Directional sensitivity of the auditory midbrain in the mustached bat to free-field tones.

To ascertain the directional characteristics of the auditory system in the mustached bat, Pteronotus parnellii, we measured the summated neural response at the lateral lemniscus (N4) in response to pure tones at 30, 60 and 90 kHz, frequencies that are typical of the harmonics of this species' biosonar signal. Stimuli were presented at various vertical and horizontal locations in the contralateral hemifield. Intensity-response functions were measured at different horizontal locations for the second harmonic, and showed no variation in shape with variations in azimuth. There was little difference in directionality measured from either threshold or amplitude of N4 potentials. Our results show that areas of maximum sensitivity (best areas) were significantly different for each of the harmonics (P less than 0.05). The centers of the best areas were: first harmonic (30 kHz), 39 degrees azimuth and -19 degrees elevation; second harmonic, 20 degrees azimuth and 0 degrees elevation; and third harmonic, 12 degrees azimuth and -11 degrees elevation. Thus, with increasing frequency best areas shifted toward the vertical midline. Directionality to first harmonic stimuli was broader than to either of the two higher harmonics.