Sound source elevation and external ear cues influence the discrimination of spectral notches by the big brown bat, Eptesicus fuscus.

Measurements of external ear transfer functions in the echolocating bat Eptesicus fuscus have revealed a prominent spectral notch that decreases in center frequency (50 to 30-35 kHz) as elevation decreases [Wotton et al., J. Acoust. Soc. Am. 98, 1423-1445 (1995)]. To examine the influence of this notch, four Eptesicus were trained to discriminate between two sets of electronically generated artificial echoes. The negative (unrewarded) stimulus contained a test spectral notch at a specific frequency that varied from 30 to 50 kHz, while the positive (rewarded) stimulus contained no test notch. The vertical position of the loudspeakers delivering these simulated echoes was changed daily. When echoes were returned from an elevation at which the external ear introduced a spectral notch at the same frequency as the test notch, then the discrimination should have been difficult. The bats' performance conformed to this prediction: All bats discriminated the presence of a 35-kHz notch at all elevations except -10 degrees. As the frequency of the synthesized notch increased, the elevation at which bats could not perform the discrimination also increased. The movement of the bat's "blind spot" for the test notch of different frequencies followed the movement of the external ear notch at different elevations.