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.     

Discrimination of sinusoidally frequency-modulated sound signals mimicking species-specific communication calls in the FM-bat Phyllostomus discolor

In the lesser spear-nosed bat, Phyllostomus discolor, maternal directive calls are characterized by an individual type of sinusoidal frequency modulation (= SFM) pattern. Beside modulation frequency, modulation depth, carrier frequency, and number of modulation cycles per call contribute to the mother's vocal signature. Since juvenile P. discolor learn to adapt their isolation calls to the corresponding call characteristics of the own mother or even to playback of a computer-stored directive call, if hand-reared in the absence of conspecifics, the bats' auditory system ought to be able to resolve interindividual differences in communication call structure. However, quantitative psychoacoustic data on the discrimination of SFM signals in this species are not available. Thus, in the present study, lesser spear-nosed bats were trained in a two-alternative forced-choice procedure to discriminate between two alternatingly presented SFM sound signals differing in modulation frequency. Other characteristics of acoustic stimuli were identical and designed to mimick the fundamental of species-specific calls. By gradually reducing the difference in modulation frequency between both stimuli within the behavioural relevant range until the animals' performance dropped below the 75%-correct level, a considerable auditory spectro-temporal resolution has been revealed. Particularly in comparison to the overall interindividual variation of this call parameter (minimal modulation frequency = 49 Hz, maximum = 100 Hz), the determined average difference limen for modulation frequency of 2.42 ± 0.29 Hz seems substantial and sufficient for labelling individuals. Accepted: 30 November 1996     

Frequency discrimination of brief tonal steps as a function of frequency in the lesser bulldog bat

In a two-alternative, forced-choice task lesser bulldog bats were trained to distinguish between a pure tone pulse and a pulse composed of a series of brief tonal steps oscillating between two different frequencies. The tone-step pulse gradually approximates the pure tone pulse as the frequency difference between the steps becomes progressively smaller. Frequency difference limens for the brief tonal frequency steps were determined for a broad range of ultrasonic frequencies. The variation in tone-step difference limens with frequency appears to be correlated to the frequency structure of the bat's short-constant-frequency/frequency-modulated echolocation sound. There was a marked decline in the value of the relative frequency difference limens (Weber ratio) over a fairly narrow range of frequencies above the constant frequency and a sharp increase in threshold above this range. The relative thresholds for frequency discrimination were small and uniform over the frequency range of the frequency-modulated sweep and increased for frequencies below the frequency- modulated sweep. Thus, the most accurate frequency-discrimination abilities occur over a narrow frequency range around the frequency of the constant-frequency component of returning echoes. Frequency discrimination over the range of frequencies of the frequency-modulated component is relatively good. Accepted: 20 March 1999     

Hearing in the FM-bat Phyllostomus discolor: a behavioral audiogram

Absolute auditory thresholds of six adult lesser spear-nosed bats Phyllostomus discolor (Chiroptera, Phyllostomidae) were determined in a two-alternative forced-choice procedure. Behavioral responses to pure tone stimuli could be elicited throughout the tested frequency range of 5–142 kHz. The shape of the average audiogram is characterized by two sensitivity peaks and a pronounced increase of thresholds around 55 kHz, and towards the limits of the tested frequency range. The spectral extent of both sensitivity peaks shows a close relation to the bandwidth of two types of species-specific vocalizations. The first low threshold area (> 10 and < 55 kHz) of the audiogram seems perfectly adapted to the directive call used for intraspecific communication, whereas the second sensitivity peak, centered around 85 kHz, covers most of the bandwidth of the species' echolocation calls.Abbreviations CF constant frequency - FM frequency modulation - l left - r right - SPL Sound pressure level     

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     

Perceptual interaction between carrier periodicity and amplitude-modulation in the gerbil (<Emphasis Type="Italic">Meriones unguiculatus</Emphasis>)

