Echolocation sound features processed to provide distance information in the CF/FM bat,Noctilio albiventris: evidence for a gated time window utilizing both CF and FM components

Summary Bats of the speciesNoctilio albiventris, trained to discriminate differences in target distance, emitted pairs of pulses at a rate of 7–10/s, the first a constant frequency (CF) pulse of about 8 ms duration and 75 kHz frequency, followed after about 28 ms by a CF/FM pulse having a 6 ms, 75 kHz CF component that terminates in a 2 ms FM sweep to about 57 kHz.Loud free-running artificial pulses, simulating the bat's natural CF/FM echolocation sound, interfered with distance discrimination at repetition rates exceeding 5/s. Systematic modifications in the temporal and frequency structure of the artificial pulses resulted in orderly changes in the degree of interference. Artificial pulses simulating the natural CF or FM components alone had no effect, nor did 10/s white noise pulses, although constant white noise of the same intensity masked the behavior.Interference occurred when the CF of the artificial pulses was between 52 and 77 kHz, ending with a downward FM sweep of 25 kHz from the CF. For interference to occur there was a much more critical requirement that the FM sweep begin at approximately the frequency of the CF component. The FM sweep needed to be 11 kHz or greater bandwidth. Interference occurred when the duration of the CF component of the CF/FM artificial pulse was between 2 and 30 ms, with maximal effect between 10 and 20 ms. However, a brief (2.0 ms) CF signal 2–27 ms before an isolated FM signal was as effective as a continuous CF component of the same duration.When coupled with the bat's own emissions, artificial CF/FM pulses interfered if they occurred after the bat's CF/FM pulse and before the next natural emission. A 2 ms FM sweep alone was effective in interfering with distance discrimination when it came 8–27 ms after the onset of the bat's own CF/FM pulse. Neither CF/FM nor FM artificial pulses interfered when they began during the bat's own emission. A 10 ms CF pulse alone had no effect at any time.These findings indicate thatN. albiventris uses both the CF and FM components of its short-CF/FM echolocation sound for distance discrimination. The CF onset activates a gating mechanism that, during a narrowly defined subsequent time window, enables the nervous system to process FM pulse-echo pairs for distance information, within a fairly broad frequency range, as long as the frequencies of the CF and the beginning of the FM sweep are nearly identical.Abbreviations CF constant frequency - FM frequency modulation     

Frequency tracking and Doppler shift compensation in response to an artificial CF/FM echolocation sound in the CF/FM bat,Noctilio albiventris

Summary Bats of the speciesNoctilio albiventris emit short-constant frequency/frequency modulated (short-CF/FM) pulses with a CF component frequency at about 75 kHz. Bats sitting on a stationary platform were trained to discriminate target distance by means of echolocation. Loud, free-running artificial pulses, simulating the bat's natural CF/FM echolocation sounds or with systematic modifications in the frequency of the sounds, were presented to the bats during the discrimination trials. When the CF component of the artificial CF/FM sound was between 72 and 77 kHz, the bats shifted the frequency of the CF component of their own echolocation sounds toward that of the artificial pulse, tracking the frequency of the artificial CF component.Bats flying within a large laboratory flight cage were also presented with artificial pulses. Bats in flight lower the frequency of their emitted pulses to compensate for Doppler shifts caused by their own flight speed and systematically shift the frequency of their emitted CF component so that the echo CF frequency returns close to that of the CF component of the artificial CF/FM pulse, over the frequency range where tracking occurs.Abbreviations CF constant frequency - FM frequency modulation     

Harmonic and frequency structure used for echolocation sound pattern recognition and distance information processing in the rufous horseshoe bat

