Coding of interaural time differences in the microsecond range by auditory units of echolocating bats

Abstract

The coding of interaural time differences (Δt) by single auditory units was investigated by extracellular recordings from the brainstem in the echolocating bat, Molossus ater. Phase-locked constant frequency tone bursts (2 ms duration, 0.5 ms rise/decay-time) delivered through earphones could be time delayed with an accuracy of <1 μs by using two “bucket-brigade-devices.” Nine percent of the 79 binaural units studied showed dependence on Δt-variations in the animals' relevant time range (±50 μs 17 mm interaural distance). From the steepest Δt-impulse count functions measured for a fixed interaural intensity difference, a minimum detectable time difference of less than 4 μs could be calculated. These small time differences were coded from the envelope of signals with comparatively long rise times. The use of phase differences can be excluded. Due to the high best frequencies of the units (18–61 kHz) the half periods of the sine waves were always shorter than the working ranges of the Δt-impulse count functions measured. Furthermore, the Δt-functions were never periodic. [Work supported by DFG: Br 593/2, Ne 146/10, and SFB 45.]