Hearing Research 203 (2005) 201–215 www.elsevier.com/locate/heares 0378-5955/$ - see front matter  2005 Elsevier B.V. All rights reserved. doi:10.1016/j.heares.2004.12.008 Corticofugal modulation of directional sensitivity in the midbrain of the big brown bat, Eptesicus fuscus Xiaoming Zhou, Philip H.-S. Jen ¤ Division of Biological Sciences and Interdisciplinary Neuroscience Program, University of Missouri, 208 Lefevre Hall, Columbia, MO 65211, USA Received 3 November 2004; accepted 22 December 2004 Available online 21 January 2005 Abstract In our recent study of corticofugal modulation of collicular amplitude sensitivity of the big brown bat, Eptesicus fuscus, we sug- gested that the corticofugal modulation is based upon the best frequency (BF) diVerences and the relative amplitude sensitivity diVer- ence between collicular (IC) and cortical (AC) neurons but not the absolute amplitude sensitivity of IC and AC neurons. To show that corticofugal modulation is systematic and multiparametric, we studied corticofugal modulation of directional sensitivity in 89 corticofugally inhibited IC neurons in the same bat species under free Weld stimulation conditions. A neuron’s directional sensitivity was expressed with the azimuthal range (AR) at 50% below the maximum of each directional sensitivity curve and the best azimuth (BAZ) at which the neuron discharged maximally. Cortical electrical stimulation did not aVect the directional sensitivity of 40 (45%) neurons with BF IC–AC diVerences of 7.3 § 4.4 kHz but sharpened the directional sensitivity of other 49 (55%) neurons with BF IC–AC diVerences of 2.3 § 1.8 kHz. Corticofugal modulation sharpened directional sensitivity curves of IC neurons by decreasing the AR and shifting collicular BAZ toward cortical BAZ. The decrease in AR and the shift in BAZ increased signiWcantly with AR IC–AC and BAZ IC–AC diVerences but not with absolute AR and BAZ of IC and AC neurons or BF IC–AC diVerences. Corticofual modulation also shifted collicular BF toward cortical BF. The shift in BF increased signiWcantly with BF IC–AC diVerences but not with the BF of IC and AC neurons or BAZ shift. Consonant with our previous study, these data indicate that corticofugal modulation of collicular directional sensitivity is based on topographic projections between the IC and the AC and the diVerence in directional sensitivity but not the absolute directional sensitivity of IC or AC neurons.  2005 Elsevier B.V. All rights reserved. Keywords: Auditory cortex; Bat; Corticofugal modulation; Directional sensitivity; Frequency; Inferior colliculus; Inhibition 1. Introduction The fact that sound direction aVects the responses of auditory neurons has been demonstrated in many stud- ies. For example, using the bat as the mammalian model system, numerous studies have shown that most neurons in the central nucleus of the inferior colliculus (abbrevi- ated as IC neurons) increase threshold and decrease the number of impulses when a sound azimuth changes from 40° contralateral (c40°) to 40° ipsilateral (i40°) within the frontal auditory space (Fuzessery and Pollak, 1985; Grinnell, 1963; Grinnell and Griennell, 1965; Grothe et al., 1996; Jen and Sun, 1984; Jen and Wu, 1993; Jen et al., 1987, 1989; Makous and O’Neill, 1986; Poon et al., 1990; Schlegel, 1977; Schlegel et al., 1988; Shimozawa et al., 1984; Sun and Jen, 1987; Wu and Jen, 1996; Zhou and Jen, 2000c, 2002, 2004). These studies also showed Abbreviations: AC, auditory cortex; AR, azimuthal range; BA, best amplitude; BAZ, best azimuth; BF, best frequency; IC, inferior collicu- lus; MT, minimum threshold; AR IC–AC , BAZ IC–AC , and BF IC–AC , diVer- ences in AR, BAZ, and BF between IC and AC neurons; pps, pulses per second; PST, peri-stimulus time; SPL, sound pressure level ¤ Corresponding author. Tel.: +1 573 882 7479; fax: +1 573 884 5020. E-mail address: jenp@missouri.edu (P.H.-S. Jen).