Interaural time vs. interaural intensity in a lateralization paradigm

In a two-interval lateralization procedure, observers judged whether a stimulus presented with an interaural intensive difference was right or left in lateral space of the same stimulus presented with only an interaural temporal difference. The stimuli were pure tones of 500 and 1,000 Hz and 1,000-Hz low-pass noise. All stimuli were presented at both 65 and 55 dB SPL. For each of several values of interaural time (ranging from 0 to 1,000 microsec across all stimuli), a function was determined which related proportion of “right” relative position judgments to the value of the interaural intensive difference. The intercepts of these functions indicated that a progressively smaller amount of interaural intensive difference was required for the two stimuli to occupy a similar lateral location as the interaural temporal difference was increased. The slopes of the function suggested that the images associated with larger values of the interaural temporal differences are less distinct and blend together more than the images associated with small values of the temporal difference. Thus, the procedure provided a means for comparing the lateral location of images produced by interaural differences of time and intensity.     

The detection of anisochrony in monaural and interaural sound sequences

Nakao and Axelrod (1976) and van Noorden (1975) showed that the threshold for discriminating an anisochronous duple rhythm (a series of clicks with a temporal offset on every other one) from an isochronous rhythm (no offset) is poorer when the clicks are presented alternately to the two ears than when they are presented to the same ears. Van Noorden reported that the difference between the thresholds in the alternating and nonalternating conditions varied with the tempo of the sequence. Nakao and Axelrod found invariance of this threshold difference with sequence speed. According to our quantification of temporal processing of interaural sequences, the latter result should be expected. We carried out five psychophysical experiments to establish interaural and monaural discrimination between isochronous and anisochronous rhythms. Across experiments, base time intervals of 60–720 msec were spanned. The main result was that we replicated the poorer discrimination for interaural sequences. This deterioration in discrimination was the same for all sequence speeds. It was also the case that the thresholds were almost constant up to a sound repetition rate of about 3 per second, but increased linearly with slower rates. This result supports evidence in the literature that temporal processing of sequences faster than about 3–4 sounds per second differs from temporal processing of slower sequences.     

Lateralization of narrow-band noise by blind and sighted listeners

The effects of varying interaural time delay (ITD) and interaural intensity difference (IID) were measured in normal-hearing sighted and congenitally blind subjects as a function of eleven frequencies and at sound pressure levels of 70 and 90 dB, and at a sensation level of 25 dB (sensation level refers to the pressure level of the sound above its threshold for the individual subject). Using an 'acoustic' pointing paradigm, the subject varied the IID of a 500 Hz narrow-band (100 Hz) noise (the 'pointer') to coincide with the apparent lateral position of a 'target' ITD stimulus. ITDs of 0, +/-200, and +/-400 micros were obtained through total waveform delays of narrow-band noise, including envelope and fine structure. For both groups, the results of this experiment confirm the traditional view of binaural hearing for like stimuli: non-zero ITDs produce little perceived lateral displacement away from 0 IID at frequencies above 1250 Hz. To the extent that greater magnitude of lateralization for a given ITD, presentation level, and center frequency can be equated with superior localization abilities, blind listeners appear at least comparable and even somewhat better than sighted subjects, especially when attending to signals in the periphery. The present findings suggest that blind listeners are fully able to utilize the cues for spatial hearing, and that vision is not a mandatory prerequisite for the calibration of human spatial hearing.     

Coherence masking protection for speech in children and adults

In three experiments, we tested the hypothesis that children are more obliged than adults to fuse components of speech signals and asked whether the principle of harmonicity could explain the effect or whether it is, instead, due to children's implementing speech-based mechanisms. Coherence masking protection (CMP) was used, which involves labeling a phonetically relevant formant (the target) presented in noise, either alone or in combination with a stable spectral band (the cosignal) that provides no additional information about phonetic identity and is well outside the critical band of the target. Adults and children (8 and 5 years old) heard stimuli that were either synthetic speech or hybrids consisting of sine wave targets and synthetic cosignals. The target and cosignal either shared a common harmonic structure or did not. An adaptive procedure located listeners' thresholds for accurate labeling. Lower thresholds when the cosignal is present indicate CMP. Younger children demonstrated CMP effects that were both larger in magnitude and less susceptible to disruptions in harmonicity than those observed for adults. The conclusion was that children are obliged to integrate spectral components of speech signals, a perceptual strategy based on their recognition of when all components come from the same generator.     

