Comparative studies of auditory physiology in reptiles

Single-unit recordings from the cochlear nuclei of representative species of several reptilian groups show some significant correlations with basila; membrane morphology. 1. Basilar membrane morphology varies conspicuously between species. 2. With regard to frequency range, all species have single-units with characteristic frequencies as low as 100-200 Hz. For a given membrane width, the relatively longer basilar membranes, such as those in Caiman and the Western Banded Gecko, have higher characteristic frequencies. 3. The minimum single unit thresholds are lower in animals with longer basilar membranes. 4. All species showed some grouping of units at certain characteristic frequency bands, this being more pronounced in animals with relatively shorter membranes such as the turtle. 5. In the species with relatively short basilar membranes neuron tuning curves tend to be sharpest at the characteristic frequencies where the units are grouped. 6. Many tuning curves of individual neurons are complex, showing multiple sensitivity peaks, although there is usually only one characteristic frequency. Minor sensitivity peaks often correspond in frequency with the characteristic frequencies of another group of units. 7. Selective tuning is thus shown by neuron responses from all animals. Comple tuning curves have not been found in Caiman and are rare in the Western Banded Gecko. Some speculations are made as to the mechanisms of frequency analysis in the various membranes. 8. The relatively long membrane of Caiman is associated with a clear tonotopic organization of the auditory neurons in the cochlear nuclei.