Encoding of steady-state vowels in the auditory nerve: representation in terms of discharge rate

Responses of large populations of auditory-nerve fibers to synthesized steady-state vowels were recorded in anesthetized cats. Driven discharge rate to vowels, normalized by dividing by saturation rate (estimated from the driven rate to CF tones 50 dB above threshold), was plotted versus fiber CF for a number of vowel levels. For the vowels /I/ and /e/, such rate profiles showed a peak in the region of the first formant and another in the region of the second and third formants, for sound levels below about 70 dB SPL. For /a/ at levels below about 40 dB SPL there are peaks in the region of the first and second formants. At higher levels these peaks disappear for all the vowels because of a combination of rate saturation and two-tone suppression. This must be qualified by saying that rate profiles plotted separately for units with spontaneous rates less than one spike per second may retain peaks at higher levels. Rate versus level functions for units with CFs above the first formant can saturate at rates less than the saturation rate to CF to-es or they can be nonmonotonic; these effects are most likely produced by the same mechanism as that involved in two-tone suppression.     

Responses to amplitude-modulated tones in the auditory nerve of the cat.

Sinusoidally amplitude-modulated (AM) tones are frequently used in psychophysical and physiological studies, yet a comprehensive study on the coding of AM tones in the auditory nerve is lacking. AM responses of single auditory-nerve fibers of the cat are studied, systematically varying modulation depth, frequency, and sound level. Synchrony-level functions were nonmonotonic with maximum values that were inversely correlated with spontaneous rate (SR). In most fibers, envelope phase-locking showed a positive gain. Modulation transfer functions were uniformly low pass. Their corner frequency increased with characteristic frequency (CF), but changed little for CFs above 10 kHz. The highest modulation frequencies to which phase locking occurred were more than 0.8 oct lower than the highest frequencies to which phase locking to pure tones occurs. Cumulative, or unwrapped, phase increased linearly with modulation frequency: The slope was inversely related to CF, and slightly higher than group delays reported for pure tones. High SR, low CF fibers showed the poorest envelope phase locking. In some low CF fibers, phase locking increased at high levels, associated with "peak-splitting" phenomena. Changes in average rate due to modulation were small, and could be enhancement or suppression.     

Suppression of auditory nerve responses. II. Suppression threshold and growth, iso-suppression contours

Two-tone "synchrony suppression" was studied in responses of single auditory nerve fibers recorded from anesthetized cats. Suppression thresholds for suppressor tones set to a fiber's characteristic frequency (CF) were approximately equal to discharge rate thresholds for CF tones. Suppression thresholds above and below CF were usually lower than the corresponding discharge rate thresholds. However, at all frequencies studied (including CF), suppression thresholds were higher than the corresponding thresholds for discharge synchronization. Across fibers, rates of suppression growth for suppressors at CF were greatest in low-CF fibers and least in high-CF fibers, and there was a systematic decrease in suppression growth rate at CF as CF increased. Within fibers, rates of suppression growth above CF were typically less than at CF, and slopes were monotonically decreasing functions of frequency. Within-fiber rates of suppression growth below CF were variable, but they usually were greater than rates of growth at CF. Iso-suppression contours (frequencies and intensities producing criterion amounts of suppression) indicated that tones near CF are the most potent suppressors at near-threshold intensities, and that the frequency producing the most suppression usually shifts downward as the amount of suppression increases. These data support the notion that synchrony suppression arises primarily as a passive consequence of hair cell activation.     

Noise susceptibility and immunity of phase locking in amphibian auditory-nerve fibers

