Acoustic pattern recognition in a grasshopper: processing in the time or frequency domain?

Abstract.

Males of the grasshopper Chorthippus biguttulus produce songs which consist of the stereotyped and rhythmic iteration of a sound unit (termed syllable) separated by distinct syllable pauses. Virgin females respond to this signal, and to similar artificial signals, with song phrases of their own. In behavioural experiments the response probability of virgin females can be measured with artificial acoustic stimuli. The stimuli consisted of an amplitude modulated noise the envelope of which was altered. We investigated several hypotheses on the mechanisms of conspecific song recognition with special emphasis on the question whether recognition occurs in the frequency domain or in the time domain. (1) Females of Ch. biguttulus required only the first five Fourier components of the envelope function (corresponding to 50 Hz for a fundamental frequency of 10 Hz) to detect the syllable/pause structure. In addition, they detected small gaps within syllables if the signal contained at least ca. 15 Fourier components (corresponding to a frequency of 150 Hz). Further experiments showed that the correct phase information of the Fourier components is necessary for recognition, indicating that pattern recognition is not achieved merely on the basis of band pass filtering. (2) A cross correlation between the signal and an assumed internal template yields only inconsistent predictions of the response probabilities. (3) The recognizer system probably works in the time domain, possibly by direct comparison of adjacent syllable and pause durations. It is not yet clear whether the duration of a syllable is evaluated with respect to the preceding or succeeding pause. We emphasize that the neural recognizer of the grasshopper does not only examine a signal for its similarity to an internal template, but that it also takes into account features that indicate an incorrect signal. This may be a general feature of neuronal pattern recognition systems which have been shaped by natural selection.