Lateralization of bands of noise as a function of combinations of interaural intensive differences, interaural temporal differences, and bandwidth

Listeners indicated the intracranial position of bands of noise (from 50 to 400 Hz in width) for several combinations of interaural intensive differences (IID), and interaural temporal differences (ITD), and/or interaural phase differences (IPD). All ITD and IPD combinations produced an interaural delay of 1500 microseconds at the center frequency of the noise. The interaural phase spectra were constructed to produce several patterns of putative cross-correlation functions. Potency of IIDs depended greatly on particular combinations of bandwidth, ITD and IPD. For some combinations, changing the IID by only 3 dB resulted in large shifts in laterality (sometimes moving the image from near one ear to near the other). The complex interactions observed make the results incompatible with the traditional notion that IIDs simply act as weights or scalars. Rather, IIDs act in two distinct manners: (1) as independent scalar quantities and (2) by interacting with specific combinations of bandwidth and ITD/IPD, which is believed to reflect an action within the cross correlation surface.     

Perceived lateral position of narrow-band noise in hearing-impaired and normal-hearing listeners under conditions of equal sensation level and sound-pressure level

The perceived lateral position of narrow-band noise (NBN) was studied in a graphic pointer task as a function of the method of compensation for interaural threshold asymmetries in hearing-impaired and normal-hearing subjects. The method of compensation consisted of equal sensation level (EqSL) or equal sound-pressure level (EqSPL) at the two ears within the same subject. The NBN signals were presented at 11 center frequencies with interaural intensity differences (IIDs) that varied from -20 to +20 dB. When equalizing by SL, the perceived lateral position is essentially linearly dependent on the degree and direction of asymmetry in asymmetric normal-hearing and hearing-impaired listeners. Equalizing by SPL shows no such dependency but produces images that are lateralized close to the midline. These results reveal that subjects may have adapted to their threshold asymmetries. These results will be discussed in terms of the fitting of binaural hearing aids.     

Human long-latency responses to brief interaural disparities of intensity

The electroencephalographic responses to abrupt changes in interaural differences of time (ITD) and intensity (IID) should provide important information on the dynamic characteristics and integrity of the binaural mechanisms detecting the azimuthal shifts of a sound image. However, a change in either or both of these cues to sound lateralization would stimulate not only the binaural mechanisms but also the monaural ones. There are several reports evidencing that in the case of ITD changes this problem can be overcome by using time-shifted noise or repetitive clicks. Any change in IID, however, will inevitably have a stimulating effect also on purely monaural mechanisms. Therefore, the stimulation techniques described in the literature so far for recording the long-latency responses related to IID mechanism cannot be regarded as being specific for binaural mechanisms. We used dichotically presented 100/s click trains which were amplitude modulated with a random sequence of 50 or 100 ms square wave-intervals, so that the sound intensities at the two ears simultaneously alternated between 60 dB and 80 dB levels except during brief periods of time (50 ms) in which the interaural intensity balance was impaired, leading to an IID of 20 dB every 2 s. Owing to the fact that the cortical mechanisms remain unresponsive to repetitive stimuli presented with intervals shorter than a certain recovery period, this stimulus did not evoke any significant potential when it was presented monotically or diotically, yet it could produce lateral sound image shifts and therefore evoke pronounced long-latency responses when presented dichotically. The main components N1 and P2 of these shift responses and those of the pip responses, also recorded from the same subjects, were compared with respect to their midline distributions and hemispheric or bilateral asymmetries. The significant differences found between the shift and pip responses indicated that those evoked by the IID stimulation we designed should not be considered simply as a non-specific vertex potential.     

