Differences in velocity-information processing between two areas in the auditory cortex of mustached bats
The bio-sonar pulse of the mustached bat, Pteronotus parnellii parnellii, consists of four harmonics of constant frequency (CF1-4) and frequency-modulated (FM1-4) components. The CF and FM components carry velocity and distance information, respectively. In the auditory cortex of mustached bats, the CC (“C” stands for constant frequency) and DIF (dorsal intrafossa) areas consist of CF/CF neurons tuned to a combination of pulse CF1 and echo CFn (n = 2 or 3). They show facilitative responses to pulse-echo stimuli with specific frequency differences, corresponding to Doppler shifts. Their facilitative responses are sharply tuned to a specific relative target velocity (best velocity). Compared with CC neurons, DIF neurons are tuned to higher velocities and to larger CF1 amplitudes, and adapt faster to repetitive pulse-echo stimuli. The great majority of CC neurons are suited for the processing of velocity information during cruising and target-directed flight, whereas the majority of DIF neurons are suited for the processing of velocity information when the bat is emitting loud pulses at low repetition rates during cruising flight. CC and DIF neurons are broadly tuned to 0–2-ms echo delays and not suited for ranging.