Due to its extended low-frequency hearing, the Mongolian gerbil (Meriones unguiculatus) has become a well-established animal model for human auditory processing. Here, two experiments are presented which quantify the gerbil’s sensitivity to amplitude modulation (AM) and carrier periodicity (CP) in broad-band stimuli. Two additional experiments investigate a possible interaction of the two types of periodicity. The results show that overall sensitivity to AM and CP is considerably less than in humans (by at least 10 dB). The gerbil’s amplitude-modulation sensitivity is almost independent of modulation frequency up to a modulation frequency of 1 kHz. Above, amplitude-modulation sensitivity deteriorates dramatically. On the basis of individual animals, carrier-periodicity detection may improve with increasing fundamental frequency up to about 500 Hz or may be independent of fundamental frequency. Amplitude-modulation thresholds are consistent with the hypothesis that intensity difference limens in the gerbil may be considerably worse than in humans, leading to the relative insensitivity for low modulation frequencies. Unlike in humans, inner-ear filtering appears not to limit amplitude-modulation sensitivity in the gerbil. Carrier-periodicity sensitivity changes with fundamental frequency similar to humans. Unlike in humans, there is no systematic interaction between AM and CP in the gerbil. This points to a relatively independent processing of the perceptual cues associated with AM and CP.     

Stream Segregation in the Perception of Sinusoidally Amplitude-Modulated Tones

Amplitude modulation can serve as a cue for segregating streams of sounds from different sources. Here we evaluate stream segregation in humans using ABA- sequences of sinusoidally amplitude modulated (SAM) tones. A and B represent SAM tones with the same carrier frequency (1000, 4000 Hz) and modulation depth (30, 100%). The modulation frequency of the A signals (f_modA) was 30, 100 or 300 Hz, respectively. The modulation frequency of the B signals was up to four octaves higher (Δf_mod). Three different ABA- tone patterns varying in tone duration and stimulus onset asynchrony were presented to evaluate the effect of forward suppression. Subjects indicated their 1- or 2-stream percept on a touch screen at the end of each ABA- sequence (presentation time 5 or 15 s). Tone pattern, f_modA, Δf_mod, carrier frequency, modulation depth and presentation time significantly affected the percentage of a 2-stream percept. The human psychophysical results are compared to responses of avian forebrain neurons evoked by different ABA- SAM tone conditions [1] that were broadly overlapping those of the present study. The neurons also showed significant effects of tone pattern and Δf_mod that were comparable to effects observed in the present psychophysical study. Depending on the carrier frequency, modulation frequency, modulation depth and the width of the auditory filters, SAM tones may provide mainly temporal cues (sidebands fall within the range of the filter), spectral cues (sidebands fall outside the range of the filter) or possibly both. A computational model based on excitation pattern differences was used to predict the 50% threshold of 2-stream responses. In conditions for which the model predicts a considerably larger 50% threshold of 2-stream responses (i.e., larger Δf_mod at threshold) than was observed, it is unlikely that spectral cues can provide an explanation of stream segregation by SAM.     

Processing of frequency-modulated stimuli in the chick auditory cortex analogue: evidence for topographic representations and possible mechanisms of rate and directional sensitivity

Summary Responses of units in the auditory forebrain (field L/hyperstriatum ventrale-complex) of awake domestic chicks were studied to frequency-modulated (FM) signals and isointensity tone bursts, presented to the ear contralateral to the recording sites. FM signals, linear frequency sweeps in the range of 50 Hz to 10.25 kHz, differed in the rate of change of frequency (RCF) and in the direction of modulation. The majority of RCF response functions obtained could be classified as predominantly ascending and bell shaped. Best rates of change of frequency (BRCFs), assigned to these functions, covered a range of nearly 3 orders of magnitude. BRCFs of the same units for upward (positive BRCFs) and for downward modulations (negative BRCFs) were correlated. The lowest BRCF encountered among all units for a given isointensity ON-response bandwidth (F _on ) increased as a function of F _on . F _on was derived from the responses to tone bursts of various frequencies at 70 dB SPL. As F_ON tended to increase with the best frequency (BF) of units the lowest BRCF encountered among all units for a given BF also increased as a function of BF. Positive and negative BRCFs of a unit were also correlated with the slopes of onset latency-frequency relationships below and above BF, respectively. FM responses were optimal, when the frequency-specific latency differences at a given unit were compensated by the direction and rate of frequency change in the signal. FM-directional sensitivity varied with BF. Most units with BFs below about 2 kHz preferred upward modulations, while those with BFs above 2 kHz preferred downward modulations. Directional preference and sensitivity correlated with asymmetric distributions of inhibitory sidebands around BF, as derived from the analysis of OFF-responses. Maximum directional sensitivity for a given BRCF increased with BRCF. BRCF and FM-directional sensitivity were topographically organized on neuronal planes harboring units with similar BFs (isofrequency planes). Highest BRCFs were observed in the input-layer L₂ of field L. BRCF declined along a rostrocaudal isofrequency axis in all 4 subdivisions of the auditory forebrain. Similarly, response strength shifted from rostral to caudal as a function of RCF. FM-directional sensitivity was organized in a subdivision-specific fashion. Units in the input-layer of field L (L₂), and even more so in the hyperstriatum ventrale, were fairly insensitive to the direction of modulation, whereas units in the postsynaptic layers of field L (L₁ and L₃) exhibited higher degrees of directional sensitivity. Directional sensitivity also declined along the rostrocaudal isofrequency axis of field L. Two simple models of connectivity in the chick auditory forebrain are presented, which could be sufficient to explain these results. One is based on a tonotopic arrangement of afferent synapses on dendrites and somata of units in L₂, the other on local lateral inhibition in the postsynaptic layers of field L.Abbreviations BF best frequency (kHz) - BRCF best rate of change of frequency (kHz/s) - DS index of FM-directional sensitivity - F _on ON-response bandwidth (kHz) - F _off OFF-response bandwidth (kHz) - FM frequency modulation - RCF rate of change of frequency (kHz/s)     