Recordings from auditory-nerve fibers in the anesthetized frog revealed that addition of broadband noise results in a reduction in the ability of a fiber to phase lock to a continuous pure tone. In particular, our results suggest that: (i) there is a threshold below which masking noise has little or no effect on vector strength (VS); then with increasing masking noise level, VS appears to decrease monotonically for all test frequencies (TFs); (ii) there exist subpopulations of auditory-nerve fibers in the frog for which the deterioration of phase locking to tones in wideband noise depends critically on the relationship of the TF to the fiber's CF. Specifically, in one subpopulation (43% of the fibers studied), the rate of VS decrease with increasing levels of masking noise is greater for CF tones than it is for TFs greater than CF. The net result is a "crossing" of the VS versus masking noise functions (e.g., Fig. 6); (iii) there exists a small subpopulation of amphibian papillar (a.p.) fibers for which the rate of VS decrease with increasing levels of masking noise is less for TFs less than CF than it is for CF tones (e.g., Fig. 5); (iv) there is a pronounced noise-induced phase lead for TFs greater than CF, whereas, for stimulus tones at or below CF, the preferred firing phase is nearly noise-level independent; (v) the remainder of the sample consists of fibers in which the VS-falloff rates appear to be test-frequency independent; (vi) addition of wideband masking noise to a CF tone, and increasing the CF-tone level in the absence of noise, produced (qualitatively) similar effects on the preferred firing phase of auditory-nerve fibers (e.g., Figs. 1 and 7). Thus amphibian auditory-nerve fibers appear to be energy detectors, i.e., exhibit phase shifts corresponding to the total energy within the filter passband defined by the frequency-threshold curve.     

Rate and timing cues associated with the cochlear amplifier: level discrimination based on monaural cross-frequency coincidence detection.

The perceptual significance of the cochlear amplifier was evaluated by predicting level-discrimination performance based on stochastic auditory-nerve (AN) activity. Performance was calculated for three models of processing: the optimal all-information processor (based on discharge times), the optimal rate-place processor (based on discharge counts), and a monaural coincidence-based processor that uses a non-optimal combination of rate and temporal information. An analytical AN model included compressive magnitude and level-dependent-phase responses associated with the cochlear amplifier, and high-, medium-, and low-spontaneous-rate (SR) fibers with characteristic frequencies (CFs) spanning the AN population. The relative contributions of nonlinear magnitude and nonlinear phase responses to level encoding were compared by using four versions of the model, which included and excluded the nonlinear gain and phase responses in all possible combinations. Nonlinear basilar-membrane (BM) phase responses are robustly encoded in near-CF AN fibers at low frequencies. Strongly compressive BM responses at high frequencies near CF interact with the high thresholds of low-SR AN fibers to produce large dynamic ranges. Coincidence performance based on a narrow range of AN CFs was robust across a wide dynamic range at both low and high frequencies, and matched human performance levels. Coincidence performance based on all CFs demonstrated the "near-miss" to Weber's law at low frequencies and the high-frequency "mid-level bump." Monaural coincidence detection is a physiologically realistic mechanism that is extremely general in that it can utilize AN information (average-rate, synchrony, and nonlinear-phase cues) from all SR groups.     

Changes in the phase of excitor-tone responses in cat auditory-nerve fibers by suppressor tones and fatigue

The responses of single auditory-nerve fibers in anesthetized cats to two-tone stimuli were studied. One of the two tones, F1, was near, above, or below characteristic frequency (CF). The second tone, F2, was located above CF. With sufficient care, F2 was made purely suppressive, eliciting no synchrony responses by itself. The vector phases of the associated period histogram calculated for F1 were carefully studied. For 78% of the fibers under study, a statistically significant increase in phase lag was consistently observed when a suppression of rate discharge occurred. The phase-intensity curve did not approximate a horizontally shifted version of the unsuppressed curve, as is seen for the related rate- and synchrony-intensity curves; rather, the amount of phase shift at any one stimulus condition tended to be monotonically related to the amount of rate suppression generated (vertical shift). Using two different measures, a significant correlation was found between the added phase lag and the discharge-rate reduction caused by F2. The amount of phase lag, along with the corresponding rate reduction, increases with the increasing intensity of F2 within the suppression area, and decreases as F2 moves away from it. These phase-lag effects were found to be uncorrelated with a fiber's CF, with its spontaneous rate, with its threshold, or with its Q value. By contrast, a reduction of discharge rate due to adaptation was not accompanied by any significant phase shift. Fatigue of the fiber due to lengthy sound exposure was found to have strong effects on the shift of response phase to single-tone stimuli.     

