Comparative auditory neurophysiology of neotropical bats employing different echolocation signals
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Five species of neotropical bats, which emit echolocation pulses different than those employed by bats previously studied, were investigated in an attempt to find corresponding differences in mechanisms of neural analysis. Evoked potentials were recorded from the posterior colliculi and more peripheral levels in anesthetized specimens.
Each species was found to be most sensitive in approximately the same frequency range it uses in its emitted pulses: Chilonycteris rubiginosa at 63 kc/s, Pteronotus suapurensis at 50 kc/s, Saccopteryx bilineata at 42 kc/s, Phyllostomus hastatus at 50 kc/s, and Carollia perspicillata at 80 kc/s. Sensitivity was especially sharply “tuned” in the bats employing predominantly constant frequency pulses (Chilonycteris and Saccopteryx).
Chilonycteris, Saccopterys and Pteronotus have prominent evoked potential responses to the end of a tone pip. In Chilonycteris and Saccopteryx these “off”-responses are extremely sharply tuned to a 2–5 kc/s frequency band centered just below the frequency of greatest sensitivity of the “on” response. The presence of an “off”-response appears to be correlated with the use of long, predominantly constant frequency pulses.
The rate of recovery of responsiveness following an initial signal varied, being rapid at the level of the colliculus in Phyllostomus and Carollia (100% in 2–3 msec, slight supranormal responsiveness, and no subsequent depression), also rapid in Pteronotus, (but with an unusual degree of facilitation of responsiveness at intervals of 1–2 msec, followed by 5–10 msec of deep depression), and comparatively slow in Chilonycteris and Saccopteryx (full recovery in 10 msec or more).
In bats using long orientation pulses which overlap with returning echoes (Chilonycteris and Saccopteryx), the “off”-responses are found to provide a more visible record of the presence of echoes and their time of arrival than do the “on”-responses.
Chilonycteris respond to the orientation pulses of other Chilonycteris with prominent “off”-responses, which apparently are evoked by the termination of the constant frequency portion of each pulse rather than the downward FM sweep.
Phyllostomus and Chilonycteris are sharply sensitive to changes in signal angle, Pteronotus is moderately so, and Saccopteryx and Carollia are relatively insensitive to angle at the level of the colliculus. In Saccopteryx and Carrollia the auditory nerve response is more sharply sensitive to angular change of the signal.
In several species, but most prominently in Pteronotus, large DC potentials, of the same duration as the signal, were recorded in the vicinity of the cochlea and VIII nerve. These potentials were probably “summating potentials” and sometimes showed reversal of polarity with changes in signal frequency.
In one preparation (Pteronotus), a DC potential was seen that was apparently contralateral in origin, had a latency 1 msec longer than the ipsilateral “summating potential”, and showed adaptation suggestive of a neural origin. It is suggested that a direct neural pathway between the two ears may exist.
It is concluded that species differences in response patterns are at least in part a result of evolutionary adaptation governed by the type of emitted orientation sounds employed.
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