Acoustic distortion from rodent ears: a comparison of responses from rats, guinea pigs and gerbils

The oto-acoustic emissions generated in response to two-tone stimulation have been studied in the ear canal sound pressure of three species of rodent: rat (Rattus norvegicus), guinea pig (Cavia porcellus) and Mongolian gerbil (Meriones unguiculatus). The level of acoustic intermodulation distortion evoked by equal-level stimuli at different frequencies can be related to the threshold frequency response curves obtained by other workers for these species using evoked electrical responses or behavioural techniques. Gerbils produce higher levels of distortion below 9 kHz than the rat or guinea pig. This may be due to the greater efficiency of the middle ear at low frequencies. Growth of 2f1-f2 with equal-level, widely-spaced stimuli (f2/f1 = 1.3) can be divided into two regions. The low intensity part of the curve grows with a slope of 1 and saturates above 60 dB SPL. With higher level stimuli, there is rapid growth with a slope of, or approaching 3. Differences in growth rate may distinguish a low-level, saturating response which owes its response characteristics to the activity of hair cells from a high-level response attributable to passive mechanical properties of the cochlea. A broad, low-frequency spread of distortion components is seen as the stimulus frequencies converge. Sharp interruptions in the growth curves of these components may be due to interaction between out-of-phase components of 'low' and 'high' level distortion. The distortion in the gerbil can be distinguished from that of the other two species in a number of details. Structural specialisation of the gerbil cochlea may contribute to these distinctive features.