Neurophysiological Studies On Echolocation Systems in Awake Bats Producing CF-FM Orientation Sounds

ABSTRACT The bats Pteronotus parnelli, P. suapurensis and Noctilio leporinus emit orientation sounds first containing a constant-frequency (CF) and then a frequency-modulated (FM) component. P. parnellii produced a long CF with a second harmonic at 62 kHz to which its auditory system was sharply tuned. In the other two species, the CF was shorter and there was no sharp tuning. Electrical stimulation of the midbrain reticular formation and/or the central grey matter elicited vocalizations which were indistinguishable from those used for echolocation. The electrically-elicited vocalization was enhanced by acoustic stimuli. In P. parnellii, this vocal response was sharply tuned at 62–63 kHz and also to downward sweeping FM sounds. In P. suapurensis and N. leporinus, the vocal responses were prominent only to downward sweeping FM sounds. This indicates that the FM is important to echolocation in all these bats and that the CF component is more essential to echolocation in P. parnellii than to that in P. suapurensis and N. leporinus. The responses of primary auditory neurons to the onset and cessation of pure tone stimuli were due to mechanical events, not due to a rebound from neural inhibition. Masking experiments with P. parnellii indicate that the neural response at the cessation of a CF-FM sound similar to its orientation sound mainly consisted of the response to the FM component and not the off-response to the CF component. During vocalization, self-stimulation was reduced by contraction of middle-ear muscles. This was not due to the acoustic reflex which started to occur with a 6 msec latency.