Summary The rufous horseshoe bat, Rhinolophus rouxi, was trained to discriminate differences in target distance. During the discrimination trials, the bats emitted complex FM/CF/FM pulses containing first harmonic and dominant second harmonic components.Loud free running artificial pulses, simulating the CF/FM part of the natural echolocation components, interfered with the ability of the bat to discriminate target distance. Changes in the frequency or frequency pattern of the artificial pulses resulted in systematic changes in the degree of interference. Interference occurred when artificial CF/FM pulses were presented at frequencies near those of the bat's own first or second harmonic components.These findings suggest that Rhinolophus rouxi uses both the first and second harmonic components of its complex multiharmonic echolocation sound for distance discrimination. For interference to occur, the sound pattern of each harmonic component must contain a CF signal followed by an FM sweep beginning near the frequency of the CF.Abbreviations CF constant frequency - FM frequency modulated     

Discrimination of fluttering targets by the FM-batPipistrellus stenopterus?

Summary Five bats of the speciesPipistrellus stenopterus were trained in a two-alternative forced-choice procedure to discriminate between two fluttering targets. The positive target simulated an insect with a 50 Hz wingbeat rate. The negative target was varied between 0 and 48 Hz.The bats were able to discriminate a target with 41 Hz from a target with 50 Hz with 75% correct choices. In the discrimination task, they typically emitted echolocation calls of 2–4 ms duration sweeping from 60 kHz to 30 kHz. The duty cycle (i.e. fraction of time filled with echolocation sounds) increased when the targets fluttered, but was always lower than 3%.The performance ofP. stenopterus in discriminating fluttering targets is comparable to that of bats emitting longer sounds with constant-frequency (CF) components and a higher duty cycle. The FM-sounds ofP. stenopterus are short compared with the period of the fluttering targets, and therefore make it difficult for the animal to measure the time interval between two acoustic glints. Other cues may be prominent, such as the frequency modulation by Doppler shifts from the moving blades.     

Complex sound analysis in the lesser bulldog bat: evidence for a mechanism for processing frequency elements of frequency modulated signals over restricted time intervals

A stereotypical approach phase vocalization response of the lesser bulldog bat, Noctilio albiventris, to artificial echoes simulating a virtual approaching object was used to assess the ability of the bat to analyze and extract distance information from the artificial echoes. The performance of the bat was not significantly different when presented with naturally structured CF/FM echoes containing FM elements that sweep continuously from about 75-55 kHz in 4 ms or with CF/FM echoes containing FM components constructed from a series of 98 pure tone frequency steps, each with a duration of 0.04 ms. The performance of the bat remained unchanged when the duration of the tone steps was increased up to 0.08 ms but declined sharply to a level that was significantly below that seen with a naturally structured echo when the steps were 0.09 ms or longer. The performance of the bat depended on the duration of the individual tone steps, which could not exceed a specific upper limit of about 0.08 ms. The study suggests that the bats have adaptations for processing individual narrow band segments of FM signals over specific time intervals.Abbreviations CF constant frequency - FM frequency modulation     

Discrimination of wingbeat motion by bats,correlated with echolocation sound pattern

Summary Bats of the species Rhinolophus rouxi, Hipposideros lankadiva and Eptesicus fuscus were trained to discriminate between two simultaneously presented artificial insect wingbeat targets moving at different wingbeat rates. During the discrimination trials, R. rouxi, H. lankadiva and E. fuscus emitted long-CF/FM, short-CF/FM and FM echolocation sounds respectively. R. rouxi, H. lankadiva and E. fuscus were able to discriminate a difference in wingbeat rate of 2.7 Hz, 9.2 Hz and 15.8 Hz, respectively, between two simultaneously presented targets at an absolute wingbeat rate of 60 Hz, using a criterion of 75% correct responses.The performance of the different bat species is correlated with the echolocation signal design used by each species, particularly with the presence and relative duration of a narrowband component preceding a broadband FM component. These results provide behavioral evidence supporting the hypothesis that bats that use CF/FM echolocation sounds have adaptations for the perception of insect wingbeat motion and that long-CF/FM species are more specialized for this task than short-CF/FM species.Abbreviations CF constant frequency - FM frequency modulation     

