Fluid Motion in Cochlear Models

Abstract

Bekesy-type, transparent models of the cochlea were used in the present study. Suspensions of aluminum dust in glycerin-water solutions provided frequency responses sufficiently high to improve the display of Bekesy''s "eddies" and to allow demonstration of particle motion within the "perilymphatic" fluid. The membrane displacement varied with frequency, whereas the revolving velocity of the "dc" eddy currents varied with the square of frequency. The "bed" of each eddy was narrowest along the membrane and much wider elsewhere, as evidenced by a reciprocal variation in "ac" particle motion (in response to pure-tone signals) and was longitudinal proximal to and trochoidal (elliptical) alongside the membrane. In general, the elliptical orbits flattened out with distance from the membrane or from the eddy''s main-stream upon which particle motion was superimposed. Along the membrane, the trochoidal pattern conformed to the changes of the traveling wave, while in the rebound portion of the eddy it became almost longitudinal. No "ac" particle motion was seen to occur beyond the eddy. A phase lag of "ac" particle motion, totalling 360[degree], accumulated gradually throughout the eddy so that particles in the in- bound and rebound portions of the eddy were in phase opposition to each other. Orbits of "ac" particle motion resulting from stimulation by complex tones were observed also and analyzed as Lissajou figures. A given phase relation between the partials of complex orbits remained unchanged throughout the eddy, although their relative magnitude did alter. In the case of beats, two separate patterns were observed in response to the beat frequency and to the intermediate frequency. This indicated that the model reacted nonlinearly to such stimulation.