Frequency tuning and latency organization of responses in the inferior colliculus of Japanese house bat, Pipistrellus abramus

Pipistrellus abramus emits quasi-constant frequency pulses during search, which extend the end frequency portion of the downward frequency-modulated sweep (terminal frequency; TF). If the narrowed frequency range is important for detecting a small frequency change caused by insect fluttering, the bats may need much finer frequency resolution at the TF. To test this hypothesis, the distribution of the best frequencies (BFs) in the inferior colliculus (IC) was electrophysiologically measured. The TF of the echolocation pulse was 41.44+/-2.62 kHz. The frequency range of 35-45 kHz was overrepresented in the IC (n=50/105; 48%), and a faint second peak was seen at 75-85 kHz (the second harmonic of the TF) in the BF distribution. The BF increased as a function of recording depth along the dorsoventral axis, except for the BFs of 35-45 and 75-85 kHz, which were found at a wide range of depths. The response latency ranged between 3.7 and 23.2 ms for the BFs of 35-45 kHz, and the maximum target range was estimated to be 3.3 m from the delay line observed in the IC. These electrophysiological measures suggest the importance of a target distance within approximately 3 m, which is consistent with behavioral measures during foraging in this species.