A gating mechanism for sound pattern recognition is correlated with the temporal structure of echolocation sounds in the rufous horseshoe bat

Summary The rufous horseshoe bat, Rhinolophus rouxi, was trained to discriminate differences in target distance. Loud free running artificial pulses, simulating the bat's natural long-CF/FM echolocation sounds, interfered with the ability of the bat to discriminate target distance. Interference occurred when the duration of the CF component of the CF/FM artificial pulse was between 2 and 70 ms. A brief (2.0 ms) CF signal 2–68 ms before an isolated FM signal was as effective as a continuous CF component of the same duration. When coupled with the bat's own emissions, a 2 ms FM sweep alone was effective in interfering when it came 42 to 69 ms after the onset of the bat's pulse. The coupled FM artificial pulses did not interfere when they began during the bat's own emissions.It appears that the onset of the CF component activates a gating mechanism that establishes a time window during which FM component signals must occur for proper neural processing. A comparison with a similar gating mechanism in Noctillo albiventris, which emits short-CF/FM echolocation sounds, reveals that the temporal parameters of the time window of the gating mechanism are species specific and specified by the temporal structure of the echolocation sound pattern of each species.Abbreviations FM frequency modulated - CF constant frequency     

The echolocation and hunting behavior of the bat,Pipistrellus kuhli

Summary The echolocation and hunting behavior ofPipistrellus kuhli was studied in the field using multi-exposure photography synchronized with high-speed tape recordings. During the search phase, the bats used 8–12 ms signals with sweeps (sweep width 3–6 kHz) and pulse intervals near 100 ms or less often near 200 ms (Figs. 1 and 2). The bats seemed to have individual terminal frequencies that could lie between 35 and 40 kHz. The duty cycle of searching signals was about 8%. The flight speed of hunting bats was between 4.0 and 4.5 m/s. The bats reacted to insect prey at distances of about 70 to 120 cm. Given the flight speed, the detection distance was estimated to about 110 to 160 cm. Following detection the bat went into the approach phase where the FM sweep steepened (to about 60 kHz bandwidth) and the repetition rate increased (to about 30 Hz). The terminal phase or buzz, which indicates prey capture (or attempted capture), was composed of two sections. The first section contained signals similar to those in the approach phase except that the pulse duration decreased and the repetition rate increased. The second section was characterized by a sharp drop in the terminal frequency (to about 20 kHz) and by very short pulses (0.3 ms) at rates of up to 200 Hz (Figs. 1 and 3). Near the beginning of the buzz the bat prepared for capturing the prey by extending the wings and forming a tail pouch (Fig. 4). A pause of about 100 ms in sound emission after the buzz indicated a successful capture (Fig. 4). Pulse duration is discussed in relation to glint detection and detection distance. It is argued that the minimum detection distance can be estimated from the pulse duration as the distance where pulse-echo overlap is avoided.Abbreviations CF constant frequency - FM frequency modulated     

Complex sound analysis in the FM bat Eptesicus fuscus,correlated with structural parameters of frequency modulated signals

Big brown bats, Eptesicus fuscus, were presented with artificial frequency modulated (FM) echoes that simulated an object becoming progressively closer to the bat. A stereotyped approach phase behavioral response of the bat to the virtual approaching target was used to determine the ability of the bat to analyze FM signals for target distance information. The degree to which the bats responded with approach phase behavior to a virtual approaching target was similar when they were presented with either a naturally structured artificial FM echo or an artificial FM echo constructed from a series of brief pure tone steps. The ability of the bats to respond to an FM signal structured from a sequence of pure tone elements depended on the number of pure tone steps in the series; the bats required the presentation of tone-step FM signals containing about 83 or greater pure tone elements. Moreover, the duration of the individual tone steps of the tone-step FM signals could not exceed a specific upper limit of about 0.05 ms. Finally, it appears that the bats were able to independently resolve individual tone steps within the tone-step FM signals that were separated by about 450 Hz or more.Abbreviations CF constant frequency - FM frequency modulation     

