Unit responses of the inferior colliculi to changes in the intensity of an amplitude-modulated signal

Unit responses of the inferior colliculi of anesthetized rats to amplitude-modulated sounds during a change in the carrier intensity were investigated. The following unit response characteristics were assessed: the number of spikes in the response, the range of reproduction of the modulation frequency, the response duration, and the pattern of the spike response relative to the envelope of the amplitude-modulated stimulus. The following parameters of the stimulus were varied: carrier intensity (usually of optimal frequency or noise), frequency of modulation (from 2 to 100 Hz), and carrier frequency. With a decrease in the intensity of the carrier in the case of monotonic neurons, and also with an increase or decrease in the intensity of the carrier relative to its optimal level in nonmonotonic neurons, the following changes in the discharge were regularly observed: the number of spikes in the response and its duration were reduced down to the appearance of only one initial response, the range of reproduction of the rhythm of modulation was narrowed, and the response pattern was sharply modified.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR. I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 5, No. 4, pp. 355–366, July–August, 1973.     

Peripheral lateral line responses to amplitude-modulated sinusoidal wave stimuli

This report describes the responses of single afferent fibers in the posterior lateral line nerve of the goldfish, Carassius auratus, to pure tone and to amplitude-modulated sinusoidal wave stimuli generated by a dipole source (stationary vibrating sphere). Responses were characterized in terms of output-input functions relating responses to vibration amplitude, peri-stimulus time histograms relating responses to stimulus duration, and the degree of phase-locking to both the carrier frequency and the modulation frequency of the amplitude-modulated stimulus. All posterior lateral line nerve fibers responded to a pure sine wave with sustained and strongly phase-locked discharges. When stimulated with amplitude-modulated sine waves, fibers responded with strong phase-locking to the carrier frequency and, in addition, discharge rates were modulated according to the amplitude modulation frequency. However, phase-locking to the amplitude modulation frequency was weaker than phase-locking to the carrier frequency. The data indicate that the discharges of primary lateral line afferents encode both the carrier frequency and the modulation frequency of an amplitude-modulated wave stimulus. Accepted: 2 June 1999     

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.     

Responses of midbrain lateral line units of the goldfish, Carassius auratus, to constant-amplitude and amplitude-modulated water wave stimuli

 Responses of mechanosensory lateral line units to constant-amplitude hydrodynamic stimuli and to sinusoidally amplitude-modulated water movements were recorded from the goldfish (Carassius auratus) torus semicircularis. Responses were classified by the number of spikes evoked in the unit's dynamic range and by the degree of phase locking to the carrier- and amplitude-modulation frequency of the stimulus. Most midbrain units showed phasic responses to constant-amplitude hydrodynamic stimuli. For different units peri-stimulus time histograms varied widely. Based on iso-displacement curves, midbrain units prefered either low frequencies (≤33 Hz), mid frequencies (50–100 Hz), or high frequencies (≥200 Hz). The distribution of the coefficient of synchronization to constant-amplitude stimuli showed that most units were only weakly phase locked. Midbrain units of the goldfish responded to amplitude-modulated water motions in a phasic/tonic or tonic fashion. Units highly phase locked to the amplitude modulation frequency, provided that modulation depth was at least 36%. Units tuned to one particular amplitude modulation frequency were not found. Accepted: 10 July 1999     

Responses of auditory medullary units to sinusoidally amplitude-modulated tones in frogs

Spike discharges of medullary units ofRana ridibunda in response to tones of optimal frequency for the neuron, with sinusoidal amplitude modulation, was studied. Reproduction of sound modulation in unit activity was assessed by the use of phase histograms of responses corresponding to the period of modulation. Amplitude modulation was reproduced in the firing pattern of neurons of the dorsal nucleus over a wide range of modulation frequencies and carrier levels. Accentuation of small changes of amplitude for modulation frequencies of 70–150 Hz was observed in many neurons of the superior olives. The phase of the response was a linear function of modulation frequency both in the dorsal nucleus and in the superior olives. The greatest enhancement of amplitude changes corresponded to low modulation indices.Academician N. N. Andreev Acoustics Institute, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 17, No. 3, pp. 390–396, May–June, 1985.     

