単一刺激による電気聴覚

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When an electric current passes through an excitable tissue, electrical potential of the tissue which is referred to as the "local potential" V (Hill, 1936) rises, and when V reaches a constant value U, "excitation" takes place. V reverts to its resting value when an electric current, not sufficiently strong to induce excitation, is removed from an excitable tissue. The time-constant of this decay which is assumed to be exponential, is the time-constant in terms of the rate of change of V which is independent of the particular form of the current applied. i. e. , κ is the time-constant of excitation. When an electric current passing through an excitable tissue lasts for a very short time, the threshold value of the "local-potential" required for excitation is constant. If the lasts for a longer time as in the cases of constant current near the rheobase, linearly or exponentially increasing current or low frequency alternating current, the critical value for excitation rises the threshold for a slowly increasing current is welknown to be higher than that for a rapidly increasing current. This Change in threshold is called "accommodation" (Nernst, 1908). Assuming that the rise of the critical value is caused by the rise in "local potential", the critical value will revert to its resting value when the "local potential" reverts to its resting value. The time-constant of the return of the critical value to its resting level which is assumed to be exponential is λ, the time-constant of "accommodation". when an electric current is passed through a normal ear via a salt solution in the external ear canal, the excitatory effect of alternating current varies with frequency. The curve of threshold voltage plotted against the logarithm of frequency is a "U" shaped curve with a difinite minimum. It appears that there is an optimum frequency of alternating current for excitation. Well below the optimum frequency the threshold becomes large in proportion to reciprocal of the frequency excitation becomes difficult owing to "accommodation". Well above the optimum the threshold rises in proportion to frequency the rise of the threshold is caused by the duration of stimulus being short, consequently the element of "accommodation" is represented as a high-pass network (a integral circuit). Thus we may devise a circuit analogy as shown in Fig. 4, which is usefull in obtaining a better understanding of the excitablility of the electrophonic hearing. When the voltage e=v(t) is supplied to the input of this circuit, the voltage of the output e_2 may be obtained from Eq. (3) Let us assume that when e_2 reaches a critical value e_c, "excitation" takes place. We have several facts at hand which prove that the threshold is little dependent upon pulse rate, but determined by the pulse duration, in case the pulse rate of the stimulus current in electrophonic hearing is less than about 100 p. p. s. . The subjects in this experiment were therefore stimulated with currents of low pulse rate (20 p. p. s. ). 1. Observation employing rectangular current In the circuit of Fig. 4, e_2 can be calculated from Eq. (4) if e_1 is a constant voltage E. If λ=∞ and duration t=∞, the threshold is called a "true-rheobase" in Hill's theory. If λ=∞, the differential circuit of the equivalent circuit shown in Fig. 4 can be neglected, in which case, when a pulse of amplitude E_0 with a very long duration is applied to the input, the final value of e_2 at t=∞ becomes E_0. Since E_0 is the threshold at t=∞, E_0 is equal to e_c. In Eq. (4), e_2 rises up to a maximum and then falls it takes the maximum at the time t_<max> of Eq. (5). For t<t_<max>, the threshold varying with duration t can be calculated from Eq. (6). Fig. 10 shows a comparison of computation (λ=3m. Sec. , κ=0. 3m. Sec. ) with experimental results. The observed values are nearly on the solid curve computed. 2. Observation employing exponentially increasing current Inserting Eq. (7) in Eq. (3),
社団法人日本音響学会の論文
1969-05-30

単一刺激による電気聴覚

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