Frequency modulated sound pattern analysis in the lesser bulldog bat: the role of interactions between adjacent frequency elements of complex sounds

A stereotyped approach phase vocalization response of Noctilio albiventris to artificial echoes simulating a virtual approaching object was used to assess the ability of the bat to analyze and extract distance information from the artificial echoes. The performance of the bats depended on the temporal pattern of frequency change of the continuously sweeping frequency modulated (FM) component of the signals. When the bats were presented with a CF/FM signal containing a time-reversed upward FM sweep, they responded with approach phase behavior at a performance level that was significantly below that seen with a CF/FM signal containing a naturally structured downward FM sweep. When the FM sweep was divided into a series of brief pure tone steps, the extent to which the bats showed a difference in their capability to process upward versus downward FM sweeps depended on the difference in frequency between the pure tone steps. The bats effectively processed downward but not upward FM sweeps when the difference in frequency between pure tone frequency elements of the FM sweeps was from about 100–200 Hz, but they effectually processed both downward and upward FM sweeps when the tonal elements composing the FM sweeps were separated by more than about 200 Hz. This suggests that the ability of the bats to effectively process downward but not upward FM sweeps is based on local interactions between adjacent frequency elements of the complex sounds.Abbreviations CF constant frequency - FM frequency modulated     

Spectral selectivity of FM-FM neurons in the auditory cortex of the echolocating bat,Myotis lucifugus

1. Spectral sensitivity was examined in delay-sensitive neurons in the auditory cortex of the awake FM bat, Myotis lucifugus. FM stimuli sweeping 60 kHz downward in 4 ms were used as simulated pulse-echo pairs to measure delay-dependent responses. At each neuron's best delay, the pulse and/or echo were divided into 4 FM quarters (Ist, IInd, IIIrd, and IVth), each sweeping 15 kHz in 1 ms, and quarters essential for delay sensitivity were determined for both pulse and echo. 2. For the pulse, the IVth quarter was essential for delay sensitivity in the majority of neurons. For the echo, the essential quarter for most neurons was the IInd, IIIrd, or IVth. 3. Different quarters of the pulse and echo were essential for delay sensitivity in 68% of the neurons examined. 4. This study provides neurophysiological evidence linking both spectral and temporal processing in delay-sensitive neurons of Myotis. Since spectral cues can provide target-shape information, sensitivity to both spectral and temporal parameters in single neurons may endow these neurons in FM bats with the potential for target analysis other than echo-ranging.     

Target ranging and the role of time-frequency structure of synthetic echoes in big brown bats,Eptesicus fuscus

Summary Echolocating bats judge the distance to a target on basis of the delay between the emitted cry and the returning echo. In a phantom echo set-up it was investigated how changes in the time-frequency structure of synthetic echoes affect ranging accuracy of big brown bats, Eptesicus fuscus.A one channel phantom target simulator and a Y/N paradigm was used. Five Eptesicus fuscus were trained to discriminate between phantom targets with different virtual distances (delays). The phantom echo was stored in a memory and broadcast from a loudspeaker after a certain delay following the bat's triggering of the system via a trigger microphone. The ranging accuracy was compared using 5 different signals with equal energy as phantom echoes: a standard cry (a natural bat cry), two kinds of noise signals, a high pass, and a low pass filtered version of the standard cry.The standard cry was recorded from one of the bats while judging the distance to a real target. The duration was 1.1 ms, the first harmonic swept down from 55 to 25 kHz and there was energy also in the second and third harmonic. Both noise signals had the same duration, power spectrum, and energy as the standard cry. One noise signal was stored in a memory and hence was exactly the same each time the bat triggered the system. The other variable noise signal was produced by storing the envelope of the standard cry and multiplying on-line with band pass filtered noise. The time-frequency structure (e.g. rise time) of this noise signal changed from triggering to triggering. The filtered signals were produced by either 40 kHz high pass or 40 kHz low pass filtering of the standard cry.The range difference thresholds for the 5 bats were around 1–2 cm (51–119 us) using the standard cry as echo. The range difference threshold with both noise signals was 7–8 cm (around 450 s delay difference). The 40 kHz high pass filtered cry increased the threshold to approximately twice the threshold with the standard cry. With the 40 kHz low pass filtered cry the threshold was increased 2.5–3 times relative to the threshold with the standard cry. A single bat was tested with a signal filtered with a 55 kHz low pass filter leaving the whole first harmonic. The threshold was the same as that with the standard signal.The reduced ranging accuracy with the filtered signals indicates that the full band width of the first harmonic is utilised for ranging by the bats. The substantial reduction in accuracy with the noise signals indicates that not only the full band width but also the orderly time-frequency structure (the FM sweep) of the cry is important for ranging in echolocating bats.Abbreviations FM frequency modulated - CF constant frequency - peSPL peak equivalent sound pressure level - SD standard deviation - SE standard error of mean - EPROM erasable programmable read only memory - FFT fast Fourier transform - S/N signal-to-noise ratio     

