A stochastic model of the repetitive activity of neurons

A recurrent model of the repetitive firing of neurons responding to stimuli of long duration is given. The model assumes a deterministic threshold potential and a membrane potential which is composed of both deterministic and random components. The model accurately reproduces interval statistics obtained from different neurons discharging repetitively over a wide range of discharge rates. It is shown that the model has three important parameters; the time course of threshold recovery following a discharge, the variance of the random component, and the level of excitatory drive. The model is extended, by the use of hyperpolarizing afterpotentials, to include negative correlation between successive interspike intervals.

Further results with the 'uniquantal EPSP' hypothesis

The hypothesis of Geisler (Brain Res. 212 (1981) 198-201), in which the different spontaneous-rate classes of primary auditory neurons were accounted for by the different sizes of uniquantal EPSPs relative to the gap between resting membrane and threshold potentials, was represented with an expanded model which included relative refractory effects. The spike rates generated by the expanded model, when plotted vs. estimated sound level, are qualitatively similar to those of experimentally obtained rate-level curves. The hypothesis is also consistent with recent ultrastructural data which suggest that average quantal-release rates for any particular primary auditory neuron are inversely related to its spontaneous rate. The model's recovery processes following spike generation (hazard functions) are also similar to those observed experimentally.

Two-tone suppression of basilar membrane vibrations in the base of the guinea pig cochlea using "low-side" suppressors

The responses of the basilar membrane (BM) in the basal section of the guinea pig cochlea were measured by laser interferometry. The stimuli were pairs of harmonically related tones, presented simultaneously. One tone, at the BM's characteristic frequency (CF) of about 17 kHz, was presented at a low intensity. The other tone, presented at various intensities, was a "low-side" suppressor, with a frequency of 0.2-8 kHz. As observed by many others, the suppressor tone, when presented at high enough intensity, reduced the magnitude of the CF component of BM displacement, sometimes dramatically. However, regardless of whether the CF component was suppressed or not, the sum of the displacement amplitudes of the CF and suppressor components was always greater than the displacement amplitude of the unsuppressed CF component. For suppressor frequencies up to 4 kHz, the suppression was both tonic and phasic, and synchronized to the suppressor period. For higher suppressor frequencies, principally tonic suppression was seen.

Suppression in auditory-nerve fibers of cats using low-side suppressors. III. Model results

A phenomenological model which simulates auditory-nerve (AN) two-tone suppression was developed. The model uses the output of the outer hair cell (OHC) to control the gain of the cochlear amplifier, which presumably affects only frequencies near the characteristic frequency (CF). Among other things, the model can simulate basic AN suppression patterns including the 1/4 to 1/2 cycle relationships which exist between phase of suppression and phase of excitation to the suppressor (SUP) tone alone (Cai and Geisler, 1996a). Without any changes, it is also able to simulate the experimental low-frequency biasing data and the suppression of CF component by the low-frequency SUP tone in the OHC outputs (Cheatham and Dallos, 1994). These successful simulations of the suppression patterns support the basic assumption in the model, that the saturation of OHC transduction current produces two-tone suppression. However, the amplitude behavior of the model fits that obtained only from AN fibers with high spontaneous rates (and from inner hair cells (IHC)), but not fibers with lower spontaneous rates. It appears, therefore, that other unknown mechanism(s) operating at stages following the IHC potential are important in determining the magnitude of low-side suppression.

Temporal patterns of the responses of auditory-nerve fibers to low-frequency tones

The temporal response patterns of auditory-nerve fibers to low-frequency tones were studied in anesthetized cats using period histograms. 'Peak-splitting' was observed mostly in fibers with lower characteristic frequencies (CF < 2 kHz) and with lower-frequency stimulation (< or = 500 Hz). The occurrence of peak-splitting, the number of peaks, and the time between the peaks were all dependent upon the stimulus frequency. The phases of responses, although complex functions of stimulus frequency, intensity, and the fiber's CF, clearly showed traveling-wave characteristics for all frequencies at or above 100 Hz. The amount of phase change with intensity was generally small for lower-frequency stimuli (< approximately 50 degrees), although larger phase changes (e.g., approximately 180 degrees) were occasionally seen with higher-frequency stimuli. At 50 and 100 Hz, the phase of neural responses in the basal region roughly corresponds to the maximum velocity of the basilar membrane towards scala tympani (as inferred from cochlear microphonic recordings).