Psychophysical evidence for auditory compression at low characteristic frequencies

Psychophysical estimates of compression often assume that the basilar-membrane response to frequencies well below characteristic frequency (CF) is linear. Two techniques for estimating compression are described here that do not depend on this assumption at low CFs. In experiment 1, growth of forward masking was measured for both on- and off-frequency pure-tone maskers for pure-tone signals at 250, 500, and 4000 Hz. The on- and off-frequency masking functions at 250 and 500 Hz were just as shallow as the on-frequency masking function at 4000 Hz. In experiment 2, the forward masker level required to mask a fixed low-level signal was measured as a function of the masker-signal interval. The slopes of these functions did not differ between signal frequencies of 250 and 4000 Hz for the on-frequency maskers. At 250 Hz, the slope for the 150-Hz masker was almost as steep as that for the on-frequency masker, whereas at 4000 Hz the slope for the 2400-Hz masker was much shallower than that for the on-frequency masker. The results suggest that there is substantial compression, of around 0.2-0.3 dB/dB, at low CFs in the human auditory system. Furthermore, the results suggest that at low CFs compression does not vary greatly with stimulation frequency relative to CF.     

Coding of spectral fine structure in the auditory nerve. II: Level-dependent nonlinear responses

Phase-locked discharge patterns of single cat auditory-nerve fibers were analyzed in response to complex tones centered at fiber characteristic frequency (CF). Signals were octave-bandwidth harmonic complexes defined by a center frequency F and an intercomponent spacing factor N, such that F/N was the fundamental frequency. Parameters that were manipulated included the phase spectrum, the number of components, and the intensity of the center component. Analyses employed Fourier transforms of period histograms to assess the degree to which responses were synchronized to the frequencies present in the acoustic stimulus. Several nonlinearities were observed in the response as intensity was varied between threshold and 80-90 dB SPL. Response nonlinearities were strong for all signals except those with random phase spectra. The most commonly observed nonlinearity was an emphasis of one or more stimulus components in the response. The degree of nonlinearity usually increased with intensity and signal complexity and decreased with fiber frequency selectivity. Half-wave rectification introduced synchronization to the missing fundamental. The strength of the response at the fundamental was related to stimulus crest factor. Signals with low center frequencies and high crest factors often elicited instantaneous discharge rates at the theoretical maximum of pi CF. This suggests that the probability of spike generation approaches one during high-amplitude waveform segments. Response nonlinearity was interpreted as arising from three sources, namely, cochlear mechanics, compression of instantaneous discharge rate, and saturation of average discharge rate. At near-threshold intensities, fibers with high spontaneous rates exhibited responses that were linear functions of stimulus waveshape, whereas fibers with low spontaneous spike rates produced responses that were best described in terms of an expansive nonlinearity.     

Speech coding in the auditory nerve: V. Vowels in background noise

Responses of auditory-nerve fibers to steady-state, two-formant vowels in low-pass background noise (S/N = 10 dB) were obtained in anesthetized cats. For fibers over a wide range of characteristic frequencies (CFs), the peaks in discharge rate at the onset of the vowel stimuli were nearly eliminated in the presence of noise. In contrast, strong effects of noise on fine time patterns of discharge were limited to CF regions that are far from the formant frequencies. One effect is a reduction in the amplitude of the response component at the fundamental frequency in the high-CF regions and for CFs between F1 and F2 when the formants are widely separated. A reduction in the amplitude of the response components at the formant frequencies, with concomitant increase in components near CF or low-frequency components occurs in CF regions where the signal-to-noise ratio is particularly low. The processing schemes that were effective for estimating the formant frequencies and fundamental frequency of vowels in quiet generally remain adequate in moderate-level background noise. Overall, the discharge patterns contain many cues for distinctions among the vowel stimuli, so that the central processor should be able to identify the different vowels, consistent with psychophysical performance at moderate signal-to-noise ratios.     

Dynamic rheological and dynamic thermomechanical properties of poly(trimethylene terephthalate)/short carbon fibre composites

The dynamic rheology and thermomechanical properties of poly(trimethylene terephthalate) (PTT)/short carbon fibre (CF) composites at different mechanical states were investigated by a rotational rheometer and a dynamic mechanical analyzer (DMA). At molten state, the composite melts were pseudo-plastic fluids, and the complex viscosity of the composite melts decreased much with increasing CF content because of the poor adhesion at the fiber/matrix interface. The viscous behavior was predominant rather than elastic behavior in the composites melt and viscous behavior was increased with increasing CF at low shearing frequency. An apparent slope change in storage modulus and loss modulus plot suggested that a structure change occurred in the melt that was dependent on shearing frequency. At glassy state, the storage modulus increased with increasing CF content, suggesting that CFs had good reinforcing effect on PTT. At glass transition region, the increasing loss modulus indicated a better toughness of the composites, and the elastic behavior was predominant rather than viscous behavior. Moreover, the glass-transition temperatures of the composites increased with 10% CF content. The composites have larger cold-crystallization rate than pure PTT.     

