Detection of frequency modulation in the FM-bat Phyllostomus discolor

In a two-alternative forced-choice procedure lesser spear-nosed bats, Phyllostomus discolor, had to discriminate between a pure tone stimulus and a sinusoidally frequency-modulated signal generated at the same carrier frequency as the tone. Modulation depths of the SFM stimuli were reduced until the animals' performance dropped below the 75%-correct level which was used to determine difference limens for detection of frequency modulation (FMDL). The dependence of FMDLs on modulation and carrier frequency was systematically investigated. For a carrier frequency of 18.5 kHz, average FMDLs increased from 95 Hz at a modulation frequency of 10 Hz to 820 Hz at a modulation frequency of 2000 Hz which corresponds to Weber ratios (2f/f) of 0.005 and 0.044 respectively. Further, difference limens were found to increase linearly in proportion to carrier frequency throughout a major part (9–74 kHz) of the species' hearing range. In comparison to other mammals, P. discolor has a pronounced capability for frequency discrimination which might be related to the extensive use of individually distinct frequency-modulated communication calls and audio-vocal learning.Abbreviations FM frequency modulation - SFM sinusoidal frequency modulation - FMDL frequency modulation difference limen - CF constant frequency - DLF difference limen for frequency - VCO voltage-controlled oscillator - SPL sound pressure level - WR Weber ratio     

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     

Frequency discrimination threshold at search call frequencies in the echolocating bat, Eptesicus fuscus

While searching for prey in open spaces, Epteisicus fuscus emits long-duration, downward frequency-modulated calls which cover a frequency band of about 28-22 kHz. In the ascending auditory pathways of E. fuscus, neurons tuned to these search call frequencies are characterised by a remarkably high frequency selectivity and very sensitive absolute thresholds. We investigated whether this narrow tuning is reflected in an exceptional psychoacoustic frequency discrimination ability. The average frequency difference limen of E. fuscus at search call frequencies determined in a two-alternative, forced-choice experiment amounted to about 420 Hz, corresponding to a Weber ratio of 0.017. This value is similar to those found in non-echolocating mammals, and an order of magnitude larger than the frequency difference limens of bats emitting constant-frequency call components. We discuss these differences in frequency difference limen, and relate them to different echolocation strategies.     

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     

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     

FM signals produce robust paradoxical latency shifts in the bat’s inferior colliculus

Previous studies in echolocating bats, Myotis lucifugus, showed that paradoxical latency shift (PLS) is essential for neural computation of target range and that a number of neurons in the inferior colliculus (IC) exhibit unit-specific PLS (characterized by longer first-spike latency at higher sound levels) in response to tone pulses at the unit’s best frequency. The present study investigated whether or not frequency-modulated (FM) pulses that mimic the bat’s echolocation sonar signals were equally effective in eliciting PLS. For two-thirds of PLS neurons in the IC, both FM and tone pulses could elicit PLS, but only FM pulses consistently produced unit-specific PLS. For the remainder of PLS neurons, only FM pulses effectively elicited PLS; these cells showed either no PLS or no response, to tone pulses. PLS neurons generally showed more pronounced PLS in response to narrow-band FM (each sweeping 20 kHz in 2 ms) pulse that contained the unit’s best frequency. In addition, almost all PLS neurons showed duration-independent PLS to FM pulses, but the same units exhibited duration-dependent PLS to tone pulses. Taken together, when compared to tone pulses, FM stimuli can provide more reliable estimates of target range.     

Characteristics of borderline modulation frequencies in the differentiation of tonal and amplitude-modulated stimuli following the removal of the inferior colliculus in rats

Elaboration of differentiation between sound stimuli was carried out in 15 laboratory rats. After bilateral ablations of auditory inferior colliculi the border frequency of stimulus amplitude modulation was determined for all rats when they still could differentiate between tonal and amplitude-modulated stimuli. Decrease in frequency of modulation by 2 Hz and more from the border frequency caused a complete loss of ability to differentiate. In all rats bilateral inferior colliculi ablations completely disturbed differentiation between tonal and amplitude-modulated signals with modulation frequency below 183-191 Hz (the range of border frequencies). The surgery however did not affect differentiation between tonal and amplitude-modulated signals with the modulation frequencies above 183-191 Hz. The data suggest that the functions of completion of coding of amplitude-modulated stimuli in the auditory system is strictly linked with definite structures.     

