The upper frequency range in pigeon hearing: Response properties in auditory nerve and cohlear nucleus—is tuning different in birds?

Information on responses of auditory nerve fibers in birds is generally limited to the low and mid-frequency ranges of hearing, due to anatomical constraints using the standard ganglion cell approach through the opened tympanic recess. Intracranially recorded responses of single units from the auditory nerve and the overlying cochlear nuclei in the pigeon (Columba livia) were compared to those recorded from the ganglion. Characteristic frequencies (CFs) of auditory units ranged from 0.08–5.7 kHz with intracranial compared to 0.06–1.6 kHz with intrascalar recordings. Response properties were qualitatively and quantitatively comparable in the overlapping CF ranges (0.08–1.6 kHz). Mean spontaneous firing rate decreased monotonically with CF. The CF thresholds were lowest (0–5 dB SPL) in the CF range 0.6–1.5 kHz. Mean sharpness of tuning (Q10 dB) varied nonmonotonically with CF, with a maximum near 2 kHz. In addition, Q10 dB of pigeon auditory units up to CFs of 2 kHz was much higher than that of gerbil (Meriones unguiculatus) primary fibers in the same CF range, recorded in the same setup. As it is highly improbable that mechanical tuning in birds would by far exceed that in mammals, this finding is further evidence for a difference in avian and mammalian tuning mechanisms. [Work supported by the DFG, SFB 269.]