Biological Psychology 56 (2001) 83 – 111 Prepulse effects as a function of cortical projection system William M. Perlstein a, *, Robert F. Simons b , Frances K. Graham b a Departments of Clinical and Health Psychology and Psychiatry and McKnight Brain Institute, Uniersity of Florida, HSC, P.O. Box 100165, Gainesille, FL 32610, USA b Department of Psychology, Uniersity of Delaware, Newark, DE 19716, USA Received 5 August 2000; received in revised form 20 December 2000; accepted 24 January 2001 Abstract A putative gating mechanism reduces startle blink, midline scalp potentials beginning with P50, and perceived loudness of startling stimuli. Tactile prestimuli were paired with auditory startle stimuli to determine if: (1) P50 inhibition is due to an extrinsic mechanism, (2) pairing differentially affects potentials reflecting modality specific and nonspecific system activity, and (3) crossmodal pairing modifies perceptual magnitudes of both pair members. Stimuli were presented alone and in pairs separated by 60 or 360 ms. Event-related potentials (ERPs) were recorded from midline and lateral sites; EMG was recorded from several facial and scalp muscles. Pairing reduced blink, and midline P50, N100 and P200 amplitudes; reduc- tions were greater at the longer interval. P30 was largely unaffected by pairing. Pairing also differentially affected lateral N100 components reflecting later activity in specific and nonspecific systems. Results show that prestimulus inhibition of ERPs is not due to intrinsic refractoriness and that pairing differentially affects ERPs associated with modality specific and nonspecific projection systems. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Prepulse inhibition; Startle; P30; P50; N100; Auditory event-related potentials; Magnitude estimation www.elsevier.com/locate/biopsycho * Corresponding author. E-mail address: wmp@grove.ufl.edu (W.M. Perlstein). 0301-0511/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0301-0511(01)00075-8