Tonotopic organization in the medial geniculate body (MGB) of lightly anesthetized cats

In the medial geniculate body (MGB) of nitrous oxide anesthetized cats, the pars lateralis (PL) was the only nucleus to show a clear topographic arrangement of its neurons according to their characteristic frequency (CF). When compared to barbiturate anesthetized cats (Imig and Morel 1985a), the tonotopic organization in PL appeared less strict and was characterized by a significant local CF disparity. Furthermore, the degree of tonotopic organization varied along the rostrocaudal axis of the nucleus: it was lower in its caudal than in its rostral half. In the pars ovoidea, the rostral half of the pars magnocellularis (PM) and the suprageniculate nucleus, CF sequences and quantitative evaluations of the tonotopicity indicated the presence of some degree of tonotopic organization which was lower than in PL. No such organization was observed in the caudal part of PM nor in the ventrolateral nucleus, while in the dorsal nucleus, the proportion of tone-responding units was too low for a significant analysis.