1. The bats Pteronotus parnellii, P. suapurensis and Noctilio leporinus emit orientation sounds first containing a constant-frequency (CF) and then a frequency-modulated (FM) component. 2. 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. 3. Electrical stimulation of the midbrain reticular formation and/or the central grey matter elicited vocalizations which were indistinguishable from those used for echolocation. 4. 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 . 5. 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. 6. 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. 7. 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.
1. Radiation patterns of the 55, 75 and 95 kHz components in frequency-modulated sounds emitted by the grey bat ( Myotis grisescens ) were studied. FM sounds similar to species-specific orientation sounds were elicited by electrical stimuli applied to the midbrain while the head of the animal was immobilized by a nail cemented to its skull. The main beam was emitted 5-10° downward from the eye-nostril line. The radiation angle at one half of maximum amplitude was 38° lateral, 18° up and 50° down at 55 kHz, 34° lateral, 8° up and 32° down at 75 kHz, and 30° lateral, 5° up and 25° down at 95 kHz. At 95 kHz, two prominent side lobes were present. 2. The directional sensitivity of the auditory system (DSA) measured in terms of the potential evoked in the lateral lemniscus was studied with the grey bat ( M. grisescens ) and the little brown bat ( M. lucifugus ). The maximally sensitive direction moved toward the median plane with the increase in frequency from 35-95 kHz. The slope of the DSA curve increased from 0.3-0.6 dB/degree with frequency. 3. The directional sensitivity of the echolocation system (DSE) was calculated using both the DSA curve and the radiation pattern of the emitted sound. The maximally sensitive direction of the echolocation system was 15° lateral to the median plane at 55kHz and 2.5° lateral at 95 kHz. The slope of the DSE curve increased from o.6 to 1.0 dB/degree with frequency. Thus, the higher the frequency of sound, the sharper was the directional sensitivity of the echolocation system. 4. The interaural pressure difference (IPD), which appeared to be the essential cue for echolocation in Myotis , changed linearly with the azimuth angle from 0-30° lateral regardless of the frequency of sound, at respective rates of 0.4, 0.7, 0.3 and 0.4 dB/degree for 35, 55, 75 and 95 kHz sounds. Beyond 30°, the change in IPD was quite different depending on frequency. For 75 and 95 kHz sounds, the IPD stayed nearly the same between 30° and 90°. Thus, the 75-95 kHz components in FM orientation sounds were not superior to the 35 and 55 kHz components in terms of the IPD cue for echolocation. 5. Assuming the just-detectable IPD and ITD to be 0.5 dB and 5µsec respectively, as in man, the just-detectable azimuth difference of Myotis around the median plane would be 0.7-1.7° with the IPD cue and 11° with the ITD cue.