DIRECT EVIDENCE OF CUBIC DIFFERENCE TONE PROPAGATION BY INTRACOCHLEARACOUSTIC PRESSURE MEASUREMENTS IN THE GUINEA-PIG

The fine tuning mechanisms involved in the normal processing of sound in the cochlea are non-linear, hence combination tones are generated i nside the cochlea when a pair of low-level pure tones with neighbourin g frequencies f(1) and f(2) is used as a stimulus. Their detection as sounds in the ear canal proves that they undergo backward propagation in the cochlea and through the middle ear, and the non-invasive measur ement of the combination tone at 2f(1)-f(2), called the cubic differen ce tone (CDT), has become a routine method of monitoring cochlear func tion. In order to gain information on the hypothetical places where CD Ts are generated, on their intracochlear levels and propagation veloci ties, direct measurements of CDT pressure waves were carried out in sc ala vestibuli and tympani of the first and second turn of the guinea-p ig cochlea, Cubic difference tones at 2f(1)-f(2) varied from 0.75 to 9 kHz and were measured with a miniature piezoresistive transducer, Its high sensitivity allowed the detection of CDTs whenever their levels exceeded 5 dB SPL in the ear canal, i.e. 40 dB SPL (re: 20 mu Pa) insi de the cochlea. The levels of CDTs were similar in scala vestibuli of the first and second turn. Phase comparisons between measurements at 2 f(1)-f(2) in the first and second turn allowed determination of the pl ace where the CDT phase was minimum, it provided an estimation of the generation site of the CDT, which appeared to be close to the place tu ned to f(2) for stimulus levels lower than 70 dB SPL. Forward and back ward travel times from one turn to the other were assessed at several frequencies, and both values were shorter than 0.2 ms. In contrast, th e overall 'round-trip' delay of CDTs, measured in the ear canal, was a bout five times larger, suggesting that local filtering processes rath er than propagation delays account for the overall CDT delay.