Suppression in auditory-nerve fibers of cats using low-side suppressors. II. Effect of spontaneous rates

The responses of auditory nerve fibers with different spontaneous rates were studied in anesthetized cats, using harmonically related characteristic frequency (CF) tone and suppressor (SUP) tone (50-2000 Hz) as stimuli. The relative-response index, defined as the ratio of the maximum response level in the two-tone segment to the response level in the CF-alone segment, at or near the intensity of maximum suppression (i.e., where the two-tone rate was lowest), was dependent on fiber's spontaneous rate (SR). For all the SUP frequencies used, lower-SR fibers almost always showed values less than unity, while high-SR fibers almost always gave values near or greater than unity. The phase of maximum suppression was not dependent upon fiber SR. In one experiment, a pair of low- and high-SR fibers with the same CF (12 kHz) were recorded consecutively in the same electrode penetration, and were studied with the same stimulus parameters. Their temporal responses showed dramatic temporal resemblances, with very similar phases of suppression and response. But the relative-response indexes were different. The similarities in the lower- and high-SR fibers support the idea that the basic response and suppression patterns in all fibers are formed at or before the inner hair cell (IHC) stage, while the differences suggest that processes more central than the IHC receptor potential are important in determining the magnitudes of suppression, particularly in the lower-SR fibers.

Long-term suppression of the responses of auditory nerve fibers to a characteristic-frequency tone by a low-frequency suppressor

The classical two-tone suppression requires the characteristic-frequency (CF) tone and the suppressor (SUP) tone to act simultaneously. We report a novel phenomenon whereby the responses to the CF tone alone were 'suppressed' by a preceding low-side SUP tone. Increasing the repetition interval to about 3000 ms or longer eliminated such suppression. The magnitude of this 'long-term' suppression was not dependent upon fiber CF, but fibers with low spontaneous rates (SR) generally showed more suppression than high-SR fibers did. The suppression threshold was not dependent upon fiber SR. This suppression of the CF responses did not affect the phases of responses to either the CF or SUP tone, or the phase of suppression. This phenomenon is not due to adaptation or fatigue, but due to the presence of the preceding SUP tone. The efferent system, particularly the 'slow' effect, might be responsible for it.

Suppression in auditory-nerve fibers of cats using low-side suppressors. I. Temporal aspects

Two-tone suppression was studied in the auditory nerve fibers of anesthetized cats, using low-frequency suppressors (50-2000 Hz). The response to the characteristic-frequency (CF) tone was suppressed in a phase-specific manner, attaining one or two minimums in 1 cycle of the suppressor (SUP) tone. The suppression phase-lead (i.e., the phase of maximum suppression leading the phase of response to the SUP tone) was about 1/4 cycle for lower-frequency suppressors (50, 100 and 200 Hz), and was close to 1/2 cycle for higher-frequency suppressors (500, 1000 and 2000 Hz). Both the phase of suppression and the suppression phase-lead are independent of fiber spontaneous rate (SR). Some fibers also show a secondary (minor) suppression at higher SUP intensities, which is always about 1/2 cycle away from the first (major) one. Fibers with higher CFs (> 2 kHz) are more likely to show a secondary suppression than those with lower CFs. The threshold difference between the major and minor suppressions is CF-dependent: lower CF fibers usually show differences of 10 dB or greater, while higher CF fibers show smaller differences. The secondary suppression is suppressor-frequency-dependent, usually restricted to lower-frequency suppressors (< or = 200 Hz). No fibers showed a secondary suppression with a SUP frequency 1000 Hz or greater. The phases of suppressions (both the major and minor suppressions) are not affected by the intensity of the CF tone. Non-excitatory, low-frequency suppressors can also give rise to significant suppression. The threshold of synchronization to the SUP tone in the two-tone part was usually the lowest, while the SUP-alone rate threshold was highest. The threshold of synchronization in the SUP-alone segment and threshold of suppression were in between. In some low-SR fibers, complete suppression can be seen.

Relationships between frequency-tuning and spatial-tuning curves in the mammalian cochlea

The tuning curve of a single auditory-nerve fiber is a measure of the intensity levels producing that fiber's threshold response at each of a number of frequencies. For many purposes it is desirable to know cochlear responses as a function of cochlear location (spatial distance from the stapes). Using assumptions of uniformity, the relationships between auditory-nerve-fiber frequency-tuning curves, basilar-membrane frequency-response curves, and basilar-membrane spatial-response curves are obtained. From the spatial-response characteristics which are then inferred from neural frequency-tuning curves, it appears that the tuning properties of the apical cochlea are fairly uniform and differ from the tuning properties of the basal cochlea.