Masking of tones by tones and of noise by noise

Using a two-alternative temporal forced-choice technique, two binaural detection experiments were performed. In the first, the detectability of a 250-Hz 128-msec tonal signal masked by a gated 70-dB SPL tone of the same frequency and duration was measured as a function of the level of the signal, the phase angle at which the signal was added to the masker, and the interaural phase difference of the signal. In the second experiment, the signal was a wideband (100-3,000 Hz) 128-msec Gaussian noise masked by a continuous Gaussian noise of the same bandwidth and coherent with the signal. The detectability of this noise signal was measured as a function of the same variables investigated in the first experiment. In both experiments detectability was found to follow a simple energy- or power-detection model when the interaural phase difference was 0 deg. When the interaural phase difference was 180 deg, the function relating the signal level required for a constant level of performance to the signal-masker phase angle is such that neither the Webster-Jeffress hypothesis nor Durlach’s E-C model accounts for the data. The data are reasonably well fit by a model proposed by Hafter and Carrier.     

Auditory temporal summation in infants and adults: Effects of stimulus bandwidth and masking noise

A visually reinforced operant procedure was employed to determine the behavioral thresholds of 6- to 7-month-old infants and adults for stimuli of various bandwidths and durations. Experiment 1 compared absolute thresholds for broadband and 1/3-octavefiltered clicks and 300-msec noise bursts. For adult subjects, the difference in threshold for clicks and noise bursts was -quite comparable in the two bandwidth conditions, but infants’ click-noise threshold differences were significantly larger for broadband than for 1/3-octave stimuli. In Experiment 2, 2-point threshold-duration functions were compared for 4-kHz tones and octave-band noise bursts presented in backgrounds of quiet and continuous noise. Infants’ threshold-duration function for octave-band noise bursts was significantly steeper than the comparable adult function in quiet, but not in masking noise. These results suggest that young infants may have particular difficulty detecting low intensity broadband sounds when durations are very short.     

Temporal processing in deaf signers

The auditory and visual modalities differ in their capacities for temporal analysis, and speech relies on more rapid temporal contrasts than does sign language. We examined whether congenitally deaf signers show enhanced or diminished capacities for processing rapidly varying visual signals in light of the differences in sensory and language experience of deaf and hearing individuals. Four experiments compared rapid temporal analysis in deaf signers and hearing subjects at three different levels: sensation, perception, and memory. Experiment 1 measured critical flicker frequency thresholds and Experiment 2, two-point thresholds to a flashing light. Experiments 3-4 investigated perception and memory for the temporal order of rapidly varying nonlinguistic visual forms. In contrast to certain previous studies, specifically those investigating the effects of short-term sensory deprivation, no significant differences between deaf and hearing subjects were found at any level. Deaf signers do not show diminished capacities for rapid temporal analysis, in comparison to hearing individuals. The data also suggest that the deficits in rapid temporal analysis reported previously for children with developmental language delay cannot be attributed to lack of experience with speech processing and production.     

Correlations among within-channel and between-channel auditory gap-detection thresholds in normal listeners

We obtained data on within-channel and between-channel auditory temporal gap-detection acuity in the normal population. Ninety-five normal listeners were tested for gap-detection thresholds, for conditions in which the gap was bounded by spectrally identical, and by spectrally different, acoustic markers. Separate thresholds were obtained with the use of an adaptive tracking method, for gaps delimited by narrowband noise bursts centred on 1.0 kHz, noise bursts centred on 4.0 kHz, and for gaps bounded by a leading marker of 4.0 kHz noise and a trailing marker of 1.0 kHz noise. Gap thresholds were lowest for silent periods bounded by identical markers--'within-channel' stimuli. Gap thresholds were significantly longer for the between-channel stimulus--silent periods bounded by unidentical markers (p < 0.0001). Thresholds for the two within-channel tasks were highly correlated (R = 0.76). Thresholds for the between-channel stimulus were weakly correlated with thresholds for the within-channel stimuli (1.0 kHz, R = 0.39; and 4.0 kHz, R = 0.46). The relatively poor predictability of between-channel thresholds from the within-channel thresholds is new evidence on the separability of the mechanisms that mediate performance of the two tasks. The data confirm that the acuity difference for the tasks, which has previously been demonstrated in only small numbers of highly trained listeners, extends to a population of untrained listeners. The acuity of the between-channel mechanism may be relevant to the formation of voice-onset time-category boundaries in speech perception.     

Spatial stimulus cue information supplying auditory saltation

Auditory saltation is a misperception of the spatial location of repetitive, transient stimuli. It arises when clicks at one location are followed in perfect temporal cadence by identical clicks at a second location. This report describes two psychophysical experiments designed to examine the sensitivity of auditory saltation to different stimulus cues for auditory spatial perception. Experiment 1 was a dichotic study in which six different six-click train stimuli were used to generate the saltation effect. Clicks lateralised by using interaural time differences and clicks lateralised by using interaural level differences produced equivalent saltation effects, confirming an earlier finding. Switching the stimulus cue from an interaural time difference to an interaural level difference (or the reverse) in mid train was inconsequential to the saltation illusion. Experiment 2 was a free-field study in which subjects rated the illusory motion generated by clicks emitted from two sound sources symmetrically disposed around the interaural axis, ie on the same cone of confusion in the auditory hemifield opposite one ear. Stimuli in such positions produce spatial location judgments that are based more heavily on monaural spectral information than on binaural computations. The free-field stimuli produced robust saltation. The data from both experiments are consistent with the view that auditory saltation can emerge from spatial processing, irrespective of the stimulus cue information used to determine click laterality or location.     

