ACOUSTIC DISTORTION PRODUCTS FROM THE EAR OF A GRASSHOPPER

Distortion-product otoacoustic emissions were recorded from the tympan um of the grasshopper, Locusts migratoria. The hearing organ of this i nsect is in direct contact with the tympanum and does not contain sens ory hair cells. 2f1 - f2 distortions were measured for stimulus freque ncies 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 cur ves calculated from distortion growth functions at different f2 freque ncies are most sensitive between 3-9 kHz. These thresholds match the a uditory sensitivity of low frequency receptor neurons in the ear [Rome r, J. Comp. Physiol. 109, 101-122 (1976)]. In contrast to vertebrates, the dependence of the 2S1 - f2 level on the frequency ratio f2/f1 did not show distinct maxima for most f2 frequencies. The distortion leve ls were largest for small ratios close to 1. The behavior of 2f1 - f2 was significantly different for stimulus frequencies below and above 1 0 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 CO2-induced hypoxia. Nonlinear mechanical processing may therefore be a general feature of sensitive hearing organs, even if these involve very different morpho logies. Our results suggest that the the ciliated dendrites of the rec eptor cells of the insect may play a role in distortion generation. (C ) 1998 Acoustical Society of America.