Diversity of adaptation patterns in responses of eighth nerve fibers in the bullfrog, Rana catesbeiana

The firing patterns of eighth nerve fibers in the bullfrog, Rana catesbeiana, were analyzed for responses to long duration tone bursts at best excitatory frequency ( BEF ) and at frequencies along the upper and lower boundaries of the excitatory tuning curve of each fiber. These firing patterns were used as an index of the degree of short-term adaptation of each fiber. Amphibian papilla fibers (with BEFs 100-1000 Hz) exhibited marked diversity in their firing patterns to BEF tones, ranging from very flat or tonic (sustained responses throughout the duration of the stimulus) to very peaked or phasic (responding primarily or exclusively to stimulus onset). Moreover, the degree of short-term adaptation shown by an individual fiber varied with stimulating frequency. The firing patterns of amphibian papilla fibers tended to become more tonic as stimulus frequency was lowered below BEF ; conversely, as stimulus frequency was increased above BEF , firing patterns either showed little change from that at BEF , or became more phasic. A similar frequency dependence of adaptation has not been reported in responses of mammalian eighth nerve fibers with comparable BEFs . The firing patterns of basilar papilla fibers ( BEFs greater than 1000 Hz) remained similar in response to both BEF and non- BEF tones. These data reveal that the firing patterns and degrees of short-term adaptation of amphibian papilla fibers vary considerably across the tuning curve, whereas those of basilar papilla fibers remain relatively more constant with changes in stimulating frequency.(ABSTRACT TRUNCATED AT 250 WORDS)     

Speech coding in the auditory nerve: III. Voiceless fricative consonants

Responses of auditory-nerve fibers in anesthetized cats were recorded for synthetic voiceless fricative consonants. The four stimuli (/x/, /s/, /s/, and /f/) were presented at two levels corresponding to speech in which the levels of the vowels would be approximately 60 and 75 dB SPL, respectively. Discharge patterns were characterized in terms of PST histograms and their power spectra. For both stimulus levels, frequency regions in which the stimuli had considerable energy corresponded well with characteristic-frequency (CF) regions in which average discharge rates were the highest. At the higher level, the profiles of discharge rate against CF were more distinctive for the stimulus onset than for the central portion. Power spectra of PST histograms had large response components near fiber characteristic frequencies for CFs up to 3-4 kHz, as well as low-frequency components for all fibers. The relative amplitudes of these components varied for the different stimuli. In general, the formant frequencies of the fricatives did not correspond with the largest response components, except for formants below about 3 kHz. Processing schemes based on fine time patterns of discharge that were effective for vowel stimuli generally failed to extract the formant frequencies of fricatives.     

Neural representation of sound amplitude by functionally different auditory receptors in crickets

The physiological characteristics of auditory receptor fibers (ARFs) of crickets, a model system for studying auditory behaviors and their neural mechanisms, are investigated. Unlike auditory receptor neurons of many animals, cricket ARFs fall into three distinct populations based on characteristic frequency (CF) [Imaizumi and Pollack, J. Neurosci. 19, 1508-1516 (1999)]. Two of these have CFs similar to the frequency component of communication signals or of ultrasound produced by predators, and a third population has intermediate CF. Here, sound-amplitude coding by ARFs is examined to gain insights to how behaviorally relevant sounds are encoded by populations of receptor neurons. ARFs involved in acoustic communication comprise two distinct anatomical types, which also differ in physiological parameters (threshold, response slope, dynamic range, minimum latency, and sharpness of tuning). Thus, based on CF and anatomy, ARFs comprise four populations. Physiological parameters are diverse, but within each population they are systematically related to threshold. The details of these relationships differ among the four populations. These findings open the possibility that different ARF populations differ in functional organization.     