Functional mapping of the U3 small nucleolar RNA from the yeast Saccharomyces cerevisiae

The 2 f1-f2 distortion product otoacoustic emission (DP) was measured in 20 normal hearing subjects and 15 patients with moderate cochlear hearing loss and compared to the pure-tone hearing threshold, measured with the same probe system at the f2 frequencies. DPs were elicited over a wide primary tone level range between L2 = 20 and 65 dB SPL. With decreasing L2, the L1-L2 primary tone level difference was continuously increased according to L1 = 0.4L2 + 39 dB, to account for differences of the primary tone responses at the f2 place. Above 1.5 kHz, DPs were measurable with that paradigm on average within 10 dB of the average hearing threshold in both subject groups. The growth of the DP was compressive in normal hearing subjects, with strong saturation at moderate primary tone levels. In cases of cochlear impairment, reductions of the DP level were greatest at lowest, but smallest at highest stimulus levels, such that the growth of the DP became linearized. The correlation of the DP level to the hearing threshold was found to depend on the stimulus level. Maximal correlations were found in impaired ears at moderate primary tone levels around L2 = 45 dB SPL, but at lowest stimulus levels in normal hearing (L2 = 25 dB SPL). At these levels, 17/20 impaired ears and 14/15 normally hearing ears showed statistically significant correlations. It is concluded that for a clinical application and prediction of the hearing threshold, DPs should be measured not only at high, but also at lower primary tone levels.     

Lateralization of High Frequency Sounds as a Function of Interaural Amplitude Disparity.

Twenty-five subjects made graphic ratings of the perceived lateral position within the head of sounds presented through headphones. The stimuli were high frequency, pure tones and amplitude modulated sounds. For the amplitude modulated sounds, a 200 Hz modulation frequency was combined with carrier frequencies of 2200 Hz, 3200 Hz, 4200 Hz, and 5200 Hz, which were also the pure tone frequencies. Interaural level differences in the signals ranged from zero to 12 dB. The rate of lateralization was defined as the slope of the linear trend relating laterality ratings to interaural level differences. The rate of lateralization was found to be a decreasing function of frequency. The laterality ratings of amplitude modulated signals were nearly identical to those for pure tones. This result suggests that, for high frequency signals, conflicting temporal information that a source is centered is suppressed in favor of information from level differences that the source is off-center. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Pathophysiology of aquaporin-2 in water balance disorders

The binaural interaction component (BIC) of the 500-Hz human frequency-following response (FFR) was evaluated as a function of interaural intensity difference (IID) using a lateralization paradigm. The robust FFR interaction component (FFR-BIC) was shown to decrease systematically with increasing IID with no discernible FFR-BIC for IID values larger than about 20 dB. These findings are similar to that observed for the high-frequency auditory brainstem response interaction component (ABR-BIC). Thus, like the ABR-BIC, the FFR-BIC may be correlated with binaural fusion and the perceived location of the fused image of the sound. These results taken together suggest that the binaural neurons in the brainstem are able to utilize IID cues presented in both low-frequency and high-frequency sounds.     

Effects of click intensity and frequency on the brain-stem auditory evoked potentials in the common marmoset (Callithrix jacchus)

Brain-stem auditory evoked potentials (BAEPs) were recorded in 20 common marmosets (Callithrix jacchus) to investigate the effects of click frequency up to 99 kHz, in consideration of the higher hearing range of the marmoset, and intensity on wave forms and peak latencies. According to the results of BAEP recordings at frequencies of 4, 32, and 99 kHz, the number of components recorded was affected by the stimulus intensity and the clicks at an intensity of 80 dB peak equivalent sound pressure level (pe SPL) had the maximum number of clear components. Therefore, it was indicated that click stimulations at an intensity of 80 dB pe SPL over a broad range of frequencies appears to be useful for recording the maximum number of components in marmosets and may increase the information obtainable from BAEPs. BAEP latencies were prolonged as the stimulus intensity decreased from 100 to 50 dB pe SPL. The effects of stimulus frequency on the wave latencies and amplitudes in response to 80 dB pe SPL at frequencies between 0.5 and 99 kHz revealed various changes: the amplitude of wave I increased at 16 and 32 kHz, but that of waves III and V increased at 4-8 and 64-99 kHz. These increases in amplitudes of the waves may correlate with higher synchronous activity of the peripheral or central auditory pathways.     