Neuronal reactions of the auditory cortex to sounds of various frequencies in rabbits in ontogeny

In 7-12-day rabbits, the auditory neurones respond to a narrow range of tonal signals of low frequency (500 Hz-4 KHz), but do not change at all, or only slightly change their parameters during the increase in stimulation frequency. The range of frequencies increases with age; the number of impulses in a discharge and the duration of the latent period are monotonously dependent on the frequency of a signal. At the end of the 4th week, the auditory neurones respond by non-monotonous patterns of impulsation and latent period to changes in tonal characteristic of the signal.     

Reaction of neurons of the medial geniculate body to acoustic stimulation

Responses of medial geniculate body (MGB) neurons to pure tones and clicks were studied in acute experiments in immobilized cats, preliminary operations being performed under calypsol anaesthesia. MGB units were identified by their reactions to cortical zone AI and brachium of inferior colliculus stimulations. When tonal stimuli were applied relay neurons of pars principalis of MGB usually demonstrated either unimodal tuning curves with narrow frequency band or fragmental ones with several narrow bands. On-response with subsequent inhibition of the background activity or without such an inhibitory period was most frequent type of the reaction (66.6%) of relay MGB neurons to tonal stimulation. The group of relay neurons with the tonic type of reaction (9.1%) was classified for which the duration of tonic response depends on the duration of tonal stimulus. Change of the excitatory reaction to the inhibitory one when the characteristic tone frequency is changed by non-characteristic++ ones is supposed to be a mechanism supplying sharpness of tuning at relay MGB neurons. It is concluded that responses of acoustic cortical neurons to sound stimulation depend to a great extent on the pattern of impulsation that comes from MGB relay units.     

Dynamic properties of Renshaw cells: Frequency response characteristics

The dynamic properties of Renshaw cells located in the lumbar spinal cord of intercollicular decerebrate cats were measured. The responses of these interneurones were recorded extracellularly, while the ventral root was stimulated with sinusoidally frequency-modulated trains of electrical pulses. The frequency of the Renshaw cell discharges resulting from such stimulation varied sinusoidally. The amplitude of modulation about the average (or carrier) rate of discharge exhibited a linear dependence on the modulation amplitude of the stimulus pulse train. Renshaw cells were able to follow modulated stimulus trains in the entire range of modulation frequencies (0.2 to 80 Hz) encompassed by the present study. Above modulation frequencies between 20 and 50 Hz, the amplitude of modulation of the responses declined. Frequency responses measured at low average frequencies of the stimulus pulse train (centre frequencies 30 and 40 Hz) showed comparatively little dependence on modulation frequency. The higher the centre frequency, however, the greater was the enhancement of the modulation amplitudes at high modulation frequencies compared with those observed at low modulation frequencies. Some aspects of the functional implications of these results are considered and an approximate formula for the transfer function of Renshaw cells is presented.     

Spatial hearing in Cope’s gray treefrog: II. Frequency-dependent directionality in the amplitude and phase of tympanum vibrations

Anuran ears function as pressure difference receivers, and the amplitude and phase of tympanum vibrations are inherently directional, varying with sound incident angle. We quantified the nature of this directionality for Cope’s gray treefrog, Hyla chrysoscelis. We presented subjects with pure tones, advertisement calls, and frequency-modulated sweeps to examine the influence of frequency, signal level, lung inflation, and sex on ear directionality. Interaural differences in the amplitude of tympanum vibrations were 1–4 dB greater than sound pressure differences adjacent to the two tympana, while interaural differences in the phase of tympanum vibration were similar to or smaller than those in sound phase. Directionality in the amplitude and phase of tympanum vibration were highly dependent on sound frequency, and directionality in amplitude varied slightly with signal level. Directionality in the amplitude and phase of tone- and call-evoked responses did not differ between sexes. Lung inflation strongly affected tympanum directionality over a narrow frequency range that, in females, included call frequencies. This study provides a foundation for further work on the biomechanics and neural mechanisms of spatial hearing in H. chrysoscelis, and lends valuable perspective to behavioral studies on the use of spatial information by this species and other frogs.     