Speech coding in the auditory nerve: IV. Sounds with consonant-like dynamic characteristics

Discharge patterns of auditory-nerve fibers in anesthetized cats were obtained for two stimulus levels in response to synthetic stimuli with dynamic characteristics appropriate for selected consonants. A set of stimuli was constructed by preceding a signal that was identified as /da/by another sound that was systematically manipulated so that the entire complex would sound like either /da/, /ada/, /na/, /sa/, /sa/, or others. Discharge rates of auditory-nerve fibers in response to the common /da/-like formant transitions depended on the preceding context. Average discharge rates during these transitions decreased most for fibers whose CFs were in frequency regions where the context had considerable energy. Some effect of the preceding context on fine time patterns of response to the transitions was also found, but the identity of the largest response components (which often corresponded to the formant frequencies) was in general unaffected. Thus the response patterns during the formant transitions contain cues about both the nature of the transitions and the preceding context. A second set of stimuli sounding like /s/ and /c/ was obtained by varying the duration of the rise in amplitude at the onset of a filtered noise burst. At both 45 and 60 dB SPL, there were fibers which showed a more prominent peak in discharge rate at stimulus onset for /c/ than for /s/, but the CF regions that reflected the clearest distinctions depended on stimulus level. The peaks in discharge rate that occur in response to rapid changes in amplitude or spectrum might be used by the central processor as pointers to portions of speech signals that are rich in phonetic information.     

Nonlinear phenomena as observed in the ear canal and at the auditory nerve

We report here several measures of nonlinear effects in the mammalian ear made in the external auditory meatus and in single neurons of the auditory nerve. We have measured the 2f1-f2 and the f2-f1 distortion products and we have found that the neural distortion product threshold curve for 2f1-f2 mirrors the low-frequency side of the frequency threshold curve, when the neural distortion product threshold curve of 2f1-f2 is plotted versus log(f2/f1) its slope is about 50 dB/oct and its intercept is 10-20 dB above the frequency threshold at the characteristic frequency CF, substantial 2f1-f2 distortion was seen in all animals studied while the f2-f1 distortion product was only rarely found at substantial levels, and the distortion product pressure observed in the ear canal was at a level equal to that detected at threshold by the neural units under study. We have also made measurements of two-tone rate suppression thresholds using two new and consistent threshold paradigms. We find that for high and intermediate characteristic frequency neural units the suppression threshold is independent of frequency and at a level of about 70 dB SPL, the suppression above CF is much less than below CF, and the tip of the frequency tuning curve can be suppressed by up to 40 dB by a low-frequency suppressor.     

Effects of acoustic overstimulation on spectral and temporal processing in the amphibian auditory nerve

Activity of isolated auditory-nerve fibers in tree frogs (Eleutherodactylus coqui) exposed to continuous 3-min tones of different intensities at their characteristic frequencies (CFs) was recorded. Period histograms show a retardation in the preferred phase of discharge during and after the cessation of the exposure. Postexposure phase shift is concomitant with an elevation in CF thresholds and related to the level of tone exposure above threshold. Vector strength does not show comparable trends of change; postexposure shifts are related to preexposure CF thresholds. Recovery of phase retardation is rapid; units exposed to successive 3-min tones of the same intensities with intervals of 10-14 min between exposures experienced similar changes in their patterns of temporal discharge. Micromechanical changes affecting stereocilia stiffness or structural alterations in the tectorial membrane of the amphibian papilla may underly the transitory phase shifts observed in traumatized anuran auditory fibers.     

Photostimulated luminescence of corrugated fiberboard as an additional screening method for detecting radiated foods

In this study, PSL of non-irradiated and irradiated corrugated fiberboards (CFs) was investigated to evaluate the possibility that CFs can be used as alternative specimens for the screening detection method of food irradiation. The irradiation at a dose of only 0.15 kGy increased PSL signals of the CF over 1 order of magnitude. The PSL signals increased with increasing in gamma irradiation dose and became almost saturated at a dose of 5 kGy. The core of CFs showed PSL signals sufficient for distinguishing irradiated from non-irradiated at least 6 months after irradiation even though the CF was exposed to light and the environmental temperature increased to 50 °C. These results suggest that the PSL property of the core of CFs is useful for detecting irradiation. However, the large variation of PSL signals among CFs made it difficult to set a well-defined “positive” threshold limit to distinguish irradiated from non-irradiated completely. All of the non-irradiated CFs showed PSL signals above 10³ counts, which is much higher than that detected for foods. It is apparent that the threshold limit of EN 13751 is not applicable to detecting irradiated CFs. More detailed collaborative research with large number of samples is needed to establish new threshold limits involving “intermediate” classification.     

