Compressive nonlinearity of human hearing in sound spectra discrimination

In the psychophysical experiments reported here, cochlear compression function was derived by comparing on-frequency and off-frequency masking. The signal was rippled spectrum noise. The ripple density discrimination threshold was measured in the ripple phase reversion test. An increase in masker intensity led to a decrease in a resolvable ripple density threshold. The on-frequency masker level at threshold increased proportionally to the signal intensity. The off-frequency masker level at threshold also increased proportionally to the signal at signal intensity levels below 50 dB, whereas at signal levels above 60 dB SPL, the ratio of the masker level at threshold gradient to signal level gradient was 1 : 5 dB/dB, revealing cochlear compression.     

Hearing Sensitivity to Shifts of Rippled-Spectrum Sound Signals in Masking Noise

The goal of the study was to enlarge knowledge of discrimination of complex sound signals by the auditory system in masking noise. For that, influence of masking noise on detection of shift of rippled spectrum was studied in normal listeners. The signal was a shift of ripple phase within a 0.5-oct wide rippled spectrum centered at 2 kHz. The ripples were frequency-proportional (throughout the band, ripple spacing was a constant proportion of the ripple center frequency). Simultaneous masker was a 0.5-oct noise below-, on-, or above the signal band. Both the low-frequency (center frequency 1 kHz) and on-frequency (the same center frequency as for the signal) maskers increased the thresholds for detecting ripple phase shift. However, the threshold dependence on the masker level was different for these two maskers. For the on-frequency masker, the masking effect primarily depended on the masker/signal ratio: the threshold steeply increased at a ratio of 5 dB, and no shift was detectable at a ratio of 10 dB. For the low-frequency masker, the masking effect primarily depended on the masker level: the threshold increased at a masker level of 80 dB SPL, and no shift was detectable at a masker level of 90 dB (for a signal level of 50 dB) or 100 dB (for a signal level of 80 dB). The high-frequency masker had little effect. The data were successfully simulated using an excitation-pattern model. In this model, the effect of the on-frequency masker appeared to be primarily due to a decrease of ripple depth. The effect of the low-frequency masker appeared due to widening of the auditory filters at high sound levels.     

Binaural masking and sensitivity to interaural delay in the inferior colliculus.

The binaural masking level difference (BMLD) is a psychophysical effect whereby signals masked by a noise at one ear become unmasked by sounds reaching the other. BMLD effects are largest at low frequencies where they depend on signal phase, suggesting that part of the physiological mechanism responsible for the BMLD resides in cells that are sensitive to interaural time disparities. We have investigated a physiological basis for unmasking in the responses of delay-sensitive cells in the central nucleus of the inferior colliculus in anaesthetized guinea pigs. The masking effects of a binaurally presented noise, as a function of the masker delay, were quantified by measuring the number of discharges synchronized to the signal, and by measuring the masked threshold. The noise level for masking was lowest at the best delay for the noise. The mean magnitude of the unmasking across our neural population was similar to the human psychophysical BMLD under the same signal and masker conditions.     

Effect of harmonicity on the detection of a signal in a complex masker and on spatial release from masking

The amount of masking of sounds from one source (signals) by sounds from a competing source (maskers) heavily depends on the sound characteristics of the masker and the signal and on their relative spatial location. Numerous studies investigated the ability to detect a signal in a speech or a noise masker or the effect of spatial separation of signal and masker on the amount of masking, but there is a lack of studies investigating the combined effects of many cues on the masking as is typical for natural listening situations. The current study using free-field listening systematically evaluates the combined effects of harmonicity and inharmonicity cues in multi-tone maskers and cues resulting from spatial separation of target signal and masker on the detection of a pure tone in a multi-tone or a noise masker. A linear binaural processing model was implemented to predict the masked thresholds in order to estimate whether the observed thresholds can be accounted for by energetic masking in the auditory periphery or whether other effects are involved. Thresholds were determined for combinations of two target frequencies (1 and 8 kHz), two spatial configurations (masker and target either co-located or spatially separated by 90 degrees azimuth), and five different masker types (four complex multi-tone stimuli, one noise masker). A spatial separation of target and masker resulted in a release from masking for all masker types. The amount of masking significantly depended on the masker type and frequency range. The various harmonic and inharmonic relations between target and masker or between components of the masker resulted in a complex pattern of increased or decreased masked thresholds in comparison to the predicted energetic masking. The results indicate that harmonicity cues affect the detectability of a tonal target in a complex masker.     