Paired associative stimulation of the auditory system: a proof-of-principle study

Background

Paired associative stimulation (PAS) consisting of repeated application of transcranial magnetic stimulation (TMS) pulses and contingent exteroceptive stimuli has been shown to induce neuroplastic effects in the motor and somatosensory system. The objective was to investigate whether the auditory system can be modulated by PAS.

Methods

Acoustic stimuli (4 kHz) were paired with TMS of the auditory cortex with intervals of either 45 ms (PAS(45 ms)) or 10 ms (PAS(10 ms)). Two-hundred paired stimuli were applied at 0.1 Hz and effects were compared with low frequency repetitive TMS (rTMS) at 0.1 Hz (200 stimuli) and 1 Hz (1000 stimuli) in eleven healthy students. Auditory cortex excitability was measured before and after the interventions by long latency auditory evoked potentials (AEPs) for the tone (4 kHz) used in the pairing, and a control tone (1 kHz) in a within subjects design.

Results

Amplitudes of the N1-P2 complex were reduced for the 4 kHz tone after both PAS(45 ms) and PAS(10 ms), but not after the 0.1 Hz and 1 Hz rTMS protocols with more pronounced effects for PAS(45 ms). Similar, but less pronounced effects were observed for the 1 kHz control tone.

Conclusion

These findings indicate that paired associative stimulation may induce tonotopically specific and also tone unspecific human auditory cortex plasticity.     

Prepulse Inhibition of Acoustic Startle Reflex as a Function of the Frequency Difference between Prepulse and Background Sounds in Mice

Background

Prepulse inhibition (PPI) depicts the effects of a weak sound preceding strong acoustic stimulus on acoustic startle response (ASR). Previous studies suggest that PPI is influenced by physical parameters of prepulse sound such as intensity and preceding time. The present study characterizes the impact of prepulse tone frequency on PPI.

Methods

Seven female C57BL mice were used in the present study. ASR was induced by a 100 dB SPL white noise burst. After assessing the effect of background sounds (white noise and pure tones) on ASR, PPI was tested by using prepulse pure tones with the background tone of either 10 or 18 kHz. The inhibitory effect was assessed by measuring and analyzing the changes in the first peak-to-peak magnitude, root mean square value, duration and latency of the ASR as the function of frequency difference between prepulse and background tones.

Results

Our data showed that ASR magnitude with pure tone background varied with tone frequency and was smaller than that with white noise background. Prepulse tone systematically reduced ASR as the function of the difference in frequency between prepulse and background tone. The 0.5 kHz difference appeared to be a prerequisite for inducing substantial ASR inhibition. The frequency dependence of PPI was similar under either a 10 or 18 kHz background tone.

Conclusion

PPI is sensitive to frequency information of the prepulse sound. However, the critical factor is not tone frequency itself, but the frequency difference between the prepulse and background tones.     

