Pairs of very small electrodes were placed in two or more turns of the cochlea of the guinea pig. The cochlear microphonic from a short segment (about 1 mm) of the cochlear partition can thus be recorded, and without contamination by action potentials. The outputs of the second, third, and fourth turns were compared with that of the first turn with respect to both amplitude and phase as a function of frequency. The space-time pattern thus revealed is a traveling wave which passes up the cochlea to a distance that depends on the frequency. The pattern agrees well with that of mechanical movement (Bdkdsy) except that the cochlear microphonic shows relatively greater amplitude (voltage) in the basal turn. Low frequencies travel farther up the cochlea than do high. The velocity (for a 750 cps tone) is about 100 m/sec in the basal turn and about 2 m/sec in the fourth turn. Phase differences of as much as 5r were observed, by means of Lissajous patterns, between the responses of the first and the third turn. Certain discontinuities in the input-output relationships and in phase relationships were found to be a function of frequency and of position along the cochlear partition. These discontinuities occurred at or near the frequencies that showed a phase difference of 2r from the basal end of the cochlea. The relation of these critical frequencies to position along the cochlea constitutes a new "frequency-map" of the cochlea. The space-time pattern of the cochlear microphonic proved to be very little affected by removal of portions of the bony wall of the cochlea or delivering acoustic energy through a hole near the apex. It is concluded that, in addition to "direct driving" of the cochlear partition by pressure differences between the two scalae and to "surface waves" arising from such driving, the transmission of transverse waves along the solid structures of the cochlear partition must also be included for a satisfactory interpretation of all of the available data.
Several experiments to determine the effects of two-tone interaction in the cochlea are described. Nonlinear effects result when the cochlear partition is stimulated by a sinusoid whose frequency falls in a very narrow frequency band surrounding, but not including, the resonant frequency at the point under the recording electrode. These nonlinear effects are manifested as a negative shift in the dc potential accompanied by a partial or complete suppression of the cochlear microphonic potential to a second tone presented at the same time. This suppression is best seen in the first and second turns of the cochlea. Similar suppression effects accompanying a dc shift are obtained by artificially displacing the basilar membrane from its resting position by an asymmetrical change in hydrostatic pressure in the perilymph. The functional relations governing suppression were determined and the results compared with those obtained in two-tone inhibition experiments in auditory nerve fibers. The results support the conclusion that inhibitory effects seen in nerve fibers, stimulated by two tones simultaneously, reflect mechanical events in the cochlear partition and subsequent changes in the effective stimulating waveform triggering the auditory nerve.
The cochlear-microphonic potential of guinea pigs was recorded during stimulation by bone-conducted sounds. The amplitude and the phase of the response were measured while the ossicles were altered in various ways. At low and middle frequencies, fixation of the ossicular chain produces a decrease of amplitude and a phase advance of about 70°, while an increase of the mass yields an increase of amplitude and a phase lag greater than 180°. A progressive increase of the mass, produced by introducing paraffin oil in the middle-ear cavity, yields a progressive variation of amplitude and phase and, at a particular moment, a complete disappearance of the response. Symmetrical results were obtained by altering the intrabullar pressure. All these results are interpreted as showing the participation of several mechanisms contributing to the whole response. At low frequencies a translational mechanism involves two components: one related to the motion of the ossicular chain and the other related to the motion of the perilymphatic fluid. These two components contribute to the whole response according to their amplitude and phase. At higher frequencies a compressional mode of bone conduction, independent of the motion of the ossicles, is responsible for the stimulation. These findings agree with the alteration of bone conduction observed in clinical cases and provide an explanation for the lateralization of the sound source in an ear affected by conductive deafness, as shown in the Weber test. This lateralization may be accounted for by the observed phase shifts.
The electric source of the whole-nerve action potential recorded by cochlear electrodes is generally considered to be part of the nerve trunk which runs through the internal auditory meatus. The comparison of the APs recorded in the first and third turn, after inactivation of the central end of the nerve trunk by KC1, shows differences in shape which support this hypothesis but also indicate that the source of AP extends somehow towards the apex. Subject Classification: 65.40, 65.42; 80.50 A classical method for recording AP is to use a pair of differential electrodes penetrating into the scala vestibuli and the scala tympani of the basal turn of the cochlea. These two electrodes are connected together through a network which rejects the microphonics, and a reference electrode is located on the neck of the animal (Tasaki, Davis, and Legouix, 1052; Tasaki and Fernandez, 1052). According to Tasaki, Davis, and Goldstein (1952) andTasaki (1954), the source of this whole-nerve action potential is situated in the nerve trunk near the basal turn. Teas, Eldredge, and Davis (1962) pointed out that the nerve trunk runs through the internal auditory meatus which forms an insulating tube and that the two ends of this tube form physiological electrodes.The whole-nerve AP is the convolution of many unit potentials. Teas e! al. suggested that these unit potentials recorded by such electrodes are diphasic and comprise one negative followed by one positive deflection. Owing to the time of propagation of the mechanical wave along the cochlear partition, the latencies of these unit potentials are distributed so that, when the stimulus is a transient, the nerve discharges last more than I msec.These authors indicated also that, when the transient is short and contains high frequencies, the whole-nerve response shows a large amplitude and the typical negative wave N• followed by a positive deflection.This waveform reflects the summation of wellsynchronized unit action potentials.It follows that, as it was pointed out by Tasaki (1954), only the basal turn is able to contribute effectively to the whole-nerve AP; the more apical parts discharge unsynchronized units of APs which are difficult to detect in the experimental conditions. For these reasons, it is generally agreed that one electrode located at any place on the surface of the cochlea, or plunging into the perilymph, records the same AP, with similar waveform and latency. The cochlea appears to be isopotential and to act as a passive conductor transmitting a variation of potential that originates from a single source located at the base.The present paper reports a series of experiments on the guinea-pig cochlea. The results seem to confirm that the waveform of the AP is determined by the configuration of the auditory meatus, as suggested by Tasaki et al. (1052). They also indicate that the source of the whole-nerve AP is not limited to a short segment of the nerve trunk in the region of the internal auditory meatus, but may e'xtend up to the third turn. I. TECHNIQUEAction potent...
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