Do auditory responses recorded from awake animals re£ect the anatomical parcellation of the auditory thalamus? J.-M. Edeline a ; *, Y. Manunta a , F.R. Nodal b , V.M. Bajo b a Laboratoire de Neurobiologie de l'Apprentissage et de la Me ¨moire, CNRS URA 1491, Universite ¨ Paris-Sud, 91405 Orsay, France b Laboratorio de Neurobiolog| ¨a de la Audicio ¨n, Instituto de Neurociencias de Castilla y Leo ¨n, Universidad de Salamanca, C/Alfonso X el Sabio, s/n, Campus `Miguel de Unamuno', 37007 Salamanca, Spain Received 11 August 1998; received in revised form 23 January 1999; accepted 6 February 1999 Abstract Previous studies performed in anesthetized animals have shown differences between the acoustic responses of neurons recorded from the different divisions of the medial geniculate body (MGB). This study aimed at determining whether or not such differences are also expressed when neurons are recorded from awake animals. The auditory responses of 130 neurons of the auditory thalamus were determined in awake, restrained guinea pigs while the state of vigilance of the animals was continuously monitored. There were significantly more `on' phasic evoked responses and significantly fewer `non-responsive' or `labile' cells in the ventral division of the MGB (MGv) than in the other divisions. The response latencies and the variability of the latencies were smaller in the MGv than in the other divisions. The tuning of the neurons obtained from MGv and from the lateral part of the posterior complex were significantly sharper than those coming from the dorsal division of the MGB and the medial division. The mean threshold and the percentage of monotonic vs. non-monotonic intensity functions were not different in the subdivisions of the auditory thalamus. When compared with previous studies, the quantifications of the acoustic responses obtained in the present study gave values that differed from those reported under deep anesthesia, but were close to those reported under light anesthesia. Lastly, even if none of the physiological characteristic makes it possible, by itself, to determine the locus of recordings in the auditory thalamus, we conclude that the physiological characteristics of the evoked responses obtained in MGv differ from those of other divisions. ß 1999 Elsevier Science B.V. All rights reserved. Key words: Guinea pig; Medial geniculate body; Anatomical parcellation; Latency; Frequency tuning 1. Introduction The tripartite parcellation of the principal auditory nucleus of the thalamus, the medial geniculate body (MGB), was initially de¢ned based upon cytoarchitec- ture and cellular morphology (Morest, 1964, 1965; Ra- mo ¨n y Cajal, 1911). A large number of subsequent stud- ies have con¢rmed these initial ¢ndings in many species [cat: Morest, 1964, 1965; Morest and Winer, 1986; Winer, 1985; opossum: Morest and Winer, 1986; tree shrew: Oliver and Hall, 1978; rat: Clerici and Cole- man, 1990; Clerici et al., 1990; LeDoux et al., 1987; mustache bat: Winer and Wenstrup, 1994a,b ; rabbit: Caballero-Bleda et al., 1991; De Venecia et al., 1995; guinea pig: Strutz, 1987; monkey: Burton and Jones, 1976; Hashikawa et al., 1995; Molinari et al., 1995; human : Winer, 1984; for reviews see Jones, 1985; Winer, 1991]. The three major divisions of the MGB that have been classically described are: (1) the ventral division (MGv) which mainly contains cells with tufted dendritic trees; (2) the dorsal division (MGd, dorsal and caudo-dorsal part) which contains stellate cells with extensive den- dritic arbors and bushy neurons with dendritic branches less tufted than those observed in MGv; (3) the medial division which contains cells with the largest soma in the MGB (the prominent magnocellular neurons) but also small tufted neurons, and more classical stellate cells. Each of these MGB division receives ascending in- 0378-5955 / 99 / $ ^ see front matter ß 1999 Elsevier Science B.V. All rights reserved. PII:S0378-5955(99)00026-X * Corresponding author. Tel.: +33 (1) 69-15-49-72; Fax: +33 (1) 69-15-77-26; E-mail: jean-marc.edeline@ibaic.u-psud.fr Hearing Research 131 (1999) 135^152