Echolocation sounds and hearing of the greater Japanese horseshoe bat (Rhinolophus ferrumequinum nippon)

Summary The relationship between the orientation sounds and hearing sensitivity in the greater Japanese horseshoe bat,Rhinolophus ferrumequinum nippon was studied.An orientation pulse consisted of a constant frequency (CF) component followed by a short downward frequency-modulated (FM) component. Sometimes, an initial upward FM component preceded the CF component. Duration of pulses was about 30 ms and the CF of resting pulses (RF) averaged 65.5 kHz. The best frequency (BF) at the lowest threshold in audiograms as measured by the pinna reflex averaged 66.1 kHz. Audiograms showed remarkable sharp cut-offs on both sides near the BF. The frequency difference between the BF and the RF was about 0.6 kHz, and the RF was always below the BF. The values of RF and BF were characteristically different from those of the European subspecies,Rhinolophus ferrumequinum ferrumequinum.Abbreviations BF best frequency - CF constant frequency - FM frequency modulated - RF resting frequency     

Echolocation by the long-eared bat,Plecotus phyllotis

Summary The echolocating bat,Plecotus phyllotis (Vespertilionidae), uses long-CF/FM and FM sonar sounds in different situations. The CF component in long-CF/FM sounds occurs at 27 kHz and has a duration of 20 to 200 ms. The FM component sweeps down from 24 to 12 kHz, with a prominent second harmonic from 40 to 22 kHz. This second harmonic sweep is interrupted at 28 to 25 kHz, providing a notch in the spectrum of the FM component at the CF frequency. This notch probably permits isolation of CF and FM components in echoes for separate processing, thus avoiding mutual interference with the different kinds of target information the two components convey. The FM component is also used without the CF component as a sonar sound. Two other FM orientation sounds are used when the bat is in a confined space such as a room. One contains only the second and fourth harmonics of the 24 to 12 kHz fundamental sweep, while the other contains only the fifth harmonic. This bat's repertoire of sonar sounds closely matches the hearing capacities of the genus.We thank P.H. Dolkart and W.A. Lavender, of Washington University, and the Nevada State Parks Department for their assistance. This research was supported by Grant # BMS-72-02351-A01 from the National Science Foundation.     

Responses to pure tones and linear FM components of the CF — FM biosonar signal by single units in the inferior colliculus of the mustached bat