A cochlear model using feed-forward outer-hair-cell forces

A linear (frequency-domain) model of the cat cochlea (implemented in both 1- and 2-dimensional versions) has been developed which uses outer hair cell (OHC) forces in a geometry which includes the longitudinal (base-to-apex) tilt of the outer hair cells (OHCs). When positive (contractile) real OHC force-constants are used, very large (50 + dB) response peaks along with very rapidly accumulating phase lags (which can reach -50 pi radians) are obtained. The wider the longitudinal segmentation, the broader the peaks and the less the phase accumulation; 71-microns segmentation produced the most realistic responses. These large response peaks are achieved by a small zone of negative resistance (ca. 1 mm) just basal to the response peak and the virtual 'zeroing' of the basilar membrane's effective impedance over the entire peak region (ca. 2.5 mm). To produce these peaks, the OHCs generate about 25-times the incoming acoustic power. Inclusion of low-pass filtering in the model's OHC representation produces, by contrast, very unrealistic notch-and-peak displacement complexes accompanied by very large phase lags, for all segmentation widths used. However, when phase reversals of OHC forces are also added, achieved by imbedding a resonant system within the tectorial membrane, very realistic peaks and phase functions are produced. More power must, however, be generated by the OHCs (about 70-times the incoming). The end result is output which mimics quite closely the living basilar membrane's responses to low-intensity high-frequency tones.

A realizable cochlear model using feedback from motile outer hair cells

A physically realizable form of a recent cochlear model using feedback forces from motile outer hair cells [Geisler (1991) Hear. Res. 54, 105-117] has been developed. The model was computer-simulated in the frequency domain (necessarily linear). Its responses to pure tones are very realistic in terms of sharpness (Q10s of 3-5) and in terms of tip-to-tail ratios (50-60 dB). These large tips are due to the feedback forces, which act as negative resistances (energy-supplying elements) over restricted spatial ranges. Nyquist-criterion analysis indicates that the model is stable. The spatial patterns of the model's output also bear qualitative resemblances to several other phenomena observed in cochleas, both living and excised.

A model of stereociliary tip-link stretches

A model of the tip-link stretches produced by angular deflections of the stereocilia of vertebrate acoustico-lateralis hair cells is presented. It is shown that tip-link stretch in the model is proportional to the angle of stereociliary deflection. By contrast, the stretch of a horizontal (e.g., row-to-row) link is proportional to the square of the angle of stereociliary deflection. Possible roles of these stretches in sensory transduction are discussed.

Two-tone suppression, excitation and the after effect in rate responses in auditory nerve fibres in the cat

Responses were recorded from single, auditory nerve fibres in the anaesthetized cat. Acoustic stimuli consisted of two tones, one of which was at characteristic frequency (CF), the other (the suppressor) was at considerably lower frequency. Tones were presented in simultaneous and sequential configurations. For simultaneous presentations, well-known response properties were observed. The rising limb of the two-tone rate-intensity function closely matched that of the appropriately adapted response to the suppressor tone presented alone. Also, whether strongly suppressed relative to CF-driven rate, or equal to CF-driven rate, rate responses to the two-tone stimuli persisted unchanged when the CF tone was terminated and the suppressor tone continued alone. These results support the hypothesis that the suppressor tone has dual influences, suppressive and excitatory, that are distinct and additive. Peristimulus response histograms confirm in the cat that depression and slow recovery of sensitivity to CF may follow termination of the suppressor tone, as reported for the guinea pig [Hill, K.G. and Palmer, A.R. (1991) Hear. Res. 55, 167-176]. This delay in recovery of normal sensitivity to CF appeared to be directly related to the amount of excitation of the fibre that is attributable to the suppressor tone. A similar, delayed re-establishment of sensitivity also occurred in the response to a tone at CF, presented immediately following excitation by a suppressor tone. However, no delay occurred in the onset of response to the suppressor when preceded by the CF tone.(ABSTRACT TRUNCATED AT 250 WORDS)