Detection of sonar signals in the presence of pulses of masking noise by the echolocating bat,Eptesicus fuscus

Summary Two big brown bats (Eptesicus fuscus) were trained to report the presence or absence of a virtual sonar target. The bats' sensitivity to transient masking was investigated by adding 5 ms pulses of white noise delayed from 0 to 16 ms relative to the target echo. When signal and masker occurred simultaneously, the bats required a signal energy to noise spectrum level ratio of 35 dB for 50% probability of detection. When the masker was delayed by 2 ms or more there was no significant masking and echo energy could be reduced by 30 dB for the same probability of detection. The average duration of the most energetic sonar signal of each trial was measured to be 1.7 ms and 2.4 ms for the two bats, but a simple relation between detection performance and pulse duration was not found.In a different experiment the masking noise pulses coincided with the echo, and the duration of the masker was varied from 2 to 37.5 ms. The duration of the masker had little or no effect on the probability of detection.The findings are consistent with an aural integration time constant of about 2 ms, which is comparable to the duration of the cries. This is an order of magnitude less than found in backward masking experiments with humans and may be an adaptation to the special constraints of echolocation. The short time of sensitivity to masking may indicate that the broad band clicks of arctiid moths produced as a countermeasure to bat predation are unlikely to function by masking the echo of the moth.Abbreviations SPL sound pressure level - SD standard deviation - SE standard error - BW bandwidth     

Free-field unmasking response characteristics of frog auditory nerve fibers: comparison with the responses of midbrain auditory neurons

Previous studies in the inferior colliculus have shown that spatial separation of signal and noise sources improves signal detection. In this study, we investigated the free-field unmasking response properties of single fibers in the auditory nerve--these were compared to those of inferior colliculus neurons under the same experimental conditions to test the hypothesis that central processing confers advantages for signal detection in the presence of spatially separated noise. For each neuron, we determined the detection threshold for a probe at the unit's best azimuth under three conditions: (1) by itself, (2) when a masker at a constant level was also presented at the unit's best azimuth, and (3) when the masker was positioned at different azimuths. We found that, on average, maskers presented at a unit's best azimuth elevated the probe detection threshold by 4.22 dB in the auditory nerve and 10.97 dB in the inferior colliculus. Angular separation of probe and masker sources systematically reduced the masking effect. The maximum masking release was on average 2.90 dB for auditory nerve fibers and 9.40 dB for inferior colliculus units. These results support the working hypothesis, suggesting that central processing contributes to the stronger free-field unmasking in the inferior colliculus.     

Effect of forward masking on evoked potentials of auditory cortex in guinea pigs

Thresholds of the event-related potentials (ERPs) appearance were measured for one stationary and four moving auditory images presented in silence or under forward masking conditions. The difference between thresholds in silence and after noise masker was considered as masking level. Under the forward masking, the amplitude of the ERP to the first click in the test series decreased in guinea pig auditory cortex. Masking level decreased with the time lag between signal and masker and didn't depend on the fused auditory image localization that corresponded to the first click in different test signals. This fact can support the hypothesis that for the long test signals the initial part can be masked more than the final one. The ERPs amplitude to next clicks in test series depended on interaction of two factors: forward masking in the "masker-signal" system and interaction of separate ERPs in the series evoked by the test signal.     

