Research Report Interaural delay-dependent changes in the binaural interaction component of the guinea pig brainstem responses B Cuneyt Goksoy a , Serdar Demirtas a , Suha Yagcioglu b , Pekcan Ungan b, * a Gulhane Military Medical Academy, Faculty of Medicine, Department of Biophysics, Ankara, Turkey b Department of Biophysics, Faculty of Medicine, Hacettepe University, 06100 Ankara, Turkey Accepted 28 June 2005 Available online 28 July 2005 Abstract Auditory brainstem responses to monaural and binaural clicks with 23 different interaural time differences (ITDs) were recorded from ten guinea pigs without anesthesia. Binaural interaction component was obtained by subtracting the sum of the appropriately time-shifted left and right monaural responses from the binaural one. With increasing ITD, the most prominent peak of the binaural difference potential so obtained shifted to longer latencies and its amplitude gradually decreased. The way these changes depended on binaural delay was basically similar to that previously observed in a cat study [P. Ungan, S. Yagcioglu, B. Ozmen. Interaural delay-dependent changes in the binaural difference potential in cat auditory brainstem response: implications about the origin of the binaural interaction component. Hear. Res. 106 (1997) 66-82]. The data were successfully simulated by the model suggested in that report. We therefore concluded that the same model, which was based on the difference between the mean onset latencies of the ipsilateral excitation and contralateral inhibition in a typical neuron in the lateral superior olive, their standard deviations, and the duration of the contralateral inhibition, should also be valid for the binaural interaction in the guinea pig brainstem. The results, which were discussed in connection with sound lateralization models, supported a model based on population coding, where the lateral position of a sound source is coded by the ratio of the discharge intensity in the left and right lateral superior olives, rather than the models based on coincidence detection. D 2005 Elsevier B.V. All rights reserved. Theme: Sensory systems Topic: Auditory system (central physiology, localization) Keywords: Sound lateralization; Interaural time difference; Auditory brainstem response; Binaural interaction; Binaural difference potential 1. Introduction It is generally accepted that the neural impulses coming from the two ears first meet and interact with each other in two nuclei of the superior olivary complex (SOC) of the brainstem, namely, the lateral and medial superior olives (LSO and MSO), and that interaural intensity and time differences (IID and ITD) are encoded, respectively, in these nuclei. The bipolar neurons in MSO are mostly of excitatory/excitatory (EE) type (i.e., receive excitatory inputs from both sides), whereas those in LSO receive inhibitory inputs of contralateral origin via the medial nucleus of trapezoid body and display inhibitory/excitatory (IE) behavior [16,18,31,32]. According to the ‘‘delay line-coincidence detector’’ model, which was proposed by Jeffress [14] and later supported by various histological and electrophysiological studies [10,43,44], the EE-neurons in MSO accomplish sound lateralization by using interaural time differences (ITD). It was because of this model that the difference potentials, which have been used to demonstrate the binaural interaction (BI) effects in auditory brainstem responses (ABRs), were computed 0006-8993/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.brainres.2005.06.083 i Parts of this work were presented in the Symposium ‘‘Object formation in audition and vision: Bottom – up and top – down processing’’, 19 – 22 August 2004, Bad Zwischenahn, Germany. * Corresponding author. Fax: +90 312 3051492. E-mail address: pekcan@hacettepe.edu.tr (P. Ungan). Brain Research 1054 (2005) 183 – 191 www.elsevier.com/locate/brainres