Two-tone suppression by a saturating feedback model of the cochlear partition

A model of a small strip of cochlear partition was computer simulated. The model is composed of two elements, approximations to the transfer functions of an inner hair cell (IHC) and an outer hair cell (OHC), respectively. The IHC element was insensitive to DC stimulation. Input was one or two sinusoids. One sinusoid, at the characteristic frequency (CF), was multiplied by the gain of the 'cochlear amplifier'. A second sinusoid, representing a tone with much lower frequency, was not affected by the amplifier gain. This gain was determined by the OHC transfer function. In one form of the model ('fixed-gain'), this gain was set at a fixed number determined from the furthest point reached on the OHC transfer function. This form of the model produced very realistic single-tone responses as well as showing 'two-tone suppression': that is, the IHC DC response produced by CF stimulation was reduced when the lower-frequency sinusoid, at suitable intensities, was added to the stimulus. When a DC component was added to the two-tone stimulus, the magnitude of this two-tone suppression was enhanced. In the second form of the model ('variable-gain'), the cochlear-amplifier gain varied throughout the stimulus cycle. Its value was re-calculated at each instant, determined by the point on the OHC transfer function current at that particular instant. This form of the model showed two-tone suppression only when a DC component was added to the two-tone stimulus.

A cochlear model using feedback from motile outer hair cells

A model of cochlear vibrations based upon motile outer hair cells (OHCs) has been developed using physiologically demonstrated phenomena. Rapid longitudinally directed OHC forces are connected in such a way as to form a negative-feedback system. The responses at the higher frequencies (greater than 1 kHZ) are quite realistic: they have properly shaped amplitude curves with large tip-to-tail ratios (30-50 dB), Q10's of 2-6, and 'shoulders' at frequencies an octave below the resonant frequency. The phases are also quite realistic, though asymptoting at somewhat lower values (about -6 pi radians) than observed physiologically. The responses in the apical section are not so realistic. The form of the OHC force is physically unrealizable, but realizable forms are discussed.

Estimation of eardrum acoustic pressure and of ear canal length from remote points in the canal

Sound pressure distributions in the human ear canal, whether unoccluded or occluded with ear molds, were studied using a probe tube technique. On average, for frequencies below 6 kHz, the measuring probe tube had to be placed within 8 mm of the vertical plane containing the top of the eardrum (TOD), determined optically, in order to obtain sound pressure magnitudes within 6 dB of "eardrum pressure." To obtain that accuracy in all of the eight subjects studied, the probe had to be within 6 mm of the TOD. Since probe location relative to the drum has to be known, a purely acoustic method was developed which can be conveniently used to localize the probe-tip position, utilizing the standing wave property of the sound pressure in the ear canal. The acoustically estimated "drum location" generally lay between the optically determined vertical planes containing the TOD and the umbo. On average, the "drum location" fell 1 mm medial to the TOD. Of the 32 estimates made acoustically in various occluded and unoccluded conditions in 14 subjects, 30 estimates lay within a +/- 2-mm range of this average.

Saturation of outer hair cell receptor currents causes two-tone suppression

Zwicker [Biol. Cybern. 35, 243-250, (1979); J. Acoust. Soc. Am. 80, 163-176 (1986)] has previously proposed that many nonlinear phenomena in the mammalian cochlea can be explained by saturation of a positive feedback process which enhances mechanical sensitivity, although the site of the nonlinearity producing this saturation has so far remained obscure. In this paper we present evidence suggesting that the nonlinearity of mechano-electrical transduction in the outer hair cells is the dominant nonlinearity producing two-tone suppression in the mammalian cochlea. In particular, we show that: (i) suppression of the extracellular summating potential (SP), recorded from a particular place within the organ of Corti, has characteristics similar to the suppression of activity in the auditory-nerve; (ii) that SP suppression occurs at approximately constant basilar membrane displacement, inferred from the SP iso-response contours; and that (iii) the onset of SP suppression with suppressor tones on the tail of the frequency tuning curve closely parallels the onset of nonlinearity in the local cochlear microphonic. Since previous studies (Patuzzi et al., 1989) have demonstrated that the vibration of the basilar membrane at its characteristic frequency is very sensitive to changes in outer hair cell receptor current, we consider that interference in outer hair cell currents caused by nonlinearity in mechano-electrical transduction is an adequate explanation of two-tone suppression. This requires that outer hair cell receptor currents deviate from linearity at a suppressor tone level below that required to produce a significant DC receptor potential within the inner hair cells, and that the active process within the cochlea is distributed along a local region of the cochlea, basal of the vibration peak.