Latency, threshold and frequency selectivity of the crossed medial cochlear efferent pathways

The efferent innervation of guinea pig cochleas was sectioned medially, at the level of the floor of the fourth ventricle, to study the effects of the crossed part of the medial efferent pathway on the compound action potential (CAP) masking phenomenon. Sectioning reduced CAP masking for a masker level varying with the frequency of the masker and the time elapsed between the masker onset and the probe onset. Functional properties of the crossed part of the medial efferent tracts: latency, thresholds and frequency selectivity, could be deduced from these data. This intensification of the masking phenomenon permitting the improvement of the signal to noise ratio, may thus be attributed to the crossed part of the medial efferent bundle which innervates the outer hair cells.     

Forward masking in the evoked responses of the guinea pig auditory cortex: effects of variation of the interaural time and phase differences

In anaesthesized guinea pigs the evoked potentials of the auditory cortex were studied in a forward masking paradigm. In-phase and out-of-phase binaurally presented clicks with interaural time delay (ITD) were used as masker, in-phase click with ITD = 0 served as probe signal. Addition of the masking stimulus suppressed the probe-evoked response that followed the masker. The magnitude of the suppression correlated with the amount of the masker-evoked response: an increase in masker-evoked excitation caused a greater reduction in probe response magnitude. Amplitude of masker-evoked response was seen to be a monotonic or non-monotonic function of ITD. The non-monotonic response exhibited a sensitivity to the interaural phase differences when in-phase and out-of-phase maskers were presented, and showed the tendency to be periodic function of ITD in the expanded range of ITD values. Phase-sensitive responses differed in recovery time following the in-phase and out-of-phase masking stimuli. At near-threshold levels of a forward masker an enhancement of the probe-evoked response was observed.     

Sound reception of marine mammales depending on parameters and sound-conducting tracks

Underwater audiograms of a northern fur seal, a Caspic seal and a dolphin aphalina were measured under conditions of full or partial (the head above the water) submergence of animals using the method of instrumental conditioned reflexes with food reinforcement. The possibility and peculiarities of sound conduction through the body of marine mammals were investigated by isolating the auricle from the medium of sound spreading (under conditions of partial submergence). By the same technique, the hearing thresholds of Caspic seal were measured in the presence of broad- and narrow-band noises with different central frequencies depending on the medium (underwater or in air) the signal and the noise masker were presented and on the sound-conducting ways (under conditions of full or partial submergence of animals). It was found that aerial and underwater sound-conducting canals of the Caspic seal were functionally connected with each other. The level of hearing masking in the Caspic seal is determined by the tracts of signal and noise conduction, by the differences in sensitivity to the signal and masker, and by their spectral structure. Apparently, the tissues of the seal body considerably change the amplitude-frequency characteristics of the sound.     

Evidence that adaptation of suppression cannot account for auditory enhancement or enhanced forward masking.

Delaying the onset of a signal relative to the onset of a simultaneous notched masker often improves the ability of listeners to 'hear out' the signal at both threshold and suprathreshold levels. Viemeister & Bacon (J. acoust. Soc. Am., 71, 1502-1507 (1982)) suggested that such auditory enhancement effects could be accounted for if the suppression produced by the masker on the signal frequency adapted, thereby releasing the signal from suppression. In support of their hypothesis, Viemeister & Bacon reported that a masker preceded by an enhancer having no component at the signal frequency produced more forward masking than did the masker by itself. Here evidence is provided from five new experiments showing that adaptation of psychophysical two-tone suppression is inadequate to account either for auditory enhancement effects or for the enhanced forward masking demonstrated by Viemeister & Bacon.     

