The e¡ect of two-tone stimulation on responses of two simultaneously recorded neurons in the inferior colliculus of the big brown bat, Eptesicus fuscus Philip H.-S. Jen à , Fei Jian Wu 1 , Qi Cai Chen 1 Division of Biological Sciences, University of Missouri-Columbia, Columbia, MO 65211, USA Received 31 August 2001; accepted 6 December 2001 Abstract This study examined auditory responses of two simultaneously recorded neurons in the central nucleus of bat inferior colliculus (IC) under two-tone stimulation conditions. We specifically examined how a sound within the excitatory frequency tuning curve (FTC) of one IC neuron might affect responses of the other IC neuron in amplitude and frequency domains. Under this specific two-tone stimulation condition, responses of 82% neurons were suppressed and their excitatory FTCs sharpened. Responses of the other 18% neurons were facilitated and their excitatory FTCs broadened. Two-tone suppression was greater at low than at high stimulus amplitudes. Two-tone suppression also decreased with increasing recording depth and best frequency (BF) difference between each pair of neurons. The suppressive or facilitatory FTC of a neuron plotted under two-tone stimulation conditions was always within the excitatory FTC of the other neuron. Two-tone suppression or two-tone facilitation was weak near the BF but became increasingly strong with frequencies away from the BF. Biological significance of these findings is discussed. ß 2002 Elsevier Science B.V. All rights reserved. Key words: Facilitation; Frequency tuning curve; Inferior colliculus; Suppression; Rate^amplitude function 1. Introduction In auditory physiology, processing of auditory infor- mation carried by sounds has traditionally been ex- plained by neural interactions of divergent and conver- gent projections within the auditory system (Suga et al., 1998). The two most important neural interactions are excitation and inhibition which, respectively, increase and decrease responses of auditory neurons. For exam- ple, in the central auditory pathway, the central nucleus of the inferior colliculus (IC) receives and integrates excitatory and inhibitory inputs from many lower audi- tory nuclei (Adams, 1979; Adams and Wenthold, 1979; Adams and Mugnaini, 1984; Casseday and Covey, 1996; Covey and Casseday, 1995; Pollak and Casseday, 1989) and from the auditory cortex (Faye-Lund, 1985; Games and Winer, 1988; Herbert et al., 1991; Hu¡man and Henson, 1990; Saldan ‹a et al., 1996). These excita- tory and inhibitory inputs contribute importantly to auditory signal processing in this nucleus (Casseday and Covey, 1996; Suga et al., 1998; Jen et al., 1998; Jen and Zhang, 1999). Previous studies have shown that many IC neurons have inhibitory areas on one or both £anks of their excitatory frequency tuning curves (FTCs) or have mul- tiple inhibitory areas using a two-tone stimulation para- digm (Ehret and Merzenich, 1988; Fuzessery, 1994; Fuzessery and Hall, 1996; Lu and Jen, 2001; Suga, 1995; Vater et al., 1992). These lateral inhibitory areas sharpen excitatory FTCs (Suga, 1995). In these studies, neural inhibition of each recorded single IC neuron was examined under forward masking conditions. To further study the importance of neural interac- 0378-5955 / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII:S0378-5955(02)00369-6 * Corresponding author. Tel.: +1 (573) 882 7479; Fax: +1 (573) 884 5020. E-mail address: jenp@missouri.edu (P.H.-S. Jen). 1 Present address: Department of Biology, Central China Normal University, Wuhan, PR China. Hearing Research 168 (2002) 139^149 www.elsevier.com/locate/heares