Evidence for expansive power functions in the generation of the discharges of 'low- and medium-spontaneous' auditory-nerve fibers

Re-analysis of data from Geisler et al. [J. Acoust. Soc. Am. 77, 1102-1109, 1985] indicates that the slopes of the intensity versus discharge-rate curves of auditory nerve (AN) fibers decrease systematically with increasing spontaneous discharge rate. For 'high-spontaneous' fibers, the slope is usually less than 0.5 dB/dB, while for 'low-spontaneous' fibers the slopes reach values greater than 4.0 dB/dB. A two-stage model accounts for this behavior. The first stage is a static non-linearity based on the measured intensity-voltage characteristic of inner hair cells. The second stage, representing action-potential generation, is linear for high-spontaneous fibers, but a squaring function for low- and medium-spontaneous fibers. The output of the model displays realistic slopes for its various intensity-rate curves. There are suggestions that a nonlinearity of still higher power is needed to simulate accurately the behavior of AN fibers having the lowest spontaneous rates (less than 0.1/s). The model also accounts for other observed differences between the discharge patterns of the different fiber classes.

Responses of auditory-nerve fibers to nasal consonant-vowel syllables

Responses of single auditory-nerve fibers in anesthetized cat to spoken nasal consonant-vowel syllables were recorded. Analyses in the form of spectrograms and of three-dimensional spatial-time and spatial-frequency plots were made. Among other features, formant transitions are clearly represented in the fibers' response synchronization properties. During vocalic segments, especially those in /mu/and/ma/, at a stimulus level near 75 dB SPL, a strong dominance in the responses by frequencies near the second formant (F2) is found for most fibers whose characteristic frequencies (CFs) are at or above F2. In contrast, at more moderate levels, the same fibers may show response synchrony to frequencies closer to their own CFs. There are significant differences in the response properties of high and low/medium-spontaneous-rate fibers.

Guinea pig tectorial membrane profile in an in vitro cochlear preparation

The guinea pig cochlea was examined under high-magnification light microscopy in an in vitro preparation. After extraction of the otic capsule, the bulla was opened widely and a small hole made into the fourth turn of the scala vestibuli. The organ of Corti was visualized under artificial endolymph at 600 X magnification. Added 1-micron titanium dioxide particles settled on the upper surface of the transparent tectorial membrane. Particle positions showed that much of this upper surface lay in a flat sheet that extended centrifugally almost to the Hensen's cells, giving the impression it was attached there. The sheet extended at least to the level of the inner hair cells, where a tectorial membrane thickness of about 40 micron was reached. Titanium dioxide particles were seen regularly in immediate proximity to the hair cell cilia, indicating that scala media is continuous with the subtectorial space. Upon mechanical manipulation, Hensen's cells proved to be extremely cohesive and elastic. It is suggested that hair cell stereocilia provide major mechanical connections for the tectorial membrane.

A model of the effect of outer hair cell motility on cochlear vibrations

A model of cochlear function is presented in which deformation forces within outer hair cells are assumed to occur in synchronized response to generator potentials. Assuming a 90 degree phase lag between the generator potentials and the deformation forces, it is shown that the forces act to reduce cochlear-partition damping and thus increase frequency selectivity. A number of other experimentally observed phenomena, such as the effects of efferent-fiber stimulation and electrical polarization, can also be accounted for with this model.

Comparison of the responses of auditory nerve fibers to consonant-vowel syllables with predictions from linear models

The responses of cat auditory-nerve fibers to synthesized consonant-vowel syllables were compared with predictions from linear models based on individual fibers' threshold tuning curves. Comparisons with the linear predictions provided information about the specific effects of peripheral nonlinearities on the representation of speech sounds. Spectral peaks, such as the formants of vowels, were more prominently represented in synchronized discharge patterns than in the linear predictions. Suppression of responses to other spectral peaks and to stimulus components between spectral peaks accounted for the differences. While profiles of fibers' synchronized responses were usually dominated by a single formant, predicted linear responses often included broad responses having two or more formants as well as components near the fibers' characteristic frequencies. In contrast, when no stimulus peak fell within a fiber's response area, the agreement between the neural response and the linear prediction was quite good. The results suggest that one role for peripheral nonlinearities in the auditory system may be to enhance the neural representation of spectral features such as formants.