Spatial features of vibrotactile masking effects on airpuff-elicited sensations in the human hand

It was recently shown that the cutaneous sensitivity to airpuffs is decreased by a low-frequency vibrotactile masker in the hairy skin, and by a low-frequency but especially by a high-frequency masker in the glabrous skin. In the current study, the spatial features of this masking effect were determined in four healthy human subjects, using a reaction time paradigm. The masking effect decreased monotonically with increasing interstimulus distance, and identically in longitudinal and transverse (i.e., lateral) directions in the palm or dorsal surface of the hand. The masking effect was stronger in the glabrous than in the hairy skin, especially in the fingers. In the glabrous skin, the spread of masking effect produced by a high-frequency masker was more extensive than that produced by a low-frequency masker. The mechanical spread of high-frequency vibration was less extensive than that of low-frequency vibration in the skin. In the glabrous skin, a masker applied to the tip of the finger produced a stronger masking effect on sensations in the base of the finger than when the masker was located at the base and the test stimulus was located at the tip. It is concluded that mechanical spread of vibration in the skin is of minor importance in explaining the masking effects. Different peripheral neural mechanisms underlie the airpuff-elicited sensations in the hairy and glabrous skin. The afferent inhibitory mechanisms are stronger for signals coming from the glabrous skin of the fingers than for signals coming from the hairy skin. Furthermore, the peripheral innervation density and size of the cortical representational areas may be of importance in determining the magnitude of the masking effect.     

Spatial Features of Vibrotactile Masking Effects on Airpuff-Elicited Sensations in the Human Hand

It was recently shown that the cutaneous sensitivity to airpuffs is decreased by a low-frequency vibrotactile masker in the hairy skin, and by a low-frequency but especially by a high-frequency masker in the glabrous skin. In the current study, the spatial features of this masking effect were determined in four healthy human subjects, using a reaction time paradigm. The masking effect decreased monotonically with increasing interstimulus distance, and identically in longitudinal and transverse (i.e., lateral) directions in the palm or dorsal surface of the hand. The masking effect was stronger in the glabrous than in the hairy skin, especially in the fingers. In the glabrous skin, the spread of masking effect produced by a high-frequency masker was more extensive than that produced by a low-frequency masker. The mechanical spread of high-frequency vibration was less extensive than that of low-frequency vibration in the skin. In the glabrous skin, a masker applied to the tip of the finger produced a stronger masking effect on sensations in the base of the finger than when the masker was located at the base and the test stimulus was located at the tip. It is concluded that mechanical spread of vibration in the skin is of minor importance in explaining the masking effects. Different peripheral neural mechanisms underlie the airpuff-elicited sensations in the hairy and glabrous skin. The afferent inhibitory mechanisms are stronger for signals coming from the glabrous skin of the fingers than for signals coming from the hairy skin. Furthermore, the peripheral innervation density and size of the cortical representational areas may be of importance in determining the magnitude of the masking effect.     

Magnetoencephalographic Study on Forward Suppression by Ipsilateral,Contralateral, and Binaural Maskers

When two tones are presented in a short time interval, the response to the second tone is suppressed. This phenomenon is referred to as forward suppression. To address the effect of the masker laterality on forward suppression, magnetoencephalographic responses were investigated for eight subjects with normal hearing when the preceding maskers were presented ipsilaterally, contralaterally, and binaurally. We employed three masker intensity conditions: the ipsilateral-strong, left-right-balanced, and contralateral-strong conditions. Regarding the responses to the maskers without signal, the N1m amplitude evoked by the left and binaural maskers was significantly larger than that evoked by the right masker for the left-strong and left-right-balanced conditions. No significant difference was observed for the right-strong condition. Regarding the subsequent N1m amplitudes, they were attenuated by the presence of the left, binaural, and right maskers for all conditions. For the left- and right-strong conditions, the subsequent N1m amplitude in the presence of the left masker was smaller than those of the binaural and right maskers. No difference was observed between the binaural and right masker presentation. For left-right-balanced condition, the subsequent N1m amplitude decreased in the presence of the right, binaural, and left maskers in that order. If the preceding activity reflected the ability to suppress the subsequent activity, the forward suppression by the left masker would be superior to that by the right masker for the left-strong and left-right-balanced conditions. Furthermore, the forward suppression by the binaural masker would be expected to be superior to that by the left masker owing to additional afferent activity from the right ear. Thus, the current results suggest that the forward suppression by ipsilateral maskers is superior to that by contralateral maskers although both maskers evoked the N1m amplitudes to the same degree. Additional masker at the contralateral ear can attenuate the forward suppression by the ipsilateral masker.     

