Neural coding in the bullfrog´s auditory system a teleological approach

Anatomical and physiological studies in the peripheral auditory system of the bullfrog have shown that there are two auditory receptor organs within the frog´s inner ear which are sensitive over nearly disjoint frequency ranges. Stimuli whose frequencies lie between these ranges have mainly an inhibitory effect upon the response of the nerve fibers that innervate the low-frequency organ. It has been shown that the mating call, which is produced by the male bullfrog and functions to evoke calling in other males, often contains energy peaks in just the frequency ranges to which the peripheral receptor organs are optimally sensitive. The auditory nerve fibers from both organs respond most vigorously to signals whose fine temporal structure is like that of the mating call, that is, to pulsatile signals having a repetition rate of about 100 per second. Behavioral studies employing synthetic croaks indicate that for a stimulus to evoke calling it is necessary that sufficient energy be simultaneously present in the frequency ranges that excite the two auditory organs; evoked calling can be suppressed by signal components which inhibit the neural responses from fibers that respond to energy in the low-frequency range. When spectral conditions are met, calling is maximal for signals which have a temporal structure that produces the maximum neural response. It is concluded that the periphery of the bullfrog´s auditory system is specialized to detect the major spectral and temporal features of the mating call and that the evoked-calling response depends upon the simultaneous excitation of the peripheral auditory organs. The possible biological significance of the latter mechanism is discussed. The relation of the other calls that the bullfrog makes to the response characteristics of his peripheral auditory system is also considered, as well as comparable relations in other species of frogs.