Wiener kernel analysis of responses from anteroventral cochlear nucleus neurons

Responses to pseudo-random Gaussian white noise, tones and clics were recorded from neurons in the anteroventral cochlear nucleus (AVCN) of barbiturate anesthetized cats. The responses to white noise were used to calculate estimates of the zero-, first- and second-order Wiener kernels for these neurons. The Wiener kernels did contain useful information on the fundamental, DC and second harmonic components of the responses of AVCN neurons to tones, clicks and noise. However, they generally did not provide predictions of the difference tone distortion products found in the peripheral auditory system. Overall, the addition of the second kernel improved a prediction based on the zero- and first-order kernels, but not by very much. If the estimates of the Wiener kernels were not very good, then a second-order prediction could be worse than a first-order one. To produce good estimates of the Wiener kernels, many repetitions of very long Gaussian white noise stimuli are necessary. Therefore the technique does not permit rapid data collection. Further, exposure to long duration high intensity noise can result in acoustic trauma. This damage effects the mechanism that generates the difference tone distortion products, and it can also affect the tuning of the auditory neurons. Thus Wiener's nonlinear system identification theory has only limited usefulness in the analysis of the peripheral auditory system.

Comparison of click responses of primary auditory fibers with minimum-phase predictions

Minimum-phase impulse responses were constructed from the frequency threshold-response curves of primary auditory fibers in the anesthetized cat. These impulse responses had many of the same characteristics as the experimental click responses. The two types of responses had similar inter-peak intervals as well as similar (+/- 1.5 ms) latencies to the principal mode and similar (+/- 1.0 ms) intervals from response onset to the principal mode.

Responses of auditory-nerve fibers to consonant-vowel syllables

The discharge patterns elicited by a set of synthesized consonant-vowel (CV) syllables were studied in the auditory nerve of the cat. The syllables, heard as /ba/, /da/, or /ga/, included a 25-, 50-, or 75-ms formant transition followed by a segment of steady-state vowel. The data were analyzed in terms of average discharge rate and in terms of the synchrony of discharges with respect to various spectral components of the stimuli. The results differ slightly from those of previous reports of the responses to vowels [Sachs and Young, J. Acoust. Soc. Am. 66, 470-479 (1979); Young and Sachs, J. Acoust. Soc. Am. 66, 1381-1403 (1979)], in that average discharge rates appear to provide more information about the spectra of formant transitions than they do about the spectra of steady-state vowels. This difference reflects changes in the spectrum of the syllable produced by movements of the formants. The synchrony of discharges, however, may provide more detailed information about the spectra of CVs than does average discharge rate. Each fiber's response at a particular peristimulus time may be characterized by the "dominant response component," the largest peak in the Fourier transform of the period histogram. The trajectories of the first three formants can be inferred from changes in the "dominant components" in a sample of fibers.

Multiple reservoir model of neurotransmitter release by a cochlear inner hair cell

A probabilistic model is described for transmitter release from hair cells, auditory neuron EPSP's, and discharge patterns. The present model assumes that several reservoirs of neurotransmitter exist, having individual probability-of-release functions centered at successively higher intensities. The model accurately mimics the adaptation of successive EPSP amplitudes of the afferent neuron of the goldfish sacculus and, for mammalian auditory-nerve fibers, the adaptation of neural discharge rate, the saturation of onset and steady-state neural rate versus intensity, and the change in neural rate in response to incremental stimuli. The model also produces realistic interval and period histograms. The data shown support the hypothesis that multiple populations of neurotransmitter are involved in the afferent hair-cell synapses.

Responses of primary auditory fibers to brief tone bursts

Responses of primary auditory fibers to short triangularly modulated bursts of tone were obtained in the anesthetized cat. Based on discharge rate alone, the characteristics of the "response areas" obtained with these tone bursts were found to depend on the best frequency of the fiber. For a fiber with low best frequency (below 1 kHz), tones of greater than 10-ms duration had to be presented in order for the frequency resolution of the neuron to be as good as it was for long tones. For fibers with high best frequencies (above 10 kHz), tones of 2 or even 1 ms caused responses that were nearly as frequency selective as those obtained with long tones. A linear minimum-phase model based on the steady-state frequency selectivity of the fibers has been developed and shows generally comparable responses, but with some interesting exceptions. Synchronization of discharges to the waveform of the low-frequency tone bursts was measured and also shown to be generally compatible with the minimum-phase model. Trapezoidally modulated tone bursts were also used.