GABA-mediated modulation of the discharge pattern and rate-level function of two simultaneously recorded neurons in the inferior colliculus of the big brown bat, Eptesicus fuscus

Neurons in the central nucleus of the inferior colliculus (IC) receive excitatory and inhibitory inputs from both lower and higher auditory nuclei. Interaction of these two opposing inputs shapes response properties of IC neurons. In this study, we examine the interaction of excitation and inhibition on the responses of two simultaneously recorded IC neurons using a probe and a masker under forward masking paradigm. We specifically study whether a sound that serves as a probe to elicit responses of one neuron might serve as a masker to suppress or facilitate the responses of the other neuron. For each pair of IC neurons, we deliver the probe at the best frequency (BF) of one neuron and the masker at the BF of the other neuron and vice versa. Among 33 pairs of IC neurons recorded, this forward masking produces response suppression in 29 pairs of IC neurons and response facilitation in 4 pairs of IC neurons. The degree of suppression decreases with recording depth, sound level and BF difference between each pair of IC neurons. During bicuculline application, the degree of response suppression decreases in the bicuculline-applied neuron but increases in the paired neuron. Our data indicate that the forward masking of responses of IC neurons observed in this study is mostly mediated through GABAergic inhibition which also shapes the discharge pattern of these neurons. These data suggest that interaction among individual IC neurons improves auditory sensitivity during auditory signal processing.     

Solutions to the cocktail party problem in insects: selective filters, spatial release from masking and gain control in tropical crickets

Background

Insects often communicate by sound in mixed species choruses; like humans and many vertebrates in crowded social environments they thus have to solve cocktail-party-like problems in order to ensure successful communication with conspecifics. This is even more a problem in species-rich environments like tropical rainforests, where background noise levels of up to 60 dB SPL have been measured.

Principal Findings

Using neurophysiological methods we investigated the effect of natural background noise (masker) on signal detection thresholds in two tropical cricket species Paroecanthus podagrosus and Diatrypa sp., both in the laboratory and outdoors. We identified three ‘bottom-up’ mechanisms which contribute to an excellent neuronal representation of conspecific signals despite the masking background. First, the sharply tuned frequency selectivity of the receiver reduces the amount of masking energy around the species-specific calling song frequency. Laboratory experiments yielded an average signal-to-noise ratio (SNR) of −8 dB, when masker and signal were broadcast from the same side. Secondly, displacing the masker by 180° from the signal improved SNRs by further 6 to 9 dB, a phenomenon known as spatial release from masking. Surprisingly, experiments carried out directly in the nocturnal rainforest yielded SNRs of about −23 dB compared with those in the laboratory with the same masker, where SNRs reached only −14.5 and −16 dB in both species. Finally, a neuronal gain control mechanism enhances the contrast between the responses to signals and the masker, by inhibition of neuronal activity in interstimulus intervals.

Conclusions

Thus, conventional speaker playbacks in the lab apparently do not properly reconstruct the masking noise situation in a spatially realistic manner, since under real world conditions multiple sound sources are spatially distributed in space. Our results also indicate that without knowledge of the receiver properties and the spatial release mechanisms the detrimental effect of noise may be strongly overestimated.     

The effect of temporal structure on rustling-sound detection in the gleaning bat,Megaderma lyra

For a gleaning bat hunting prey from the ground, rustling sounds generated by prey movements are essential to invoke a hunting behaviour. The detection of prey-generated rustling sounds may depend heavily on the time structure of the prey-generated and the masking sounds due to their spectral similarity. Here, we systematically investigate the effect of the temporal structure on psychophysical rustling-sound detection in the gleaning bat, Megaderma lyra. A recorded rustling sound serves as the signal; the maskers are either Gaussian noise or broadband noise with various degrees of envelope fluctuations. Exploratory experiments indicate that the selective manipulation of the temporal structure of the rustling sound does not influence its detection in a Gaussian-noise masker. The results of the main experiment show, however, that the temporal structure of the masker has a strong and systematic effect on rustling-sound detection: When the width of irregularly spaced gaps in the masker exceeded about 0.3 ms, rustling-sound detection improved monotonically with increasing gap duration. Computer simulations of this experiment reveal that a combined detection strategy of spectral and temporal analysis underlies rustling-sound detection with fluctuating masking sounds.     