The responses of 682 single-units in the inferior colliculus (IC) of 13 mustached bats (Pteronotus parnellii parnellii) were measured using pure tones (CF), frequency modulations (FM) and pairs of CF-FM signals mimicking the species' biosonar signal, which are stimuli known to be essential to the responses of CF/CF and FM-FM facilitation neurons in auditory cortex. Units were arbitrarily classified into 'reference frequency' (RF), 'FM2' and 'Non-echolocation' (NE) categories according to the relationship of their best frequencies (BF) to the biosonar signal frequencies. RF units have high Q10dB values and are tuned to the reference frequency of each bat, which ranged between 60.73 and 62.73 kHz. FM2 units had BF's between 50 and 60 kHz, while NE units had BF's outside the ranges of the RF and FM2 classes. PST histograms of the responses revealed discharge patterns such as 'onset', 'onset-bursting' (most common), 'on-off', 'tonic-on','pauser', and 'chopper'. Changes in discharge patterns usually resulted from changes in the frequency and/or intensity of the stimuli, most often involving a change from onset-bursting to on-off. Different patterns were also elicited by CF and FM stimuli. Frequency characteristics and thresholds to CF and FM stimuli were measured. RF neurons were very sharply tuned with Q10dB's ranging from 50-360. Most (92%) also responded to FM2 stimuli, but 78% were significantly more sensitive (greater than 5 dB) to CF stimuli, and only 3% had significantly lower thresholds to FM2. The best initial frequency for FM2 sweeps in RF units was 65.35 +/- 2.138 kHz (n = 118), well above the natural frequency of the 2nd harmonic. FM2 and NE units were indistinguishable from each other, but were quite different from RF units: 41% of these two classes had lower thresholds to CF, 49% were about equally sensitive, and 10% had lower thresholds to FM. For FM2 units, mean best initial frequency for FM was 60.94 kHz +/- 3.162 kHz (n = 114), which is closely matched to the 2nd harmonic in the biosonar signal. Very few units (5) responded only to FM signals, i.e., were FM-specialized. The characteristics of spike-count functions were determined in 587 units. The vast majority (79%) of RF units (n = 228) were nonmonotonic, and about 22% had upper-thresholds.(ABSTRACT TRUNCATED AT 400 WORDS)     

Echolocation by free-tailed bats (Tadarida)

Summary The echolocation of bats in the genusTadarida is highly adaptive to different acoustic conditions. These bats use different types of sonar signals with a diversity usually observed in comparisons across families of bats.Tadarida brasiliensis andT. macrotis search for airborne prey in open, uncluttered spaces using narrow-band, short CF signals with no FM components. They add broadband FM components while dropping the CF components when approaching or capturing prey. Only one harmonic is present in these insect-pursuit signals. When flying in cluttered situations or echolocating in a laboratory room,T. brasiliensis uses multiple-harmonic FM signals. Stationary bats tend to use linear frequency sweeps and moving bats tend to use curvilinear frequency sweeps or linear period sweeps. When emerging from a roost they initially use a short-CF/FM signal, changing to an FM signal as they fly away. The acuity of perception of target range inT. brasiliensis is about 1.0 to 1.5 cm and is determined by the bandwidth of the target-ranging sonar signals as represented by their autocorrelation functions. Many less adaptable species of bats use signals corresponding to part of the sonar repertoire ofTadarida. The functions of short CF or narrowband signals for detection and FM or broadband signals for resolution and acoustic imaging identified from comparisons among such species are confirmed by observations of echolocation byTadarida. The differences observed in echolocation among many species and families of bats appear to be evolutionary adaptations to some of the same features of the acoustic environment to whichTadarida responds behaviorally.Abbreviations CF frequency modulated - FM constant frequency - LPM linear period modulation - LFM linear-frequency modulation We thank Prof. T.T. Sandel, Prof. D.R. Griffin, Dr. George Pollak, and P.H. Dolkart for their advice and assistance. This research was supported by Grant No. BMS 72-02351-A01 from the National Science Foundation and by Biomedical Research Support Grant No. RR-07054 from the Division of Research Resources, National Institutes of Health.     

