Influence of envelope rise time on neural responses in the auditory system of anurans.

The influence of envelope rise time on neural responses was investigated in the central auditory pathway of frogs. Single unit and evoked potential recordings were made from the dorsal medullary nucleus (DMN) and thalamus, respectively. It was found that phasic neurons (15% of the population) in the DMN responded preferentially to stimuli with rapid (less than 25 ms) rise times. Acoustically evoked potentials (AEPs) recorded from the thalamus, specifically the Nucleus of Bellonci, also showed more pronounced responses to stimuli with rapid, rather than slower, envelope rise times. Interestingly, the leopard frog mating call, which has a rapid onset, elicited strong neuronal discharge both within the DMN and thalamus. In contrast, the mating call of bullfrogs, a species sympatric with leopard frogs, has a characteristically slow (greater than 100 ms) envelope rise time and elicited little, if any, response. These findings indicate the presence of neural specializations within the frog's auditory pathway for the optimal detection of the conspecific mating call. The relevance of these findings for stimulus coding in the auditory pathway of other vertebrates is discussed.