Infants' detection of increments in low- and high-frequency noise

A visually reinforced operant paradigm was employed to examine the relationship between the difference limen (DL) for intensity and level of the standard during infancy. In Experiment 1, 7-month-old infants and adults detected increments in continuous noise presented via headphones at each of four levels ranging from 28 to 58 dB SPL. Noise stimuli were 2-octave bands centered at either 400 or 4000 Hz, and increments were 10 and 100 msec in duration. Infants' DLs were significantly larger than those of adult subjects and significantly larger for low- than for high-frequency stimuli. For the high-frequency noise band, infants' DLs were generally consistent with Weber's law, remaining essentially constant for standards higher than 28 dB SPL (3 dB SL) for 100-msec increments and 38 dB SPL (13 dB SL) for 10-msec increments. For low-frequency noise, infants' absolute thresholds were exceptionally high, and sensation levels of the standards were too low to adequately describe the relationship. In Experiment 2, 7-month-old infants detected 10- and 100-msec increments in 400-Hz noise stimuli presented in sound field. Infants' low-frequency DLs were large at low intensities and decreased with increases in level of the standard up to at least 30 dB SL. For both low- and high-frequency noise, the difference between DLs for 10- and 100-msec increments tended to be large at low levels of the standard and to decrease at higher levels. These results suggest that the relationship between the DL and level of the standard varies with both stimulus frequency and duration during infancy. However, stimulus-dependent immaturities in increment detection may be most evident at levels within approximately 30 dB of absolute threshold.     

Effects of sensorineural hearing loss on interaural discrimination and virtual localization

Cross-frequency binaural processing was investigated in listeners with normal hearing (NH) and with bilateral high-frequency sensorineural hearing impairment (IH). In experiment 1 just-noticeable-differences for interaural time and interaural intensity were measured using 1/3-octave narrow-band noises (NBNs) centered at 0.5 and 4 kHz. These stimuli were presented in isolation and in different cross-frequency interaural combinations. IH listeners displayed the best interaural time discrimination when the 0.5-kHz NBN was dichotic and the best intensity discrimination when both bands were dichotic. Both NH listeners (time) and IH listeners (time and intensity) displayed the poorest interaural discrimination when the NBNs were presented simultaneously with interaural differences in only the 4-kHz NBN (0.5 kHz NBN dichotic). Localization accuracy was measured in experiment 2 using the 0.5- and 4-kHz NBNs in isolation and with 0.5-kHz target/4-kHz interferer and 4-kHz target/0.5-kHz interferer conditions. Best localization of NH and IH subjects was seen for the 0.5-kHz target, with or without an interferer. Poorest localization of IH subjects was observed for the 4-kHz target and 0.5-kHz interferer. Results suggest that for these IH subjects, localization is most difficult when they are forced to rely on interaural information in a higher-frequency region with conflicting interaural information at low frequencies.     

Diotic and dichotic detection using multiplied-noise maskers

Detection thresholds were measured with a multiplied-noise masker that was in phase in both ears and a sinusoidal signal which was either in phase or out of phase (NoSo and NoS pi conditions). The masker was generated by multiplying a low-pass noise with a sinusoidal carrier. The signal was a sinusoid with the same frequency as the carrier and a constant phase offset, theta, with respect to the carrier. By adjusting the phase offset, the stimulus properties were varied in such a way that only interaural time delays (theta = pi/2) or interaural intensity differences (theta = 0) were present within the NoS pi stimulus. Thresholds were measured at a center frequency of 4 kHz as a function of bandwidth for theta = pi/2 and for theta = 0. In a second experiment thresholds were measured for a bandwidth of 25 Hz as a function of the center frequency. The results show that narrow-band BMLDs at 4 kHz can amount to 30 dB for the theta = 0 condition. For this condition, narrow-band BMLDs are also reasonably constant across frequency, in contrast to results obtained with standard Gaussian-noise maskers. For theta = pi/2, BMLDs are restricted to the frequency region below 2 kHz provided that the masker is narrow band, but BMLDs of up to 15 dB are found at 4 kHz if the masker is 50 Hz or wider. The frequency dependence of the binaural thresholds seems to be best explained by assuming that the stimulus waveforms are compressed before binaural interaction.     