Effect of the surface roughness on interfacial properties of carbon fibers reinforced epoxy resin composites

The effect of the surface roughness on interfacial properties of carbon fibers (CFs) reinforced epoxy (EP) resin composite is studied. Aqueous ammonia was applied to modify the surfaces of CFs. The morphologies and chemical compositions of original CFs and treated CFs (a-CFs) were characterized by Atomic Force Microscopy (AFM), and X-ray Photoelectron Spectroscopy (XPS). Compared with the smooth surface of original CF, the surface of a-CF has bigger roughness; moreover, the roughness increases with the increase of the treating time. On the other hand, no obvious change in chemical composition takes place, indicating that the treating mechanism of CFs by aqueous ammonia is to physically change the morphologies rather than chemical compositions. In order to investigate the effect of surface roughness on the interfacial properties of CF/EP composites, the wettability and Interfacial Shear Strength (IFSS) were measured. Results show that with the increase of the roughness, the wettabilities of CFs against both water and ethylene glycol improves; in addition, the IFSS value of composites also increases. These attractive phenomena prove that the surface roughness of CFs can effectively overcome the poor interfacial adhesions between CFs and organic matrix, and thus make it possible to fabricate advanced composites based on CFs.     

Auditory-nerve response from cats raised in a low-noise chamber

A litter of four cats, born and raised in a soundproofed chamber, was studied in an attempt to determine which, if any, features of the auditory-nerve response from routinely available cats might be due to the chronic effects of noise exposure. Two features of routine-normal response were especially suspect in this regard: (1) a "notch" in the distribution of single-unit thresholds centered at characteristic frequencies (CF's) near 3 kHz and (2) a compression of the distribution of rates of spontaneous discharge for units with CF above 10 kHz. A third feature of response in routine animals was the presence of a small number (roughly 10%) of units with virtually no spontaneous discharge and very high thresholds, sometimes 80 dB less sensitive than high-spontaneous units of similar CF. In the data from chamber-raised animals, the high-spontaneous units showed exceptionally low thresholds at all CF regions, however, there were signs of the midfrequency notch in the threshold distribution of at least two of these animals. The compression of the spontaneous rate distribution was not seen in any of the three most sensitive animals. The data suggest that there is a significant amount of "normal pathology" in the high-CF units from routine animals. Low-spontaneous, high-threshold units were present in all four chamber-raised ears with the same characteristics as in routine animals (exceptionally narrow tuning curves and exceptionally low maximum discharge rates) and at roughly the same percentage of the unit sample. A class of units with medium spontaneous rates and intermediate thresholds could also be identified. The possible significance of a classification of auditory-nerve units according to spontaneous rate is discussed.     

Stochastic resonance at the periphery of auditory system: A simulation experiment

An auditory nerve fiber model is studied. The model includes the formation of the response of the basilar membrane, formation of the receptor potential of the internal hair cell, formation of the synaptic potential of the auditory nerve fiber, and transformation of the synaptic potential into a sequence of spikes. The role of this transformation, as well as the role of changes in the excitability of the fiber after the spike generation in the coding of amplitude-modulated signals is revealed for the cases of signals of medium (i.e., corresponding to the sloping part of the curve representing the mean firing rate of an auditory nerve fiber as a function of the stimulus level) and subthreshold levels. Simulated experiments show that the coding of the envelope of a medium-level amplitude-modulated signal is a dynamic process, which includes fine tuning (adaptation) of the threshold of the auditory nerve fiber to the stimulus level. The coding of the signal envelope is little affected by the slope of the dependence of the mean firing rate on the stimulus level. However, fibers with steep input-output characteristics may exhibit stochastic resonance properties. Owing to these properties, such fibers are capable of reproducing the envelope of a subthreshold modulated signal when weak noise is added to it. Ways are considered for extending the range of subthreshold signal and noise levels within which the envelope of a modulated signal is reproduced (or the phenomenon of stochastic resonance is observed).