Acoustic distortion products from the ear of a grasshopper

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Abstract
Distortion-product otoacoustic emissions were recorded from the tympanum of the grasshopper, Locusta migratoria. The hearingorgan of this insect is in direct contact with the tympanum and does not contain sensory hair cells. 2f1−f2 distortions were measured for stimulus frequencies between 2–70 kHz. For frequencies between 3–9 kHz, the level of 2f1–f2 was 30–50 dB below the stimulus level. 2f1−f2 threshold curves calculated from distortion growth functions at different f2 frequencies are most sensitive between 3–9 kHz. These thresholds match the auditory sensitivity of low frequency receptor neurons in the ear [Römer, J. Comp. Physiol. 109, 101–122 (1976)]. In contrast to vertebrates, the dependence of the 2f1−f2 level on the frequency ratio f2/f1 did not show distinct maxima for most f2 frequencies. The distortion levels were largest for small ratios close to 1. The behavior of 2f1−f2 was significantly different for stimulus frequencies below and above 10 kHz. Below 10 kHz, the thresholds were more sensitive, the slope of distortion growth curves was shallower by a factor of at least 2, and the distortion levels reversibly decreased during CO 2 -induced hypoxia. Nonlinear mechanical processing may therefore be a general feature of sensitive hearingorgans, even if these involve very different morphologies. Our results suggest that the the ciliated dendrites of the receptor cells of the insect may play a role in distortion generation.

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