Hearing in two cricetid rodents: Wood rat (Neotoma floridana) and grasshopper mouse (Onychomys leucogaster).

Investigated the degree to which both general and specific selective pressures have played a role in the evolution of hearing in wood rats and grasshopper mice. The audiograms of 2 wood rats and 3 grasshopper mice were determined with a conditioned avoidance procedure. The wood rats were able to hear tones from 940 Hz to 56 kHz at a level of 60 db sound pressure level (SPL), with their best sensitivity of –3 db occurring at 8 kHz. The hearing of the grasshopper mice ranged from 1.85 to 69 kHz at 60 db (SPL), with their best sensitivity of 9 db also occurring at 8 kHz. Results support the relation between interaural distance and high-frequency hearing and between high- and low-frequency hearing. The inability of the grasshopper mouse and other desert rodents such as kangaroo rats and gerbils to hear low frequencies demonstrates that not all rodents in deserts have developed good low-frequency hearing. (65 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Subjective assessment of allergy relief following group hypnosis and self-hypnosis: a preliminary study

Distortion Product Otoacoustic Emissions (DPOEs) are otoacoustic emissions evoked by two pure equilevel tones (f1, f2) called primaries and are believed to provide frequency-specific information regarding cochlear function. We recorded DPOEs at 2f1-f2 frequency with a constant frequency ration (f2/f1 = 1.22) in 8 normal hearing subjects (16 ears, mean age 28 +/- 1.5) to establish the characteristics of these emissions in the adult population. DPOEs were measured at the following F2 frequencies and respective fp geometric mean frequencies: 696/632, 1001/904, 1501/1360, 2002/1809, 3003/2714, 4004/3626, 5005/4531 e 6006/5435 Hz. Detailed testing included the recording of DPOE "audiograms" and input-output functions depicting the relationship of the amplitudes of DPOE to primary-tone levels ranging from 25 to 70 dB SPL in 5 dB steps. The present findings are in good agreement with investigations based on evoked otoacoustic emissions published by other researchers. The average DPOE "audiograms" demonstrated a low-frequency maximum at 1501 Hz (f2)/1360 (fp) and a high-frequency peak at 5005 Hz (f2)/4531 (fp). The two maximum regions were separated by a minimum around 3003 Hz (f2)/2714 (fp). This study confirms the feasibility of DPOE measurements among adults and provide a normal baseline for this age group. DPOEs could be useful, in association with evoked otoacoustic emissions and with auditory brainstem responses, in obtaining a precise evaluation of the peripheral auditory system.     

Acoustic distortion as a measure of frequency selectivity: relation to psychophysical equivalent rectangular bandwidth

The magnitude of cubic intermodulation distortion generated when two tones are progressively separated in frequency reaches a broad maximum when the distortion frequency falls just over half an octave below the high-frequency stimulus (f2), when this distortion is measured with a microphone in the ear canal. For the component 2f1-f2, this peak occurs at an f2/f1 ratio of approximately 1.2. The tuning, magnitude, and mean group delay of this distortion peak was measured for a fixed f2 of 4 kHz at 40 dB SPL and a varied f1 at 55 dB SPL in eight human subjects with normal hearing. The distortion peak measures were compared with the frequency selectivity at 4 kHz of the same eight subjects derived using a forward-masking notched-noise paradigm. In the six subjects from whom good, repeatable levels of distortion were measured, a significant negative correlation was found between the tuning of the distortion peak and the psychophysical bandwidth at f2. It is concluded that the tuning of the distortion peak may provide an objective measure of frequency selectivity in the human cochlea.     

