THRESHOLD OF ACTION POTENTIALS IN EAR OF GUINEA PIG

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Abstract
A method is described for determining the threshold of auditory action potentials in the cochlea of the guinea pig for frequencies below 1000 cps. The method depends on cancelling the aural microphonic by a signal derived from the electrical current that produces the sound stimulus. The action-potential threshold in decibels for tones below 1000 cps. is a linear function of the logarithm of the frequency. The slope of this line varied from -28 db per octave in fresh prepn. to -1 db per octave in animals treated with a large dose of quinine (which nearly suppresses the action potentials with very little affect on the aural microphonic). The intensity needed to produce an aural microphonic of 1 microvolt seems to be practically independent of frequency from 1 to 5000 cps. provided the intra-aural muscles are relaxed and provided the microphonic at each frequency is measured close to its source. In a fresh prepn. the over-all "threshold of cochlear response" is detd. not by the aural microphonic at high and at low frequencies but by action potentials over the middle frequencies. The apparent "threshold" curve is irregular because of reinforcement or partial cancellation of the 2 components. The relationships to frequency of the action-potential threshold and of the microphonic differ widely (up to 28 db per octave) and vary independently. The differential sensitivity of the ear with respect to frequency is detd. in part by physical acoustic properties of the ear as a whole. For frequencies at least up to 1500 cps., nowever, this sensitivity is even more importantly detd. by a process closely related to the excitation of the nerve impulses and subsequent to (or quite independent of) the aural microphonic.
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