A model for discharge patterns of primary auditory-nerve fibers

A model for the synapse between an inner hair cell and afferent fiber is presented. The hair cell is assumed to release 25-100 quantized neurotransmitter packets per sec. Each packet causes an exponentially shaped unitary excitatory postsynaptic potential (EPSP). Any particular afferent fiber is assumed to have a discharge threshold either above or below that of the unitary EPSP. Using any reasonable distribution of threshold values, the model produces a bimodal distribution of spontaneous discharge patterns. The group producing high spontaneous rates (greater than 20/sec) all have discharge thresholds below the unitary EPSP level, while the low-spontaneous group (rates less than 20/sec) all have discharge thresholds above that level. Other physiologically realistic results are obtained from the two groups.

Auditory-nerve fiber responses to frequency-modulated tones

Discharge patterns of cat auditory-nerve fibers were obtained in response to frequency-modulated (FM) tones. The rate and direction of frequency change and the sound-pressure level of the sweep tones were systematically varied, and aspects of the discharge patterns were compared to aspects of the discharge patterns elicited by pure tones. Increases in SPL broaden the frequency range over which the fiber responds, as is the case with pure-tone stimuli. Increases in the rate of frequency change have little effect on frequency selectivity for the rates tested. In general, the pure-tone response area is a good predictor of the response area to FM. Although approximately equal numbers of spikes are elicited by ascending and descending sweeps, the discharge patterns differ slightly; for each direction of frequency change, the FM response area is shifted in the direction of the earliest-occurring frequencies. Most of this shift can be accounted for by neural adaptation. This asymmetry is small, relative to those observed in the central nervous system.

Responses of primary auditory fibers to combined noise and tonal stimuli

Responses to tonal stimuli, with and without added noise of different bandwidths, were obtained from anesthetized cat auditory-nerve fibers using glass micropipettes. When low-pass noise with a cut-off frequency at least one octave below best (or characteristic) frequency was used, every fiber tested at high enough intensities showed a suppression of the tonal response. This suppression did not cause a general reduction of neural responsiveness to all sounds, but rather took the general form of a frequency-specific reduction in the effective intensity of the tonal stimuli. The suppression mechanism(s) involved thus adjust the sensitivity of these fibers to cover higher intensity ranges in the presence of noise. The frequency of the most severely affected tones was always at or near best frequency, in confirmation of previous work (Abbas, P.J. and Sachs, M.B. (1976): J. Acoust. Soc. Am. 59, 112-122; Kiang, N.Y.-S. and Moxon, E.C. (1974): J. Acoust. Soc. Am. 55, 620-630). The suppresson is a direct but highly nonlinear function of the intensity and bandwidth of the noise. The effects on tonal response of wide-band noise were more variable, sometimes causing suppression similar to that induced by the low-pass noise and sometimes causing only 'strong-signal capture' effects. A model of noise-induced suppression has been developed whereby each sound produces both an excitatory effect, sharply tuned at best frequency, and a suppressive effect, which also has its lowest threshold at best frequency but is more broadly tuned.

Voltage-dependent elements are involved in the generation of the cochlear microphonic and the sound-induced resistance changes measured in scala media of the guinea pig

The injection of d.c. current into scale media alters both the cochlear microphonic (CM) and the acoustically synchronized changing resistance (CR) measured in scala media. Positive current increases the CM and decreases the CR. The effect on the CM is greatest at high sound pressure level (SPL), whereas the effect on CR is greatest at low SPL. Negative current has a similar but opposite effect on both the CM and the CR. The results suggest that a voltage-dependent nonlinear element exists in cochlear hair cells.

Stimulation of efferents alters the cochlear microphonic and the sound-induced resistance changes measured in scale media of the guinea pig

Electrical stimulation of the crossed olivo-cochlear bundle (COCB) increases both the cochlear microphonic and the acoustically synchronized changing resistance (CR) and it causes a decrease in the electrical impedance of scala media of the guinea pig. The similarity between the change in CR due to COCB stimulation and the change in CR due to negative d.c. polarization (Mountain, D.C., Hubbard, A.E. and Geisler, C.D. (1980): Hearing Res. 3, 215-229) suggests that the CR is dependent on the hair cell membrane potential measured with respect to scale tympani.