不同时程弱前掩蔽声对小鼠下丘神经元声反应的选择性抑制

自由声场条件下,以强度为神经元最小阈值阈上5dB,时程分别为40、60、80和100ms的纯音作为前掩蔽声,观察和记录了不同时程弱前掩蔽声对小鼠（Musmusculus Km）下丘神经元发放和声强处理的影响。实验记录到154个神经元,对其中的104个神经元做了不同时程掩蔽声影响的测试。结果发现：掩蔽声对神经元放电率的抑制作用在时间上表现为前抑制（41％）、后抑制（9％）和全抑制（50％）三种类型。改变掩蔽声时程时,大部分神经元（72％）的抑制类型不发生改变,但少部分神经元（28％）随掩蔽声时程的增加,大量的后抑制类型转变为前抑制或全抑制类型。此外,超过一半的神经元（58．06％）其强度．放电率函数曲线随掩蔽声时程的改变而发生转变,主要表现为单调型向饱和型转变及饱和型向非单调型转变,这种转变并不随掩蔽声时程增加表现出规律性的变化。结果表明,前掩蔽作用于下丘神经元声反应的时间域和强度域时具有不均衡性,推测不同时程弱前掩蔽声激活的抑制性输入能分化性调制下丘神经元声反应特性。     

Spatial unmasking in the echolocating Big Brown Bat, <Emphasis Type="Italic">Eptesicus fuscus</Emphasis>

Masking affects the ability of echolocating bats to detect a target in the presence of clutter targets. It can be reduced by spatially separating the targets. Spatial unmasking was measured in a two-alternative-forced-choice detection experiment with four Big Brown Bats detecting a wire at 1 m distance. Depth dependent spatial unmasking was investigated by the bats detecting a wire with a diameter of 1.2 mm in front of a masker with a threshold distance of 11 cm behind the wire. For angle dependent spatial unmasking the masker was turned laterally, starting from its threshold position at 11 cm. With increasing masker angles the bats could detect thinner wires with diameters decreasing from 1.2 mm (target strength −36.8 dB) at 0° to 0.2 mm (target strength −63.0 dB) at 22°. Without masker, the bats detected wire diameters of 0.16 mm (target strength −66.2 dB), reached with masker positions beyond 23° (complete masking release). Analysis of the sonar signals indicated strategies in the echolocation behavior. The bats enhanced the second harmonics of their signals. This may improve the spatial separation between wire and masker due to frequency-dependent directionality increase of sound emission and echo reception.     

Change in Auditory Image Movement Trajectories under Conditions of Direct Nonsimultaneous Masking

The authors studied fused auditory image (FAI) movement trajectories under conditions of direct nonsimultaneous masking. This movement was created by a gradual change in a dichotically presented series of clicks with interaural differences in stimulation from 0 to ±700 s or from ±700 to 0 s. Binaurally presented transmissions of wide-band noise served as maskers. The location and length of the trajectories were evaluated without a masker and with five values of the time lag between the signal beginning and masker end. When the test signal duration was 200 ms, the length of the trajectories was 33–44° without a masker. In the first test group, this trajectory lay close to the median line of the head without a masker (irrespective of the movement direction) and moved away from it under masking conditions. When the FAI moved from the median line towards the right or left ear, the initial part of the trajectory was masked; when the movement direction was opposite, the final part was masked. In the second group, the trajectories were located near the ears when the FAI moved from either ear and shifted towards the median line as a result of masking. When the movement direction was opposite, they were close to the median line and shifted towards the ear under masking conditions. When the FAI moved along all trajectories, their initial parts were masked.