Acoustic startle threshold of the albino rat (Rattus norvegicus).

The startle threshold of the albino Sprague-Dawley rat runs parallel to the curve of the hearing threshold. The difference between the startle and hearing threshold is 87 dB (SPL) at a background noise level of 75 dB (SPL). At 110 dB (SPL), the threshold has a range from 2 kHz to 50 kHz with a minimum at 10 kHz and a second minimum at 40 kHz. Amplitude and latency of the startle response are not only dependent on the sensation level of the acoustic stimulus but also on the frequency. At threshold, only the head movement component of the startle response is elicited. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Masking-level differences in older adults: the effect of the level of the masking noise

In Experiment 1, masking-level differences (MLDs) for a 500-Hz tone at five masker levels were obtained from younger and older adults. For both age groups, there were no reliable increases in MLD once the spectrum level of the masker exceeded 27 dB SPL. MLDs were larger for younger than for older adults over the range of masker levels tested. In Experiment 2, the levels of both the signal and the masker in one ear were attenuated by either 15 or 30 dB relative to their level in the other ear, which was fixed at a spectrum level of 47 dB SPL. MLDs for both age groups declined with increasing IAA and age-related differences were observed in all conditions. The findings of these experiments indicate that (1) age-related differences in MLDs exist even when the level of the masker is sufficiently high that older adults achieve their plateau performance, and (2) older listeners are not disadvantaged more than younger listeners by interaural differences in the level of the input.     

Lateralization of high frequency, amplitude modulated sounds as a function of carrier frequency

Twenty subjects made graphic ratings of the perceived laterality of amplitude modulated sounds that were presented through earphones. A 200 Hz modulation frequency was combined with carrier frequencies of 2200 Hz, 3200 Hz, 4200 Hz, and 5200 Hz. The modulator sinusoid was delayed to either ear by temporal intervals ranging from zero to 0.6 ms. A significant interaction of carrier frequency with the linear trend for interaural temporal disparity indicated that the slopes of laterality ratings on temporal disparity decreased with carrier frequency. A significant interaction of carrier frequency with the difference in ratings for the 0.6 ms delays to the two ears indicated that the range in laterality ratings decreased with carrier frequency. The results indicate that the amount of laterality is a decreasing function of carrier frequency for high frequency sounds, which may be a consequence of greater weight being given to zero intensity difference as frequency increases.     

IID sensitivity differs between two principal centers in the interaural intensity difference pathway: the LSO and the IC

Interaural intensity differences (IIDs) are the chief cues that animals use to localize high-frequency sounds. Neurons that are sensitive to IIDs are excited by sound at one ear and inhibited by sound at the other. Thus a given IID generates a combination of excitation and inhibition that is reflected in a cell's spike count. In mammals, the so-called "IID pathway" begins in the lateral superior olive (LSO), which is dominated by the type of IID-sensitive neurons just described. The LSO then sends a prominent projection to the inferior colliculus (IC), which also contains a substantial population of IID-sensitive cells. Recent pharmacological studies have suggested that the response properties of IID-sensitive neurons in the IC undergo considerable processing and thus should not simply reflect the output of the LSO. However, we have no direct evidence as to whether IID sensitivity, the defining response feature of these cells, differs at these two levels. The present study makes this direct comparison in the Mexican free-tailed bat, a species that relies greatly on high-frequency hearing and thus on IIDs for localizing sounds in space. Extracellular recording techniques were used to obtain IID functions from 50 IC neurons. Comparable data from 50 LSO cells were available from a previous study. The main result was that IID sensitivity significantly differed between cells in the LSO and the IC. Among LSO cells, sensitivity was centered approximately 0 dB (no intensity difference between the ears) whereas, in the IC, sensitivity was biased toward the inhibitory ear: on average, IC cells required a more intense signal at the inhibitory ear to reach the same degree of suppression as observed in LSO cells. Further analysis showed that the vast majority of IC cells (88%) exhibited a mismatch in the latencies of their inputs: inhibition arrived later when an equally strong excitation and inhibition were elicited; this reduced the effectiveness of the inhibition. Because latency shortens with increasing stimulus intensity, an IID with a more intense signal at the inhibitory ear could equate the latencies of excitation and inhibition, increasing the effectiveness of the inhibition. This result suggests that latency mismatches account, to a great extent, for the difference in sensitivity between the LSO and the IC; and when mismatches were negated by electronically time shifting the signals to the ears, sensitivity was no longer significantly different between the two nuclei.     