Sound-induced resistance changes in the inner ear

A new technique for measuring sound-induced resistance changes (CR) in scala media in response to pure-tone stimuli by injecting alternating current into guinea-pig cochleas was reported recently [C.D. Geisler et al., J. Acoust. Soc. Am. 61, 1557-1566 (1977)]. Detailed measurements with this technique indicate that while the CR behaves approximately as does the cochlear microphonic (CM) there can be very significant differences between the two variables under certain experimental conditions. Computer analysis of simultaneously recorded CR voltage components and CM indicates that the CR harmonics, in both amplitude and phase, behaved differently with sound intensity and with asphyxia than did the CM harmonies (A.E. Hubbard et al., J. Acoust. Soc. Am 66, 431-445 (1979)]. Direct current injection and stimulation of the crossed olivocochlear bundle (COCB) indicate further differences between CM and CR (D.C. Mountain, Ph.D. thesis, University of Wisconsin-Madison, 1978). Positive dc caused a relative augmentation of CM that grew with sound intensity, and a relative reduction in CR magnitude that decreased with intensity. Negative dc caused effects of similar magnitude but opposite sign. COCB stimulation caused enhancement of both CM and CR. Present models cannot account quantitatively for these results.

Auditory-nerve fiber encoding of two-tone approximations to steady-state vowels

Responses to two harmonically related tones, approximating the lowest formants of nine American English vowels, were recorded from single auditory-nerve fibers. Data were compiled as period histograms for tones presented singly and in combination using the fundamental frequency of the two-tone complex as the time base. The amplitudes of the primary frequency components present in a histogram were estimated by least-squares fitting a half-wave rectified sum of the stimulating sinusoids plus a constant. Nonlinear interactions resulted for most two-tone stimuli: one tone dominated the response. When one tone was equal to best frequency, that tone always controlled discharge timing, usually suppressing the response to the second tone. Complicated interactions took place when the stimulating frequencies bracketed best frequency. The tone nearest best frequency was most effective near threshold, while higher stimulus levels usually favored the low-frequency tone. Nevertheless, the suppression mechanisms appear to provide an effective spatial separation in the cochlea for the response components to each vowel approximation. Fourier analysis of the period histograms yielded qualitatively similar results.

A study of hospital based ambulance systems in Wisconsin

This study was undertaken to determine the advantages and disadvantages of a hospital-based emergency medical service system with hospital employees serving as emergency medical technicians. This type of service is operating in thirteen hospitals in Wisconsin. A series of interviews was conducted to obtain pertinent information regarding the hospital, emergency medical vehicles, ambulance attendants, finances, communications and personal reactions to the system. The data indicate that hospital-based services using hospital employees can operate with minimal interference to regular patient care, and with apparent advantages to the emergency patients. In the rural, sparsely populated areas which were studied, response times from the hospital-based systems were comparable to other rural systems, and the financial and administrative aspects of the system were reviewed.

Auditory nerve fiber response to wide-band noise and tone combinations

1. Responses of single auditory nerve fibers to combinations of noise and tone were obtained. The results were found to depend on the relative effectiveness of each stimulus when presented alone. 2. When the response rate to one stimulus presented alone was considerably greater than the response rate to the other stimulus presented alone, the more effective stimulus dominated the responses when the two stimuli were combined. The more effective stimulus captured the response of the neuron. Thus, intense noise was found to mask responses to weaker tones, and intense tones were found to mask responses to weaker noise. This masking of the weaker stimulus is thought to enhance the signal-to-noise ratio of the most prominent response component. 3. When the two stimuli had similar effectiveness, complex interactions occurred. When the tone was near best (characteristic) frequency, partial summation effects occured. The tone partially suppressed the responses to the noise if other frequencies were used. Tones above best frequency caused particularly powerful suppression. 4. The bandwidth of the noise was varied somewhat. While bandwidth affected the effectiveness of the noise, it did not affect the types of interactions observed. 5. For a neuron which was essentially silent in the absence of acoustic stimuli, adding a weak level of noise lowered the threshold of responsiveness to the tone.

A model of the peripheral auditory system responding to low-frequency tones

A model of the peripheral auditory system responding to low-frequency tone stimulation is given. The model is of the type previously introduced by Weiss (1966). It includes three interconnected parts: a linear model of the ear's mechanical system, a model of the cochlear transducer, and a stochastic model of an auditory nerve fiber. The output of the model accurately mimics many characteristics of the output of some auditory nerve neurons responding to sinusoidal stimuli but is unable to successfully match all reported aspects of data obtained from other of these neurons. Characteristics of the model neurons are discussed.