Electrosensory phase sensitivity in the weakly electric fish Eigenmannia in the detection of signals similar to its own

The electric organ discharge (EOD) of the South American knifefish Eigenmannia sp. is a permanently present wave signal of usually constant amplitude and frequency (similar to a sine wave). A fish perceives discharges of other fish as a modulation of its own. At frequency identity (F = 0 Hz) the phase difference between a fish's own electric discharge and that of another fish affects the superimposed waveform. It was unclear whether or not the electrosensory stimulus-intensity threshold as behaviourally determined depends on the phase difference between a fish's own EOD and a sine-wave stimulus (at F = 0 Hz). Also the strength of the jamming avoidance response (JAR), a discharge frequency shift away from a stimulus that is sufficiently close to the EOD frequency, as a function of phase difference was studied. Sine-wave stimuli were both frequency-clamped and phase-locked to a fish's discharge frequency (F = 0 Hz). In food-rewarded fish, the electrosensory stimulus-intensity threshold depended significantly on the phase difference between a fish's discharge and the stimulus. Stimulus-intensity thresholds were low (down to 3 V/cm, peak-to-peak) when the superimposed complex wave changed such that the shift in zero-crossings times relative to the original EOD was large but amplitude change minimal; stimulus-intensity thresholds were high (up to 16.9 V/cm, peak-to-peak) when the shift in zero-crossings times was small but amplitude change maximal. Similar results were obtained for the non-conditioned JAR: at constant supra-threshold stimulus intensities and F = 0 Hz, the phase difference significantly affected the strength of the JAR, although variability between individuals was higher than that observed in the conditioned experiments.Abbreviations ACP active phase coupling - EOD electric organ discharge - JAR jamming avoidance response - F frequency (fish) — frequency (stimulus) [Hz] - p-p peak-to-peak     

Comparative collicular tonotopy in two bat species adapted to movement detection,Hipposideros speoris andMegaderma lyra

Summary The tonotopic organization of the inferior colliculus (IC) in two echolocating bats,Hipposideros speoris andMegaderma lyra, was studied by multiunit recordings.InHipposideros speoris frequencies below the range of the echolocation signals (i.e. below 120 kHz) are compressed into a dorsolateral cap about 400–600 m thick. Within this region, neuronal sheets of about 4–5 m thickness represent a 1 kHz-band.In contrast, the frequencies of the echolocation signals (120–140 kHz) are overrepresented and occupy the central and ventral parts of the IC (Fig. 3). In this region, neuronal sheets of about 80 m thickness represent a 1 kHz-band. The largest 1 kHz-slabs (400–600 m) represent frequencies of the pure tone components of the echolocation signals (130–140 kHz).The frequency of the pure tone echolocation component is specific for any given individual and always part of the overrepresented frequency range but did not necessarily coincide with the BF of the thickest isofrequency slab. Thus hipposiderid bats have an auditory fovea (Fig. 10).In the IC ofMegaderma lyra the complete range of audible frequencies, from a few kHz to 110 kHz, is represented in fairly equal proportions (Fig. 7). On the average, a neuronal sheet of 30 m thickness is dedicated to a 1 kHz-band, however, frequencies below 20 kHz, i.e. below the range of the echolocation signals, are overrepresented.Audiograms based on thresholds determined from multiunit recordings demonstrate the specific sensitivities of the two bat species. InHipposideros speoris the audiogram shows two sensitivity peaks, one in the nonecholocating frequency range (10–60 kHz) and one within the auditory fovea for echolocation (130–140 kHz).Megaderma lyra has extreme sensitivity between 15–20 kHz, with thresholds as low as –24 dB SPL, and a second sensitivity peak at 50 kHz (Fig. 8).InMegaderma lyra, as in common laboratory mammals, Q_10dB-values of single units do not exceed 30, whereas inHipposideros speoris units with BFs within the auditory fovea reach Q_10dB-values of up to 130.InMegaderma lyra, many single units and multiunit clusters with BFs below 30 kHz show upper thresholds of 40–50 dB SPL and respond most vigorously to sound intensities below 30 dB SPL (Fig. 9). Many of these units respond preferentially or exclusively to noise. These features are interpreted as adaptations to detection of prey-generated noises.The two different tonotopic arrangements (compare Figs. 3 and 7) in the ICs of the two species are correlated with their different foraging behaviours. It is suggested that pure tone echolocation and auditory foveae are primarily adaptations to echo clutter rejection for species foraging on the wing close to vegetation.Abbreviations BF Best frequency - CF constant frequency - FM frequency modulated - IC inferior colliculus - HS Hipposideros speoris     

Resonance phenomena in the cochlea of the mustache bat and their contribution to neuronal response characteristics in the cochlear nucleus