Water wave discrimination in the surface-feeding fish Aplocheilus lineatus

The surface-feeding fish Aplocheilus lineatus uses its cephalic lateral line to detect water surface waves caused by prey insects. The ability of Aplocheilus to discriminate between surface waves with aid of the lateral line system was tested by go/no-go conditioning. Our results show that Aplocheilus can distinguish between single-frequency surface wave stimuli with equal velocity or equal acceleration amplitudes which differ only in frequency. Frequency difference limens were about 15%, i.e. fish distinguished a 20-Hz wave stimulus from a 23-Hz stimulus in 100% of the trials. Aplocheilus can also discriminate between pure sine-wave stimuli and sine waves which show abrupt frequency changes. In contrast, fish were unable to distinguish amplitude-modulated wave stimuli (carrier frequency 20, 40 and 60 Hz, modulation frequency 10 and 20 Hz) from pure sine waves of the same frequency, even if amplitude modulation depth was 80%. Accepted: 27 December 1996     

Amplitude modulation thresholds for the parakeet (Melopsittacus undulatus)

Summary Parakeets were tested for the ability to detect sinusoidal amplitude modulation of broad band noise. Instrumental avoidance conditioning and a psychophysical modified method of limits procedure were used to measure the threshold for detecting amplitude modulation at 10 modulation frequencies between 2 and 2,048 Hz. Below about 40 Hz, modulation threshold is independent of modulation rate and noise level. Above 40 Hz, modulation threshold decreases with modulation frequency at the rate of 3 dB/ octave. These results are somewhat different from amplitude modulation functions in humans suggesting different degrees of temporal resolving power in birds and humans. Thresholds for changes in modulation rate are 1–2 orders of magnitude higher than pure tone frequency difference limens.We thank Frank Cusimano, Ann Huessener, Susan Peters, Roberta Pickert, Bill Searcy, Ken Yasukawa and Tim DeVoogd for participating as subjects, and Dick Fay for providing critical comments. This research was supported by grant No. PHS MH31165 from the National Institute of Mental Health to the first author.     

Tonotopical organization and pure tone response characteristics of single units in the auditory cortex of the Greater Horseshoe Bat

Summary Tonotopical organization and frequency representation in the auditory cortex of Greater Horseshoe Bats was studied using multi-unit recordings.The auditory responsive cortical area can be divided into a primary and a secondary region on the basis of response characteristics forming a core/belt structure.In the primary area units with best frequencies in the range of echolocation signals are strongly overrepresented (Figs. 6–8). There are two separate large areas concerned with the processing of the two components of the echolocation signals. In one area frequencies between the individual resting frequency and about 2 kHz above are represented, which normally occur in the constant frequency (CF) part of the echoes (CF-area), in a second one best frequencies between resting frequency and about 8 kHz below are found (FM-area).In the CF-area tonotopical organization differs from the usual mammalian scheme of dorso-ventral isofrequency slabs. Here isofrequency contours are arranged in a semicircular pattern.The representation of the cochlear partition (cochleotopic organization) was calculated. In the inferior colliculus and auditory cortex there is a disproportionate representation of the basilar membrane. This finding is in contradiction to the current opinion that frequency representation in the auditory system of Horseshoe Bats is only determined by the mechanical tuning properties of the basilar membrane.Response characteristics for single units were studied using pure tone stimuli. Most units showed transient responses. In 25% of units response characteristics depended on the combination of frequency and sound pressure level used.Frequency selectivity of units with best frequencies in the range of echolocation sounds is very high. Q-10dB values of up to 400 were found in a small frequency band just above resting frequency.Abbreviations BF best frequency - CF constant frequency - FM frequency modulated - MT minimal threshold     

Detection of low‐frequency tones and whale predator sounds by the American sand lance Ammodytes americanus

Auditory evoked potentials (AEP) were used to measure the hearing range and auditory sensitivity of the American sand lance Ammodytes americanus. Responses to amplitude‐modulated tone pips indicated that the hearing range extended from 50 to 400 Hz. Sound pressure thresholds were lowest between 200 and 400 Hz. Particle acceleration thresholds showed an improved sensitivity notch at 200 Hz but not substantial differences between frequencies and only a slight improvement in hearing abilities at lower frequencies. The hearing range was similar to Pacific sand lance Ammodytes personatus and variations between species may be due to differences in threshold evaluation methods. AEPs were also recorded in response to pulsed sounds simulating humpback whale Megaptera novaeangliae foraging vocalizations termed megapclicks. Responses were generated with pulses containing significant energy below 400 Hz. No responses were recorded using pulses with peak energy above 400 Hz. These results show that A. americanus can detect the particle motion component of low‐frequency tones and pulse sounds, including those similar to the low‐frequency components of megapclicks. Ammodytes americanus hearing may be used to detect environmental cues and the pulsed signals of mysticete predators.     

