Auditory Response Properties of the Anuran Thalamus: Nonlinear Facilitation

Abstract

Auditory evoked responses were first recorded in the anuran diencephalon by Mudry and her colleagues (Mudry, 1978; Mudry, Constantine-Paton, and Capranica, 1977). These investigators showed, in several anuran species, that evoked responses from the dorsal thalamus exhibited a property that they termed "nonlinear facilitation". That is, thalamic evoked potentials responded preferentially to two-tone stimuli containing acoustic energy that simultaneously excited both of the peripheral auditory papillae. The magnitudes of responses to these two-tone stimuli were larger than that predicted by the linear summation of response magnitudes to each tone presented separately. Such an effect was not observed in the torus semicircu1aris of the midbrain. In this auditory center, evoked response magnitudes to two-tone stimuli are equal to the summed responses to each tone presented alone. These results have interesting implications for theories of the neural control of call detection in anurans. Mating calls of many anuran species involve the simultaneous use of both low-and high-frequency energy. Any putative neural "mating call detector" would, therefore, be required to respond preferentially to simultaneous presentations of low-and high-frequency sounds. The two peripheral papillae have restricted frequency sensitivities, the amphibian papilla responding maximally to low-and mid-frequency spectral energy, and the basilar papilla responding maximally to high-frequency energy. It is only at the level of the dorsal thalamus that convergence of the responses from the two papillae seems to occur. Therefore, the dorsal thalamus may fulfill an important condition for a putative "mating call detector" (Mudry, 1978). The dorsal thalamus does not, however, constitute an anatomically homogeneous area but, rather, consists of several distinct subnuclei (Neary, 1974, 1975). Auditory activity can be recorded throughout these various nuclei (Mege1a and Capranica, 1981). The central nucleus receives direct ascending projections from the torus and apparently no other input. This nucleus may be the homolog of the medial geniculate body in mammals. The anterior 895 J.-P. Ewert et al. (eds.), Advances in Vertebrate Neuroethology