Metal microelectrodes were used to record the activity of single cells in the dorsal medullary nucleus of the bullfrog (Rana catesbeiana) in response to acoustic stimulation of each ear independently. Electrode tracks were identified histologically. Among the 124 units that were isolated, 56% of them were responsive to stimulation from one ear alone. The majority of these monaural cells could be excited only by ipsilateral stimuli. The binaural cells consisted of two types. One type, comprising about 20% of the binaural units, could be excited by either ear. The neural latency was approximately the same for the two ears. The remaining 80% of the binaural units comprised the second type, which could be excited by one ear and inhibited by the other ear; the majority of these units were excited by the contralateral ear. The frequency to which a binaural unit was most sensitive (excitation or inhibition) was almost identical for the two ears. Cells that could be driven by the contralateral ear were generally found in the ventral part of the dorsal nucleus, whereas cells driven by the ipsilateral ear were usually located in the dorsal part of the nucleus. [Supported by NIH Grant NS-09244.]

A sensitive method for measurement of small vibrations at acoustical frequencies has been devised using optical heterodyne spectroscopy to detect the Doppler shifts of laser light diffusely scattered from the vibration object. Beginning with the geometry of a Michelson interferometer, we replace one mirror by the scatterer and mix the reference and scattered beam on a small-area photomultiplier cathode. With only simple optical alignment, acoustic vibration spectra can be measured without perturbations even on very small-size objects, since no attachment such as an interferometer mirror is required. Vibration amplitudes as low as 0.01 nm and up to ∼100 nm can be detected with a precision of a few percent in a few seconds averaging time by phase sensitive detection of the optical beat note at the excitation frequency. Application to mechanically unstable subjects such as the auditory organs of living biological specimens is facilitated by incorporation of low-frequency feedback control of the optical path length and provision for continuous amplitude calibration. The complete system has been used in measurements of tympanal membrane movements of acoustic receptor organs. [Supported by NSF and NIH].

Single auditory cells in the superior olivary nucleus of the green treefrog (Hyla cineria) were isolated with indium-filled micropipettes. Acoustic stimuli, consisting of tones and clicks, were delivered to each ear independently; electrode locations were verified histologically. About 50% of the cells in the superior olive receive binaural input. Almost all binaural units can be excited by contralateral stimuli and inhibited by ipsilateral stimuli. The majority of these units are sensitive to interaural intensity differences. Only low-frequency units are sensitive to interaural time differences and generally show greater response when the contralateral stimulus leads the ipsilateral stimulus; high-frequency units are insensitive to interaural time differences. The response properties of these cells are similar to cells in the superior olivary nucleus of mammals. Our results suggest, therefore, that this neural center in the frog's auditory system plays a role in sound localization. [Supported by NIH Grant NS-09244.]

The auditory fibers in the eighth nerve of the American toad (Bufo americanus) that have their best excitatory frequencies in the range 100–600 Hz can be totally inhibited by the addition of a second tone of appropriate frequency and intensity. Each of these fibers can also be excited by a pair of tones if their frequency difference is approximately equal to the unit's best excitatory frequency, even though neither tone by itself has any excitatory effect on the unit. The intensities of the two “difference tones” in many cases need only be 10–20 dB greater than the intensity of the best excitatory tone to evoke comparable spike rates. The excitation of these low-frequency fibers by difference tones seems to reside in nonlinear mechanical events in the inner ear. Midfrequency-sensitive units, having their best frequencies in the range 500–1000 Hz, and the high-frequency-sensitive units, with their best frequencies in the range 1100–1700 Hz, cannot be inhibited by tones nor can they be excited by pairs of difference tones. Similar results in other anuran species provide evidence that inhibition in the peripheral auditory system of frogs and toads is of mechanical origin. [Supported by NSF Grant GB-18836 and NIH Grant NS-09244.]

Responses to acoustic stimuli of neural units in the medullae of DIAL-anesthetized bullfrogs (Rana catesbeiana) have been recorded with metal microelectrodes. In a number of ways, these units appear to preserve the peripherally encoded information conveyed in the auditory portion of the eighth nerve. As in the periphery, two classes of units can be identified on the basis of frequency sensitivity, a low-frequency group that can be inhibited and a high-frequency group that cannot. The range of frequency sensitivity of these units for excitation and inhibition is the same as for the corresponding primary units. In other ways, however, the neurons of the medulla transform the eighth-nerve input. Weak facilitation of the response of a high-frequency unit by a low-frequency stimulus has been found in several units. Many low-frequency units exhibit very rapid adaptation at their frequencies of maximum sensitivity, responding only to the onset of a tonal stimulus. The maximum repetition rate at which such NO units can respond synchronously to pulses is much lower than for the corresponding peripheral units. The ON response has been found only among low-frequency units. High-frequency units respond, with adaptation, throughout a stimulus and show synchronous responses to pulses at rates as high as, or higher than, their peripheral counterparts.

A quantitative study of a highly selective behavioral response to sound in the bullfrog is described. The males of a laboratory colony respond vocally to a restricted class of natural and synthetic sounds. A necessary condition to elicit calling is the simultaneous presence of sufficient sound energy in the frequency regions of 100–400 and 1000–2000 cps. The relative amplitudes of the spectral envelopes of these two regions can independently vary over a wide range (at least 50 dB) without noticeably affecting calling performance. Calling can be suppressed by the introduction of energy in the neighborhood of 500 cps. The degree of suppression depends on the relative spectral amplitudes of the introduced energy and the energy in the lower frequency range of 100–400 cps. A comparison of temporal fine structure indicates optimum calling to waveform periodicities of approximately 100 cps. In presentations of the mating calls of 34 different species, only the call of the bullfrog evokes the vocal response. [Work supported in part by the U. S. Army, U. S. Navy, and U. S. Air Force under contract DA-36-039-AMC-03200(E); National Institutes of Health, U. S. Department of Health, Education, and Welfare (grant MH-04737-04); National Science Foundation (grant GP-2495); and National Aeronautics and Space Administration (grant NsG-496).]