The role of binaural and fundamental frequency difference cues in the identification of concurrently presented vowels

The relative importance of voice pitch and interaural difference cues in facilitating the recognition of both of two concurrently presented synthetic vowels was measured. The interaural difference cues used were an interaural time difference (400 μsec ITD), two magnitudes of interaural level difference (15 dB and infinite ILD), and a combination of ITD and ILD (400 μsec plus 15 dB). The results are analysed separately for those cases where both vowels are identical and those where they are different. When the two vowels are different, a voice pitch difference of one semitone is found to improve the percentage of correct reports of both vowels by 35.8% on average. However, the use of interaural difference cues results in an improvement of 11.5% on average when there is a voice pitch difference of one semitone, but only a non-significant 0.1% when there is no voice pitch difference. When the two vowels are identical, imposition of either a voice pitch difference or binaural difference reduces performance, in a subtractive manner. It is argued that the smaller size of the interaural difference effect is not due to a “ceiling effect” but is characteristic of the relative importance of the two kinds of cues in this type of experiment. The possibility that the improvement due to interaural difference cues may in fact be due to monaural processing is discussed. A control experiment is reported for the ITD condition, which suggests binaural processing does occur for this condition. However, it is not certain whether the improvement in the ILD condition is due to binaural processing or use of the improvement in signal-to-noise ratio for a single vowel at each ear.     

Perception of spasmodic dysphonia speech in background noise

The purpose of this investigation was to judge whether the Lombard effect, a characteristic change in the acoustical properties of speech produced in noise, existed in adductor spasmodic dysphonia speech, and if so, whether the effect added to or detracted from speaker intelligibility. Intelligibility, as described by Duffy, is the extent to which the acoustic signal produced by a speaker is understood by a listener based on the auditory signal alone. Four speakers with adductor spasmodic dysphonia provided speech samples consisting of low probability sentences from the Speech Perception in Noise test to use as stimuli. The speakers were first tape-recorded as they read the sentences in a quiet speaking condition and were later tape-recorded as they read the same sentences while exposed to background noise. The listeners used as subjects in this study were 50 undergraduate university students. The results of the statistical analysis indicated a significant difference between the intelligibility of the speech recorded in the quiet versus noise conditions (F(1,49) = 57.80, p < or = .001). It was concluded that a deleterious Lombard effect existed for the adductor spasmodic dysphonia speaker group, with the premise that the activation of a Lombard effect in such patients may detract from their overall speech intelligibility.     

Signal detectability in the presence of monotic or dichotic noise bands of equal or unequal levels

The application of the power-spectrum model of masking to the detectability of a signal masked by dichotic noise was investigated in three experiments. In each experiment, the signal was a 2-kHz sinusoid of 400-msec duration, masked by either one or two 800-Hz wide bands of noise presented singly or in pairs. In Experiment 1, we compared the detectability of a diotic signal masked by dichotic noise with the detectability of a monaural signal masked by each of the noises separately. The spectrum level of the noise was 35 dB SPL. For dichotic presentations, the signal was sent to both ears while pairs of noise bands, one below and one above the signal frequency, were presented together, one band to each ear. Threshold levels with the dichotic stimuli were lower than or equal to the thresholds with either ear's stimulus on its own. Similar dichotic stimuli were used in Experiment 2, except that the signal frequency was nearer to one or the other of the bands of masking noise, and the noise had a spectrum level of 50 dB SPL. In Experiment 3, thresholds were obtained with two sets of symmetrically and asymmetrically placed notched-noise maskers. For one of these sets, the spectrum level of both noise bands was 35 dB SPL; for the other set, interaural intensity differences were introduced in the form of an inequality in the levels of the noise bands on either side of the signal. In one ear, the spectrum level of the lower frequency noise band was 35 dB SPL and the spectrum level of the higher frequency noise band was 25 dB SPL, whereas in the other ear, the allocation of noise level to noise band was reversed. The dichotic thresholds obtained with the unequal noise maskers could be predicted from the shapes of the auditory filters derived with equal noise maskers. The data from all three experiments suggest that threshold signal levels in the presence of interaural differences in masker intensity depend principally on the ear with the higher signal-to-masker ratio at the output of its auditory filter, a finding consistent with the power-spectrum model of masking.     

