Rate-intensity functions in the emu auditory nerve

Rate-versus-intensity functions recorded from mammalian auditory-nerve fibe rs have been shown to form a continuum of shapes, ranging from saturating t o straight and correlating well with spontaneous rate and sensitivity. Thes e variations are believed to be a consequence of the interaction between th e sensitivity of the hair-cell afferent synapse and the nonlinear, compress ive growth of the cochlear amplifier that enhances mechanical vibrations on the basilar membrane. Little is known, however, about the cochlear amplifi er in other vertebrate species. Rate-intensity functions were recorded from auditory-nerve fibers in chicks of the emu, a member of the Ratites, a pri mitive group of flightless birds that have poorly differentiated short and tall hair cells. Recorded data were found to be well fitted by analytical f unctions which have previously been shown to represent well the shapes of r ate-intensity functions in guinea pigs. At the fibers' most sensitive frequ encies, rate-intensity functions were almost exclusively of the sloping (80 .9%) or straight (18.6%) type. Flat-saturating functions, the most common t ype in the mammal, represented only about 0.5% of the total in the emu. Bel ow the best frequency of each fiber, the rate-intensity functions tended mo re towards the Aat-saturating type, as is the case in mammals; a similar bu t weaker trend was seen above best frequency in most fibers, with only a sm all proportion (18%) showing the reverse trend. The emu rate-intensity func tions were accepted as supporting previous evidence for the existence of a cochlear amplifier in birds, the conclusion was drawn further that the nonl inearity observed is probably due to saturation of the hair-cell transducti on mechanism. (C) 2000 Acoustical Society of America. [S0001-4966(00)05903- 8].