Monaural middle ear destruction in juvenile and adult mice: e¡ects on responses to sound direction in the inferior colliculus ipsilateral to the intact ear Philip H.-S. Jen à , Lijing Xu 1 Division of Biological Sciences, University of Missouri-Columbia, Columbia, MO 65211, USA Received 22 July 2002; accepted 11 September 2002 Abstract This study examined the effect of monaural middle ear destruction on auditory responses to sound direction in the inferior colliculus (IC) of the laboratory mice, Mus musculus. Monaural middle ear destruction was performed on juvenile and adult mice (the experimental mice). Auditory response properties of neurons to ipsilateral and contralateral sounds (I-40‡ and C-40‡) were examined in the IC ipsilateral to the intact ear 4 weeks later. IC neurons of control mice had higher minimum thresholds (MTs), larger Q n (Q 10 , Q 30 ) values but smaller dynamic ranges at I-40‡ than at C-40‡. These direction-dependent response properties were not observed for IC neurons of experimental juvenile and adult mice. However, Q n values of IC neurons were significantly smaller in experimental juvenile than in control and experimental adult mice. Normal tonotopic organization in terms of positive correlation between recording depth and best frequency (BF) was observed in the IC of control and experimental adult mice at both sound directions but not in the IC of experimental juvenile mice. A positive correlation of increasing MT with BF was only observed for IC neurons in control mice but not in both experimental mice. Possible mechanisms for these different response properties are discussed. ß 2002 Elsevier Science B.V. All rights reserved. Key words: Middle ear destruction; Inferior colliculus; Q n value; Sound direction; Tonotopic organization 1. Introduction Previous studies have shown that abnormal auditory stimulation during early postnatal development can be manifested through anatomical and physiological changes that occur in the central auditory system. For example, cochlear ablation leads to signi¢cant reduction in the number and soma size of neurons in the cochlear nucleus (Blatchly et al., 1983; Coleman and O’Connor, 1979; Hashisaki and Rubel, 1989; Moore and Kowal- chuk, 1988; Powell and Erulkar, 1962; Trune, 1982; Webster and Webster, 1977, 1983), the trapezoid body (Pasic et al., 1994; Moore, 1992), the lateral and medial superior olivary nuclei (Feng and Rogowski, 1980; Sanes and Takacs, 1993), the nucleus of the lateral lemniscus (Powell and Erulkar, 1962) and the inferior colliculus (IC) (Hardie et al., 1998; Hardie and Shep- herd, 1999; Moore and Kitzes, 1985; Moore and Ko- walchuk, 1988; Nishiyama et al., 2000). Electrophysio- logical studies have shown that early monaural plugging, blocking or occlusion changes the auditory spatial sensitivity of neurons in bats, owls, and guinea pigs (Clements and Kelly, 1978; Jen and Sun, 1990; Knudsen et al., 1984). This monaural auditory depriva- tion also produces substantial loss of binaural interac- tion of auditory neurons in rats and cats because of reduction in the strength of inhibition from the ipsilat- eral intact ear (Clopton and Silverman, 1977, 1978; 0378-5955 / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII:S0378-5955(02)00699-8 * 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 Mathematics, University of Memphis, Memphis, TN 38152, USA. Abbreviations: BF, best frequency; DR, dynamic range; FTC, frequency tuning curve; IC, inferior colliculus; MT, minimum threshold; C-40‡ or I-40‡, contralateral 40‡ or ipsilateral 40‡ Hearing Research 174 (2002) 249^259 www.elsevier.com/locate/heares