Frequency tuning curves of the dolphin's hearing: envelope-following response study

Simultaneous tone-tone masking in conjunction with the envelope-following response (EFR) recording was used to obtain tuning curves in dolphins (Turslops truncatus). The EFR was evoked by amplitude-modulated probes of various frequencies. A modulation rate of 600 Hz was found to fit the requirement to have a narrow spectrum and evoke EFR of large amplitude. Tuning curves were obtained within the frequency range from 11.2 to 110 kHz. The Q₁₀ values of the obtained tuning curves varied from 12–14 at the 11.2 kHz center frequency to 17–20 at the 64–90 kHz frequencies.Abbreviations ABR auditory brainstem response - EFR envelope following response - ERB equivalent rectangular bandwidth     

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.     

Responses of central auditory neurons to frequency-modulated stimuli

Unit responses of the inferior colliculi of albino rats to frequency-modulated stimuli were investigated. The number of spikes, firing pattern, and duration of the discharge were determined. Parameters of unit responses obtained with different directions of frequency modulation were compared with the results of testing the effect of constant-frequency tones on the neurons. The distinguishing features of the unit responses to stimuli with different directions of frequency modulation were compared with the characteristics of the frequency-threshold curve and the lateral inhibitory zones, taken as indices of the unit responses to constant-frequency tones. With a change in stimulus frequency upward or downward from the initial level the unit responses to both directions could be similar as regards the number of spikes per discharge and the firing pattern or could differ sharply depending on the direction or, finally, they could arise only if the stimulus frequency changed in one direction. In some cases selectivity for the direction of the change in stimulus frequency was due to the width and position of the lateral inhibitory zones. However, for one-third of the neurons tested analysis of the spatial characteristics of the excitatory and inhibitory zones alone was insufficient to explain the high selectivity in the formation of a response to a stimulus with frequency modulation in one direction only.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol.6, No.3, pp.237–245, May–June, 1974.     

Dependence of responses of central auditory neurons on the depth and rate of frequency modulation

Unit responses of the inferior colliculi of anesthetized albino rats to frequency-modulated stimuli were studied. The number of spikes and the pattern and duration of the unit discharges in response to frequency modulation in different directions were shown to depend on the depth and rate of modulation. With a change in the rate of frequency modulation the number of spikes in the discharge of individual neurons could increase, decrease, or remain constant. Neurons of the inferior colliculi give their material discharge at different rates of frequency modulation. A change in the depth and rate of modulation did not change the sensitivity of most neurons to the direction of the frequency change in the stimulus within the frequency range corresponding to the center of the excitatory zone of the unit response.     

Vibratory communication in spiders

We analyzed the response of the vibration sensitive lyriform organ on the metatarsus of female spiders (Cupiennius salei) to dummies of male courtship vibrations. One of the two representative slits studied is sharply tuned to 500 Hz. Only the other slit is sensitive enough at lower frequencies to represent the parameters contained in the behaviourally effective dummies:

1. Amplitude. The physiological threshold is similar to the behavioural threshold. The stimulus acceleration amplitudes leading to a good synchronization between response and temporal stimulus pattern coincide with those effectively eliciting a behavioural response. The most frequent spike intervals remain nearly constant in this range. At acceleration amplitudes above the natural range, syllable and pause durations are misrepresented by the receptor response.
2. Frequency. Varying the carrier frequency between 35–500 Hz changes the most frequent spike intervals. Interval histograms resulting from behaviourally effective stimuli (50–200 Hz), however, are similr for carrier frequencies differing by a factor of 2.
3. Temporal pattern. Response duration reflects the temporal parameters of the stimulus. The most frequent spike interval only changes with temporal stimulus characteristics far off the natural range. The number of spikes during a syllable decreases in ongoing stimulus series. The quality of copying the temporal stimulus pattern remains unchanged, however.

     

Intracellular responses of antennal chordotonal sensilla of the American cockroach

The responses of mechanoreceptor neurons in the antennal chordotonal organ have been examined in cockroaches by intracellular recording methods. The chordotonal organ was mechanically stimulated by sinusoidal movement of the flagellum. Stimulus frequencies were varied between 0.5 and 150 Hz. Receptor neurons responded with spike discharges to mechanical stimulation, and were classed into two groups from plots of their average spike frequencies against stimulus frequency. Neurons in one group responded to stimulation over a wide frequency range (from 0.5 to 150 Hz), whereas those in a second group were tuned to higher frequency stimuli. The peak stimulus frequency at which receptor neurons showed maximum responses differed from cell to cell. Some had a peak response at a stimulus frequency given in the present study (from 0.5 to 150 Hz), whereas others were assumed to have peak responses beyond the highest stimulus frequency examined. The timing for the initiation of spikes or of a burst of spikes plotted against each stimulus cycle revealed that spike generation was phase-locked in most cells. Some cells showed phase-independent discharges to stimulation at lower frequency, but increasing stimulus frequencies spike initiation began to assemble at a given phase of the stimulus cycle. The response patterns observed are discussed in relation to the primary process of mechanoreception of the chordotonal organ.     

