus parameters on PPI, which will facilitate comparisons across startle modification studies that use different stimulus values. This should increase our ability to make sense of data obtained in a variety of research settings and from various participant populations. Methods Participants Participants (N 5 55, 40 men, 15 women, 18–22 years of age) were selected from a group of students earning credit for a Portions of this research were supported by a grant from the Wake Forest University Social Science Research Fund. Address reprint requests to: Terry D. Blumenthal, Department of Psychology, Wake Forest University, Winston-Salem, NC 27109, USA. E-mail: blumen@wfu.edu. Psychophysiology, 44 (2007), 339–342. Blackwell Publishing Inc. Printed in the USA. Copyright r 2007 Society for Psychophysiological Research DOI: 10.1111/j.1469-8986.2007.00498.x 339