Auditory pattern recognition by the CNS is fundamental to acoustic communication. For the neural processing of amplitude modulated sounds the instantaneous spike rate rather than the mean neural activity may be the appropriate coding principle. We compared both coding parameters in the simple auditory system of the cricket Gryllus bimaculatus. Since crickets communicate with stereotyped sound pulses (syllables) of constant frequency, they are established models to investigate the neuronal mechanisms of auditory pattern recognition. We demonstrate that the primary auditory afferents are not tuned to syllable patterns of certain repetition rates but they act as syllable onset detectors, which preferentially respond to the beginning of sound pulses. This explains why the duration of syllables is not relevant for phonotactic behaviour. When stimulated with different temporal sound patterns the response of a thoracic low order interneurone (Omega neurone ON1) is very similar when the average discharge rate is considered. However, analysing the instantaneous discharge rate, demonstrates that the neuron responds with prominent peaks in its instantaneous discharge rate to syllable rates close to the species-specific sound pattern. The occurrence and repetition rate of these peaks in the neuronal instantaneous discharge are sufficient to explain temporal filtering in the cricket auditory pathway since they closely match the tuning of phonotactic behaviour to different sound patterns. Temporal filtering or Êpattern recognitionË may not happen as previously proposed in the brain but may already occur at the earliest stage in the auditory pathway. This is in good agreement with robotic models of cricket phonotaxis, where pattern recognition results from simple low level auditory processing. Our data emphasize the importance of instantaneous discharge rate coding in auditory processing rather than integration of neural activity over long time periods.

This work was supported by a BBSRC research grant to BH (8/S17898)