Summary The cochlea of the mustache bat, Pteronotus parnellii, is very sensitive and sharply tuned to the frequency range of the dominant second harmonic of the echolocation call around 61 kHz. About 900 Hz above this frequency the cochlear microphonic potential (CM) reaches its maximum amplitude and lowest threshold. At exactly the same frequency, pronounced evoked otoacoustic emissions (OAE) can be measured in the outer ear canal, indicating mechanical resonance. The CM amplitude maximum and the OAE are most severely masked by simultaneous exposure to tones within the range from about 61–62 kHz up to about 70 kHz. The data suggest that the mechanism of mechanical resonance involves cochlear loci basal to the 61 kHz position.The resonance contributes to auditory sensitivity and sharp tuning: At the frequency of the OAE, single unit responses in the cochlear nucleus have the lowest thresholds. Maximum tuning sharpness occurs at frequencies about 300 Hz below the OAE-frequency, where the threshold is about 10 dB less sensitive than at the OAE-frequency. In addition, in the frequency range around the OAE-frequency several specialized neuronal response features can be related to mechanical resonance: Long lasting excitation after the end of the stimulus, asymmetrical tuning curves with a shallow high frequency slope and phasic on-off neuronal response patterns. In particular the latter phenomenon indicates the occurrence of local mechanical cancellations in the cochlea.Abbreviations CF constant frequency component of echolocation calls - CM cochlear microphonic potential - FM frequency modulated component of echolocation calls - N1 compound action potential of the auditory nerve - OAE octoacoustic emission - SEOAE synchronous evoked OAE     

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.     

Audiovocal interactions during development? Vocalisation in deafened young horseshoe bats vs. audition in vocalisation-impaired bats

Summary Horseshoe bats (Rhinolophus rouxi) were deafened in their 3rd–5th postnatal week. Subsequently their vocalisations were monitored to evaluate the impact of audition on the development of echolocation pulses. Hearing impairment affected the echolocation pulses as follows: the frequency of the constant frequency (CF) component was altered by between + 4 kHz and – 14 kHz, and the dominance of the second harmonic of the pulses was neutralised by a relative increase in intensity of the first and third harmonics.A second experiment focused on possible influences of acoustical self-stimulation with echolocation pulses on the establishment of auditory fovea representation in the inferior colliculus (IC). Frequency control of echolocation pulses was disrupted by larynx denervation. Thereafter, the bats produced multiharmonic echolocation signals (4–11 harmonics) varying in frequency. IC tonotopy, however, as monitored by stereotaxic electrophysiology, showed the same developmental dynamics as seen in control specimens (Fig. 10).Both experiments indicate that throughout postnatal development echolocation pulses are under auditory feedback control, whereas maturation of the auditory fovea and shifts in its frequency tuning represent an innate process. The significance of this postnatal development might be the adjustment of the vocal motor system of each bat to the frequency of its personal auditory fovea.Abbreviations CF constant frequency - CF1, CF2, CF3 harmonics of pure tone components of the echolocation pulses - FM frequency modulation - IC inferior colliculus of the midbrain     

Beaked whale auditory evoked potential hearing measurements

Several mass strandings of beaked whales have recently been correlated with military exercises involving mid-frequency sonar highlighting unknowns regarding hearing sensitivity in these species. We report the hearing abilities of a stranded juvenile beaked whale (Mesoplodon europaeus) measured with auditory evoked potentials. The beaked whale’s modulation rate transfer function (MRTF) measured with a 40-kHz carrier showed responses up to an 1,800 Hz amplitude modulation (AM) rate. The MRTF was strongest at the 1,000 and 1,200 Hz AM rates. The envelope following response (EFR) input–output functions were non-linear. The beaked whale was most sensitive to high frequency signals between 40 and 80 kHz, but produced smaller evoked potentials to 5 kHz, the lowest frequency tested. The beaked whale hearing range and sensitivity are similar to other odontocetes that have been measured.     

