Mapping functionally identified auditory afferents from their peripheral origins to their central terminations.

Aey wo~& auditory afferents-Lucifer yellow stalmng-vestibular fibers-acoustic nuclei An afferent tiber in the Vlllth cramal nerve of a terrestrial vertebrate exhibits exquisitely sensitive and selective response to one of the following stimuli to the ear. hnear acceleration or tilt with respect to the gravitational vector; angular acceleration ; airborne sound; and m some lower vertebrates at least, substrate-borne vlbranon 1,16. Among afferents responding to a given mode (e.g. angular acceleration), the stimulus response properties vary markedly from fiber to fiber. The degrees of freedom include the regularity of the response spike tram, the axis intercept 0.e. either the threshold level for response to weak stimuli or the zero-input firing rate), the constant of propomonahty (gain) between stimulus and response at higher stimulus levels, the degree of adaptation to sustained stlmuh, and the dynamics of adaptation :3.6,l~. Fibers exhlNting auditory or vibratory responses have frequency-dependent sensitlwties (tuning) and non-hneanties (two-tone suppression, difference tone stimulation, and the hke), with additional associated degrees of freedom, e.g. the sharpness of tuning (Q); frequency of greatest sensmvlty (center frequency); presence or absence of non-hnearitles z& Large-scale morphological dlstinctmns generally are presumed to account for the major stimulus-response divisions: e.g. presence of semicIrcular canals for sensitivity to angular acceleration; presence of otocomal membranes for sensitivity to hnear acceleratmn; t~lt or wbratmn; and presence of intimate connection to the sound-conducting apparatus of the middle ear for sensmvlty to airborne sound. The morphological variations that account for the degrees of freedom within the major divisions, on the other hand, are not known. Candidates include morphological variations known to exist among the receptor cells (hair cells) 16, morphological variations known to exist among the acellular, gelatmoub structures (tectorml membranes, otoconial membranes, cupulae) lying over the