Autonomic cardiovascular control in conscious mice

Autonomic cardiovascular control was characterized in conscious, chronically catheterized mice by spectral analysis of arterial pressure (AP) and heart rate (HR) during autonomic blockade or baroreflex modulation of autonomic tone. Both spectra were similar to those obtained in humans, but at approximately 10x higher frequencies. The 1/f relation of the AP spectrum changed to a more shallow slope below 0.1-0.2 Hz. Coherence between AP and HR reached 0.5 or higher below 0.3-0.4 Hz and also above 2.5 Hz. Muscarinic blockade (atropine) or beta-adrenergic blockade (atenolol) did not significantly affect the AP spectrum. Atropine reduced HR variability at all frequencies, but this effect waned above 1 Hz. beta-Adrenergic blockade (atenolol) slightly enhanced the HR variability only above 1 Hz. alpha-Adrenergic blockade (prazosin) reduced AP variability between 0.05 and 3 Hz, most prominently at 0. 15-0.7 Hz. A shift of the autonomic nervous tone by a hypertensive stimulus (phenylephrine) enhanced, whereas a hypotensive stimulus (nitroprusside) depressed AP variability at 1-3 Hz; other frequency ranges of the AP spectrum were not affected except for a reduction below 0.4 Hz after nitroprusside. Variability of HR was enhanced after phenylephrine at all frequencies and reduced after nitroprusside. As with atropine, the reduction with nitroprusside waned above 1 Hz. In conclusion, in mice HR variability is dominated by parasympathetic tone at all frequencies, during both blockade and physiological modulation of autonomic tone. There is a limitation for further reduction but not for augmentation of HR variability from the resting state above 1 Hz. The impact of HR on AP variability in mice is confined to frequencies higher than 1 Hz. Limits between frequency ranges are proposed as 0.15 Hz between VLF (very low frequency range) and LF (low frequency range) and 1.5 Hz between LF and HF (high frequency range).     

Frequency selectivity of hearing in the green treefrog,Hyla cinerea

Frequency selectivity of hearing was measured in the green treefrog, Hyla cinerea. A psychophysical technique based on reflex modification was used to obtain masked threshold estimates for pure tones (300-5,400 Hz) presented against two levels of broadband masking noise. A pure tone (S-1) presented 200 ms prior to a reflex-eliciting stimulus (S-2) inhibited the motor reflex response to S-2. The magnitude of this reflex modification effect varied systematically with the sound pressure level (SPL) of S-1, and threshold was defined as the SPL of S-1 at which the reflex modification effect disappeared. Masked thresholds were used to calculate critical ratios, an index of the auditory system's frequency selectivity. The frequency selectivity of the treefrog's hearing is greatest and critical ratios are lowest (22-24 dB) at about 900 and 3,000 Hz, the two spectral regions dominant in the male treefrog's species-specific advertisement call. These results suggest that the treefrog's auditory system may be specialized to reject noise at biologically-relevant frequencies. As in other vertebrates, critical ratios remain constant when background noise level is varied; however, the shape of the treefrog's critical ratio function across frequencies differs from the typical vertebrate function that increases with increasing frequency at a slope of about 3 dB/octave. Instead, the treefrog's critical ratio function resembles its pure tone audiogram. Although the shape of the treefrog's critical ratio function is atypical, the critical ratio values themselves are comparable to those of many other vertebrates in the same frequency range. Critical ratio values here measured behaviorally do not match critical ratio values previously measured physiologically in single eighth nerve fibers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pupillary dilation response as an indicator of auditory discrimination in the barn owl

The pupil of an awake, untrained, head-restrained barn owl was found to dilate in response to sounds with a latency of about 25 ms. The magnitude of the dilation scaled with signal-to-noise ratio. The dilation response habituated when a sound was repeated, but recovered when stimulus frequency or location was changed. The magnitude of the recovered response was related to the degree to which habituating and novel stimuli differed and was therefore exploited to measure frequency and spatial discrimination. Frequency discrimination was examined by habituating the response to a reference tone at 3 kHz or 6 kHz and determining the minimum change in frequency required to induce recovery. We observed frequency discrimination of 125 Hz at 3 kHz and 250 Hz at 6 kHz – values comparable to those reported by others using an operant task. Spatial discrimination was assessed by habituating the response to a stimulus from one location and determining the minimum horizontal speaker separation required for recovery. This yielded the first measure of the minimum audible angle in the barn owl: 3° for broadband noise and 4.5° for narrowband noise. The acoustically evoked pupillary dilation is thus a promising indicator of auditory discrimination requiring neither training nor aversive stimuli. Accepted: 28 February 2000