Temporal peculiarities of responses of auditory system neurons of the frog <Emphasis Type="Italic">Rana temporaria</Emphasis> to tone bursts with different modulation frequencies

Activity of medullar and midbrain auditory neurons at action of amplitude-modulated tone burst was recorded in immobilized common frogs Rana temporaria. Depth of modulation amounted to 10% or 80%, frequency of modulation varied from 5 to 150 Hz, and carrier intensity was in the range of 20–30 dB. Phasic neurons in medulla clearly reproduced the modulation frequency, but only at the 80% modulation depth. However, during presentation of signal with the 10% modulation depth, these neurons practically did not respond. Tonic neurons were able to reproduce the modulation frequency up to 10–150 Hz, but at the 10% modulation depth, the degree of reproduction of envelope was rather low, although for several first modulation periods it rose statistically significantly. In midbrain, the highest frequency of the reproduced modulation sharply fell. At greater modulation frequencies, the response of these neurons qualitatively reminds that of medullar neurons. At the low modulation frequencies, there is identified a group of midbrain neurons with a prominent increase of the signal modulation. This occurs in the frequency diapason up to 60 Hz; at an increase of the modulation frequency the time of achievement of maximal synchronization decreases. The optimal modulation frequency in many neurons of semicircular torus corresponds to parameters of the male nuptial call.     

Neurons with different temporal firing patterns in the inferior colliculus of the little brown bat differentially process sinusoidal amplitude-modulated signals

We examined how well single neurons in the inferior colliculus (IC) of an FM bat (Myotis lucifugus) processed simple tone bursts of different duration and sinusoidal amplitude-modulated (SAM) signals that approximated passively heard natural sounds. Units' responses to SAM tones, measured in terms of average spike count and firing synchrony to the modulation envelope, were plotted as a function of the modulation frequency to construct their modulation transfer functions. These functions were classified according to their shape (e.g., band-, low-, high-, and all-pass). IC neurons having different temporal firing patterns to simple tone bursts (tonic, chopper, onset-late, and onset-immediate) exhibited different selectivities for SAM signals. All tonic and 83% of chopper neurons responded robustly to SAM signals and displayed a variety of spike count-based response functions. These neurons showed a decreased level of time-locking as the modulation frequency was increased, and thereby gave low-pass synchronization-based response functions. In contrast, 64% of onset-immediate, 37% of onset-late and 17% of chopper units failed to respond to SAM signals at any modulation frequency tested (5–800 Hz). Those onset neurons that did respond to SAM showed poor time-locking (i.e., non-significant levels of synchronization). We obtained evidence that the poor SAM response of some onset and chopper neurons was due to a preference for short-duration signals. These data suggest that tonic and most chopper neurons are better-suited for the processing of long-duration SAM signals related to passive hearing, whereas onset neurons are better-suited for the processing of short, pulsatile signals such as those used in echolocation.Abbreviations C chopper - FM frequency-modulated - IC inferior colliculus - MTF modulation transfer function - O₁ onset-immediate - O_L onset-late - PAM pulsatile amplitude-modulation - PSTH peri-stimulus time histogram - SAM sinusoidal amplitude-modulation - SC synchronization coefficient - T tonic     

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.     

Auditory evoked potentials in the West Indian manatee (Sirenia:Trichechus manatus)

Summary Potentials evoked by clicks and tone pips were recorded by fine wires inserted extracranially in four West Indian manatees (Trichechus manatus) in air. Sounds were delivered via padded ear phones.Averaging a few thousand trials at 20/s reveals early peaks at N5.4 (vertex negativity to a frontal reference, at 5.4 ms), P7.6, N8.8, P9.5 — probably equivalent to waves IV and VII of the typical mammalian auditory brainstem response (ABR). Averaging 100 trials at <4/s suffices to reveal a complex sequence of later peaks including N25, P80, N150 and P190; consistent smaller peaks are visible when several hundred trials are averaged.Using tone pips with a rise and fall time of 2–5 ms the carrier frequency becomes important. Evoked potential wave forms are not the same at different frequencies, bringing out the fact that frequency is not a scalar that can be compensated for by intensity. Therefore the method was not used to obtain audiograms; however the largest EPs occur in the range of 1–1.5 kHz. EPs are found up to 35 kHz; almost no evoked potential is discernible at 40 kHz but the undistorted intensity available was limited. This is in reasonable agreement with the theoretical expectation for the upper limit of behavioral hearing from Heffner and Masterton based on head size and aquatic medium.Among several ear phone placements, that over the external auditory meatus was the most effective, but only slightly so. The external canal is presumably fluid or tissue filled and sound enters over a large area.Comparing data for two species on the most effective range of frequencies and the power spectra of their vocalizations,T. manatus is lower thanT. inunguis in both respects.The results show the utility and limitations of the method of recording extracranial evoked potentials to sounds, especially for large and valuable animals under makeshift conditions.Abbreviations ABR auditory brainstem response - AEP averaged evoked potentials - EMG electromyogram - F frontal sinuses - V vertex