Sensitivity to amplitude modulated sounds in the anuran auditory nervous system

Auditory responses were recorded from single units in the eighth nerve and in the midbrain torus semicircularis of the leopard frog (Rana pipiens). Acoustic stimuli included sinusoidally amplitude-modulated (AM) tones and noise, as well as pure tones. Mean spike rates were measured at various rates of AM, and the degree to which a unit's spikes were restricted to a particular phase of the modulation cycle was described by a synchronization coefficient. The firing rate of eighth-nerve fibers was largely independent of the rate of AM over the modulation range 10 to 150 Hz. Further, the general shape of the spike rate vs. AM-rate function was invariant with either depth of modulation or sound-pressure level (SPL). Although virtually all eighth-nerve fibers exhibited significant synchronization to the envelope of AM, the shape of the synchronization function depended on the unit's best-excitatory frequency (BEF). Fibers with highest BEF's, presumed to innervate the basilar papilla, generally showed greater synchronization as the AM rate was increased (up to 100-150 Hz). Fibers tuned to the low-and midfrequency region, which innervate the amphibian papilla, exhibited low-pass synchronization characteristics. As the depth of modulation was reduced, the degree of synchronization of eighth-nerve fibers decreased. For a given depth of modulation an increase in sound level tended to decrease the degree of synchronization, but significant synchronization could still be observed at stimulus intensities at least 65 dB above threshold. On the basis of the spike rate vs. AM-rate functions, the temporal selectivity of single cells in the torus could be characterized by five response types: AM nonselective (spike rate was largely independent of the AM rate); AM high-pass (activity increased as the AM rate was increased); AM low-pass (response was greatest for slow AM rates and decreased at high rates); AM band-suppression (these neurons responded well to low and high AM rates, but responded weakly to intermediate rates); and AM-tuned (spike rate was greatest over a narrow range of modulation rates). In these measurements the depth of modulation was held constant at 100%. The five response categories are not discrete, but rather reflect representative examples along a continuum with regard to temporal selectivity. The temporal selectivity exhibited by toral units in their firing rates was not evident in their AM-synchronization functions.(ABSTRACT TRUNCATED AT 400 WORDS)