Selective laryngeal neurotomy and the control of phonation by the echolocating bat,Eptesicus

Summary

• 1. The effect, on the subglottic pressure and on the emitted orientation sounds, of selectively cutting nerves to various laryngeal muscles of the Big Brown Bat,Eptesicus fuscus, was studied.

• 2. Bilateral inferior laryngeal neurotomy caused no change in the repetition rate, duration, initial frequency or bandwidth of downward sweeping frequency modulated (FM) pulses, but after this treatment mean subglottic pressure at pulse onset was lowered 8 to 16 cm H₂O and the mean peak pulse intensity was reduced 4 to 5 dB. The relationship of subglottic pressure to the bandwidth of the FM sweep was also altered (Table 1, Figs. 1, 2, 3).

• 3. Inferior laryngeal neurotomy also caused the bat to produce atypical rising FM pulses which began at about 20 kHz and swept upward almost one octave (Table 1, Fig. 2).

• 4. Total bilateral superior laryngeal neurotomy eliminated most of the FM and reduced the fundamental frequency to 7.9 kHz with multiple harmonics (Fig. 4). Pulse duration became highly variable and the peak sound pressure level (SPL) dropped 7 to 13 dB (Table 2). The correlation between subglottic pressure and pulse SPL was also eliminated.

• 5. Bilateral section of only the caudal branch of the superior laryngeal nerve reduced the fundamental frequency of the pulses to 10 kHz and eliminated most of the frequency modulation. Pulse SPL was reduced about 10 dB, but pulse onset subglottic pressure remained correlated with the maximum SPL (Table 2).

• 6. Paralysis of the tongue by bilateral section of the hypoglossal nerves distal to the thyrohyoid twigs had no detectable effect on the downward sweeping FM pulses, but caused the bats to also emit long duration pulses whose frequency rose and fell in a sinusoidal fashion (Figs. 5, 6).

• 7. The muscular control of pulse frequency and intensity is discussed in the light of these data. The aberrant rising FM after section of the inferior laryngeal nerves and sigmoid pulses after hypoglossal neurotomy can be understood as the result of timing errors in the opening and/or closing of the glottis relative to the contraction-relaxation cycle of the cricothyroid muscles.