Modulation discrimination interference and auditory grouping.

The detection of a change in the modulation pattern of a (target) carrier frequency, fc (for example a change in the depth of amplitude or frequency modulation, AM or FM) can be adversely affected by the presence of other modulated sounds (maskers) at frequencies remote from fc, an effect called modulation discrimination interference (MDI). MDI cannot be explained in terms of interaction of the sounds in the peripheral auditory system. It may result partly from a tendency for sounds which are modulated in a similar way to be perceptually 'grouped', i.e. heard as a single sound. To test this idea, MDI for the detection of a change in AM depth was measured as a function of stimulus variables known to affect perceptual grouping, namely overall duration and onset and offset asynchrony between the masking and target sounds. In parallel experiments, subjects were presented with a series of pairs of sounds, the target alone and the target with maskers, and were asked to rate how clearly the modulation of the target could be heard in the complex mixture. The results suggest that two factors contribute to MDI. One factor is difficulty in hearing a pitch corresponding to the target frequency. This factor appears to be strongly affected by perceptual grouping. Its effects can be reduced or abolished by asynchronous gating of the target and masker. The second factor is a specific difficulty in hearing the modulation of the target, or in distinguishing that modulation from the modulation of other sounds that are present. This factor has effects even under conditions promoting perceptual segregation of the target and masker.     

Unit responses of the superior olives in bats to single and paired ultrasonic stimuli

Single unit responses in the superior olive of the greater horseshoe bat to ultrasonic stimuli with a filling frequency within the echolocation range were investigated. Some neurons were found to have three completely unconnected response regions with characteristic frequencies of 1/2 and 1/3 of the basic frequency, which was within the 80–86 kHz band. An increase in strength of the stimulus with filling frequency equal to the characteristic frequency of the neuron changed the tonic regime of activity into phasic. Presentation of two stimuli, overlapping in time, replaced the phasic regime by tonic. The frequency of the tonic response corresponded exactly to the beating frequency up to 1200 Hz (synchronization of unit discharges with each beating cycle). The synchronized tonic regime was preserved to definite strengths and filling frequencies of the two stimuli.A. A. Zhdanov State University, Leningrad. Translated from Neirofiziologiya, Vol. 8, No. 1, pp. 30–38, January–February, 1976.     

Modulation rate transfer functions in bottlenose dolphins (<Emphasis Type="Italic">Tursiops truncatus</Emphasis>) with normal hearing and high-frequency hearing loss

Envelope following responses were measured in two bottlenose dolphins in response to sinusoidal amplitude modulated tones with carrier frequencies from 20 to 60 kHz and modulation rates from 100 to 5,000 Hz. One subject had elevated hearing thresholds at higher frequencies, with threshold differences between subjects varying from ±4 dB at 20 and 30 kHz to +40 dB at 50 and 60 kHz. At each carrier frequency, evoked response amplitudes and phase angles were plotted with respect to modulation frequency to construct modulation rate transfer functions. Results showed that both subjects could follow the stimulus envelope components up to at least 2,000 Hz, regardless of carrier frequency. There were no substantial differences in modulation rate transfer functions for the two subjects suggesting that reductions in hearing sensitivity did not result in reduced temporal processing ability. In contrast to earlier studies, phase data showed group delays of approximately 3.5 ms across the tested frequency range, implying generation site(s) within the brainstem rather than the periphery at modulation rates from 100 to 1,600 Hz. This discrepancy is believed to be the result of undersampling of the modulation rate during previous phase measurements.     

