Tonotopic organisation of the auditory midbrain nuclei of the midwife toad (Alytes obstetricans)

The torus semicircularis (TS) of Alytes obstetricans is tonotopically organized. A stereotactic system was used to obtain isointensity responses with a few-units recording method; at each recording site the dominant frequency (that eliciting maximal discharge) was noted. Neurons activated by acoustic stimuli were found in the laminar, principal and magnocellular nuclei; they were rare in the commissural nucleus, and in the subependymal nucleus no stimulus-correlated responses were recorded. High-frequency dominance (greater than or equal to 900 Hz) was found only in a particular region of the torus, extending from caudomedial to rostrolateral, and it was restricted to ventral sites in the caudal and lateral parts of this region. In a few of the more rostral penetrations high-frequency dominance was found at dorsal as well as ventral positions. In a rostromedial area high-frequency neurons predominated over the entire dorsoventral extent of the torus, and in a caudolateral area low-frequency (less than 500 Hz) neurons were similarly distributed. The maximal discharge elicited by high tones proved, almost without exception, to derive from neurons of the laminar and magnocellular nuclei. Low-frequency dominance was found at some positions in these nuclei as well as in the principal and commissural nuclei.