A Time Window for Distance Information Processing in the Bats, Noctilio Albiventris and Rhinolophus Rouxi

Abstract

Microchiropteran bats emit echolocation sounds that have structured patterns of frequency changes over time. Classes of frequency pattern have been observed among bat orientation pulses. Bat echolocation sounds generally consist of constant frequency (CF) and frequency modulated (FM) elements alone or in a combination of the two components. For example, in certain species (known as CF/FM bats), the echolocation sounds contain a CF component preceding the FM sweep; in some species the CF component is short (under 12 msec), while in others it is long (over 12 msec). The defined structures of echolocation sounds presumably reflex specific information processing requirements. One generally accepted requirement is that a broadband signal is necessary for accurate perception of target distance by neural measurement of the time interval between the emitted broadband event and a returning echo. Nevertheless, the essential elements and processing requirements of complex CF/FM echolocation sounds has mostly been a matter of conjecture. In this paper I describe experiments that demonstrate which structural elements of complex CF/FM echolocation sounds code target distance information and provide a mechanism regarding hew this information is processed by the nervous system. These studies suggest that CF/FM bats use both the CF and FM components of their CF/FM echolocation sounds for the determination of target distance, with the onset of the CF component activating a gating mechanism that establishes a time window during which FM component pulse-echo pairs are processed for distance information. In a psychophysical procedure bats of the species Noctilio albiventris (a short-CF/FM bat, Roverud and Grinnell 1985) and Rhinolophus rouxi (a long-CF/FM bat) were trained to discriminate a 5 cm and 8 cm difference in target distance respectively. Both species can discriminate these presented range differences with 90% or greater accuracy. During the discrimination trials N. albiventris emits 7-10 pairs of pulses each second. The first pulse is an 8 msec duration short CF signal that rises from about 71 to 75 kHz and then descends to 71 kHz. About 28 msec later the bat produces a short-CF/FM signal with an initial 6 msec CF component (again rising from about 71 to 75 kHz) and a terminal 2 msec FM sweep to about 57 kHz. The "CF" component of the calls of N. albiventris are actually shallow FM signals that seem to serve a narrowband-function. During similar discrimination trials R. rouxi emits pulses at a repetition rate of about 10/sec. The bats usually-emitted second harmonic FM/long-CF/FM pulses with an initial 2 msec upward FM sweep followed by an approximately 45 msec long CF component and terminating in a 2 msec downward FM sweep. The second harmonic CF frequency 513