Sensory-encoding differences contribute to species-specific call recognition mechanisms

Object recognition is a fundamental function of the auditory system, but the underlying mechanisms are not well understood. Acoustic communication in the Tettigoniid genus Neoconocephalus provides a useful system for studying these mechanisms. We examined the ascending interneuron pathway in three Neoconocephalus species with diverse calls and recognition mechanisms. This pathway processes spectral information and transmits call temporal patterns to the supraesophageal ganglion where the recognition circuits reside. For each species, we describe one local auditory interneuron (ON) and three with ascending projections (AN-1, AN-2, TN-1), which were physiologically and morphologically similar to those described in other Tettigoniids. TN-1 responded only to the beginning of call models. For AN-1, each call model pulse elicited a single action potential in N. robustus and N. bivocatus, whereas every other pulse elicited an action potential in N. triops. Individual pulses did not reliably evoke AN-2 responses in all three species. AN-1 responses were limited to frequencies<20 kHz. AN-1 tuning differed among the three species, reflecting their differences in the dominant frequency of the calls. AN-2 was broadly tuned, and responses increased with intensity in all three species. In behavioral experiments, N. robustus showed greater spectral selectivity than the other two species. Adding the second harmonic to the spectrum of call models suppressed phonotaxis in N. robustus, but not N. triops or N. bivocatus. Adding the second harmonic reduced AN-1 responses in N. robustus but not in the other two species. We discuss the potential function of the ascending neurons for call recognition.