Chronotopically Organized Target-Distance Map in the Auditory Cortex of the Short-Tailed Fruit Bat

Abstract

Topographic cortical representation of echo delay, the cue for target range, is an organizational feature implemented in the auditory cortices of certain bats dedicated to catch flying insects. Such cortical echo-delay maps provide a calibrated neural representation of object spatial distance. To assess general requirements for echo-delay computations, cortical delay sensitivity was examined in the short-tailed fruit bat Carollia perspicillata that uses frequency-modulated (FM) echolocation signals. Delay-tuned neurons with temporal specificity comparable to those of insectivorous bats are located within the high-frequency (HF) field of the auditory cortex. All recorded neurons in the HF field respond well to single pure-tone and FM-FM stimulus pairs. The neurons respond to identical FM harmonic components in echolocation pulse and delayed echo (e.g., FM(2)-FM(2)). Their characteristic delays (CDs) for low echo amplitudes range between 1 and 24 ms, which is comparable to other bat species. Maps of the topography of FM-FM neurons show that they are distributed across the entire HF area and organized along a rostrocaudal echo-delay axis representing object distance. Rostrally located neurons tuned to delays of 2-8 ms are overrepresented (66% of CDs). Neurons with longer delays (>/=10 ms) are located throughout the caudal half of the HF field. The delay-sensitive chronotopic area covers approximately 3.3 mm in rostrocaudal and approximately 3.7 mm in dorsoventral direction, which is comparable or slightly larger than the size of cortical delay-tuned areas in insectivorous constant frequency bats, the only other bat species for which cortical chronotopy has been demonstrated. This indicates that chronotopic cortical organization is not only used exclusively for precise insect localization in constant frequency bats but could also be of advantage for general orientation tasks.