Processing of behaviorally relevant temporal parameters of acoustic stimuli by single neurons in the superior olivary nucleus of the leopard frog

Summary Response characteristics of 130 single neurons in the superior olivary nucleus of the northern leopard frog (Rana pipiens pipiens) were examined to determine their selectivity to various behaviorally relevant temporal parameters [rise-fall time, duration, and amplitude modulation (AM) rate of acoustic signals. Response functions were constructed with respect to each of these variables. Neurons with different temporal firing patterns such as tonic, phasic or phasic-burst firing patterns, participated in time domain analysis in specific manners. Phasic neurons manifested preferences for signals with short rise-fall times, thus possessing low-pass response functions with respect to this stimulus parameter; conversely, tonic and phasic-burst units were non-selective and possessed all-pass response functions. A distinction between temporal firing patterns was also observed for duration coding. Whereas phasic units showed no change in the mean spike count with a change in stimulus duration (i.e., all-pass duration response functions), tonic and phasic-burst units gave higher mean spike counts with an increase in stimulus duration (i.e., primary-like high-pass response functions). Phasic units manifested greater response selectivity for AM rate than did tonic or phasic-burst units, and many phasic units were tuned to a narrow range of modulation rates (i.e., band-pass). The results suggest that SON neurons play an important role in the processing of complex acoustic patterns; they perform extensive computations on AM rate as well as other temporal parameters of complex sounds. Moreover, the response selectivities for rise-fall time, duration, and AM rate could often be shown to contribute to the differential responses to complex synthetic and natural sounds.Abbreviations SON superior olivary nucleus - DMN dorsal medullary nucleus - TS torus semicircularis - FTC frequency threshold curve - BF best excitatory frequency - PAM pulsatile amplitude modulation - SAM sinusoidal amplitude modulation - SQAM square-wave amplitude modulation - MTF modulation transfer function - PSTH peri-stimulus time histogram     

Auditory units in the medulla of the marsh frog with unusual patterns of spontaneous activity

Statistical properties of spontaneous firing were studied in 79 single auditory units located in the dorsal medullar (cochlear) nucleus of unanaesthetized curarized marsh frogs (Rana ridibunda). The great majority of these units showed irregular spontaneous activity with mean rates in the range 1–30 spikes · s^–1. In 53% of the cells the auto-renewal functions of the spontaneous activity monotonically rose to an asymptotic value, but 41% of the cells produced auto-renewal functions which showed a pronounced peak after a dead-time period. Five low-frequency auditory neurons revealed periodic firing in the absence of controlled stimuli. The preferred period did not correspond to the unit's best frequency but demonstrated a modest correlation with the best modulation frequency of the unit's response to amplitude-modulated tones and with the duration of the after-onset dip in peri-stimulus time histograms.Abbreviations AM amplitude modulation - ARF auto-renewal function - DMN dorsal medullar nucleus - PST peristimulus time - SA spontaneous activity - TID time interval distribution - RMG response modulation gain     

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     

Single unit responses in the auditory center of the frog mesencephalon to amplitude-modulated tones

Responses of most single neurons of the torus semicircularis ofRana ridibunda to stimuli of characteristic frequency and with low (10–30%) sinusoidal amplitude modulation were considerably stronger than those of the same neurons to pure tones. Analysis of phase histograms synchronized with the period of modulation was used to study dependence of the response on the frequency of modulation. In some cells the degree of modulation of the phase histogram fell steadily with an increase in modulation frequency, but in others a maximum was found in the 10–20 Hz region. Usually modulations of the phase histogram were significantly greater than stimulus modulation. The phase angle between the maximum of stimulus amplitude and the maximum of the unit response increased as an approximately linear function of the increase in modulation frequency.N. N. Andreev Acoustic Institute, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 12, No. 3, pp. 264–271, May–June, 1980.     

Response of frog midbrain neurons to tones amplitude-modulated by pseudorandom noise

Spike response in torus semicircularis units to the effects of uninterrupted characteristic frequency tones amplitude-modulated by pseudorandom noise were investigated during experiments on immobilizedRana ridibunda. Period histograms of modulating waveform of 512 msec duration (both modulating polarities) were produced for 32 units. Almost all neurons investigated responded exclusively to the positive half of the modulating signal. Difference histograms obtained by calculating period histograms for different polarities of the envelope faithfully reproduced the dynamics of signal amplitude in four units. The remainder responded only to envelope maxima, without reproducing amplitude dynamics among these; over half the units represented only some of the envelope maxima, moreover. Certain cells were found which retained their specific pattern of response to pseudorandom noise over a wide range of carrier intensities.N. N. Andreev Acoustical Institute, Moscow. Translated from Neirofiziologiya, Vol. 22, No. 2, pp. 227–235, March–April, 1990.