Role of broadcast harmonics in echo delay perception by big brown bats

Big brown bats (Eptesicus fuscus) emit frequency-modulated (FM) echolocation sounds containing two principal down-sweeping harmonics (FM₁ ~ 55–25 kHz, FM₂ ~ 105–50 kHz). To determine whether each harmonic contributes to perception of echo delay, bats were trained to discriminate between “split-harmonic” echoes that differed in delay. The bat’s broadcasts were picked up with microphones, and FM₁ and FM₂ were separated with highpass and lowpass filters at about 55 kHz, where they overlap in frequency. Both harmonics then were delivered from loudspeakers as positive stimuli in a 2-choice delay discrimination procedure with FM₁ delayed 3.16 ms and FM₂ delayed 3.46 ms (300 μs delay split). Negative stimuli contained FM₁ and FM₂ with the same filtering but no delay separation. These were presented at different overall delays from 11 down to 3 ms to measure the bat’s delay discrimination acuity for each harmonic in the split harmonic echoes. The bats determined the delays of both FM₁ and FM₂, but performance was overlaid by a broad pedestal of poor performance that extended for 800 μs. Splitting the harmonics by 300 μs appears to defocus the bat’s representation of delay, revealing the existence of a process for recognizing the normally simultaneous occurrence of the harmonics.     

Size, peripheral auditory tuning and target strength in noctuid moths

We investigated relationships among body size, the frequency of peak auditory sensitivity (best frequency) and acoustic conspicuousness (measured as target strength) to simulated bat echolocation calls in a range of tympanate moths (Lepidoptera: Noctuidae). Audiograms of Amphipyra pyramidea Linnaeus, Agrotis exclamationis Linnaeus, Omphaloscelis lunosa Haworth and Xestia xanthographa Denis and Schiffermüller are described for the first time. Best frequency was inversely related to forewing length, an index of body size. Models predict that target strength falls off rapidly once wavelength (1/frequency) exceeds some defined feature of target size (e.g. circumference for spheres). We investigated how target strength varies in relation to target size and emitted frequency for simple targets (paper discs) and for moths. Target strength fell rapidly when target radius/wavelength < 2 for paper discs of similar size to many noctuid moths. Target strength fell rapidly below wing‐length/wavelength ratios of 2 in relatively small (O. lunosa, wing‐length = 15.2 ± 0.4 mm, best frequency = 45 kHz) and large (N. pronuba, wing‐length = 24.6 ± 0.8 mm, best frequency = 15 kHz) noctuid species, and decreased rapidly at frequencies below 25 kHz in both species. These target strengths were used to predict the detection distance of the moths by bat sonar between 10 and 55 kHz. Predicted detection distances of both species were maximal for fictive call frequencies of 20 kHz, and were reduced at lower frequencies due to decreased target strength and at higher frequencies by excess atmospheric attenuation. Both relatively large and small noctuid moths are therefore strong acoustic targets to bats that echolocate at relatively low frequencies. Bats may emit allotonic calls at low frequency because the costs of reduced detection range are smaller than the benefits of reduced audibility to moths. Because best frequency scales with body size and maximum detection distance is not very sensitive to body size, noctuid moths in the size range examined do not necessarily have best frequencies that would match the call frequencies of bats that may detect the moths at greatest distance precisely. Hence, best frequency may be constrained in part by body size.     

A neural mechanism for detecting the distance of a selected target by modulating the FM sweep rate of biosonar in echolocation of bat

Most species of bats making echolocation use frequency modulated (FM) ultrasonic pulses to measure the distance to targets. These bats detect with a high accuracy the arrival time differences between emitted pulses and their echoes generated by targets. In order to clarify the neural mechanism for echolocation, we present neural model of inferior colliculus (IC), medial geniculate body (MGB) and auditory cortex (AC) along which information of echo delay times is processed. The bats increase the downward frequency sweep rate of emitted FM pulse as they approach the target. The functional role of this modulation of sweep rate is not yet clear. In order to investigate the role, we calculated the response properties of our models of IC, MGB, and AC changing the target distance and the sweep rate. We found based on the simulations that the distance of a target in various ranges may be encoded the most clearly into the activity pattern of delay time map network in AC, when the sweep rate of FM pulse used is coincided with the observed value which the bats adopt for each range of target distance.