Contralateral suppression of transiently evoked otoacoustic emissions by harmonic complex tones in humans

Variations in the amplitude of transiently evoked otoacoustic emissions (TEOAEs) produced by a contralateral complex tone were measured in 26 normal-hearing human subjects. TEOAEs were evoked using a 1-kHz tone pip at 60 dB SPL. The contralateral complex consisted of harmonic components with frequencies between 400 and 2000 Hz; it was presented at levels ranging from 40 to 50 dB SL and its fundamental frequency (F0) was varied. In experiment 1, the dependence of TEOAE amplitude variations on the F0 of the contralateral complex was measured by varying the F0 from 50 to 400 Hz in octave steps. The results revealed a nonmonotonic dependence of TEOAE amplitude variations on contralateral F0, with significantly larger TEOAE suppression for F0's of 100 and 200 Hz than for F0's of 50 and 400 Hz. Experiment 2, in which the harmonics were summed in alternating sine-cosine phase instead of constant sine phase, showed a shift of the function relating TEOAE attenuation to F0 towards lower F0's, indicating that the waveform repetition rate, rather than harmonic spacing, was the actual factor of the dependence of contralateral TEOAE attenuation on F0. Furthermore, significantly smaller suppression was observed with the alternating-phase complexes than with the sine-phase complexes, suggesting an influence of the waveform crest factor. Experiment 3 showed no difference between the contralateral TEOAE attenuation effects produced by positive and negative Schroeder-phase complexes. Overall, these results bring further arguments for the notion that contralaterally induced medial olivocochlear bundle (MOCB) activity, as measured through the contralateral suppression of TEOAEs in humans, is sensitive to the rate of temporal envelope fluctuations of the contralateral stimulus, with preferential rates around 100-200 Hz.     

How the owl resolves auditory coding ambiguity

The barn owl (Tyto alba) uses interaural time difference (ITD) cues to localize sounds in the horizontal plane. Low-order binaural auditory neurons with sharp frequency tuning act as narrow-band coincidence detectors; such neurons respond equally well to sounds with a particular ITD and its phase equivalents and are said to be phase ambiguous. Higher-order neurons with broad frequency tuning are unambiguously selective for single ITDs in response to broad-band sounds and show little or no response to phase equivalents. Selectivity for single ITDs is thought to arise from the convergence of parallel, narrow-band frequency channels that originate in the cochlea. ITD tuning to variable bandwidth stimuli was measured in higher-order neurons of the owl's inferior colliculus to examine the rules that govern the relationship between frequency channel convergence and the resolution of phase ambiguity. Ambiguity decreased as stimulus bandwidth increased, reaching a minimum at 2-3 kHz. Two independent mechanisms appear to contribute to the elimination of ambiguity: one suppressive and one facilitative. The integration of information carried by parallel, distributed processing channels is a common theme of sensory processing that spans both modality and species boundaries. The principles underlying the resolution of phase ambiguity and frequency channel convergence in the owl may have implications for other sensory systems, such as electrolocation in electric fish and the computation of binocular disparity in the avian and mammalian visual systems.