Temporal masking level differences for transients: Further evidence for a short-term integrator

Studies of binaural perception have indicated that subjects are able to use temporal information available in high-frequency regions of the spectrum to lateralize high-frequency waveforms but not to detect these waveforms in masking noise. The present experiments demonstrate that although high-frequency interaural difference cues are relatively ineffective in simultaneous and forward masking, they can be utilized in backward masking. In Experiment 1, large maskinglevel differences were found in backward masking for high-frequency transients presented either monaurally or with an interaural temporal delay. Experiments 2–4 examined fringe masking, effects of masker duration, and combined forward-backward masking for both high- and lowfrequency transients presented with interaural differences in phase and intensity. The results are interpreted as support for the view that the auditory system is organized into parallel shortand long-term integration systems specialized for processing transient and sustained aspects of acoustic stimulation. It is suggested that information from the two integrators is combined when analysis of interaural differences within each of the systems yields similar estimates of spatial location.     

Lateralization of very-short-duration tone pulses of low and high frequencies

The position and image-width of the simultaneous images produced by very short tone pulses were measured as a function of interaural time difference (ITD) at both low- (250 and 800 Hz) and high- (2500 and 8000 Hz) frequencies using a direct-estimation technique.

Primary images are lateralized towards the ear receiving the leading stimulus. At low frequencies image position is proportional to interaural phase-difference (IPD) below 90° and remains at the lead-ear for larger values. At high frequencies image position is proportional to ITD up to 500-1000 μsec. Secondary images are reported on the opposite side of the head for IPDs greater than 180° at low frequencies, and at ITDs greater than 500 μsec at high frequencies. Image width is approximately constant for all ITDs and both images at a given frequency, but becomes more compact as frequency increases.

The data are discussed in terms of onset cues and stimulus fine-structure cues. The best explanation is in terms of an onset mechanism, but one that is calibrated in terms of IPD at low frequencies. The existence of double images is explained in terms of a breakdown in the mechanism determining fusion.     

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 Ex-periment 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.     

Fast Fourier-based DSP algorithm for auditory motion experiments

A digital signal-processing (DSP) technique for rapid generation of complex auditory motion stimuli based on dynamic linear changes in interaural delay is described. In this technique, a pair of comple-mentary discrete Fourier transforms (DFTs) for which the component spacing in one series is different than that of the other is used. The appeal of this technique is its wide applicability, since it can generate real-time motion stimuli of any velocity and starting interaural delay for complex broadband or filtered noise waveforms and nonstationary sounds such as speech, music, and other natural sounds.     

Delayed auditory feedback and shadowing

On the basis of speech disturbance under binaural DAF, groups of subjects of high and low susceptibility were formed. Both groups were required to shadow messages presented under four conditions: single message presented binaurally; message presented to one ear with either white noise, an irrelevant message, or delayed feedback of the repetitition of the message, presented to the contralateral ear. For both groups the number of errors (omitted words) increased significantly in the irrelevant message and the delayed feedback conditions as compared with the binaural or white noise conditions. There was no difference between the susceptible and non-susceptible groups in the binaural and white noise conditions, but the susceptible group showed a much larger increase in the irrelevant message and delayed feedback conditions. Implications of these findings for theories of DAF are discussed.     

Auditory Pareidolia: Effects of Contextual Priming on Perceptions of Purportedly Paranormal and Ambiguous Auditory Stimuli

Reality television programs that explore purportedly paranormal phenomena with pseudoscientific research approaches have emerged in popular culture. These shows commonly feature electronic voice phenomena (EVP), whereby recording devices capture audio signals that are interpreted as paranormal messages. We compared perceptions for voices in EVP with actual speech, acoustic noise, and degraded speech. Some participants were told that the experiment was about speech intelligibility, whereas others were told that the experiment was about paranormal EVP. The paranormal prime increased the proportion of trials for which participants perceived voices in both EVP stimuli and degraded speech. When a voice was detected, low agreement was found regarding the content of EVP messages. In both priming conditions, participants reported general skepticism in the paranormal. Results are discussed in the context of theoretical perspectives on paranormal events, trait‐versus‐state accounts of paranormal beliefs, and pseudoscientific approaches to research. Copyright © 2014 John Wiley & Sons, Ltd.     

Temporal summation of 500-Hz tones and octave-band noise bursts in infants and adults

A visually reinforced operant procedure was employed to obtain 2-point threshold-duration functions in 7-month-old infants and adults in two experimental paradigms. In Experiment 1, thresholds were determined for 10- and 100-msec, 500-Hz tones and octave-band noise bursts presented in quiet and noise backgrounds. Threshold-duration functions were significantly steeper for infants than for adults under all experimental conditions, and did not differ in slope as a result of differences in either stimulus bandwidth or masking noise. In Experiment 2, thresholds for trains of brief 500-Hz tone pulses were examined in infant and adult subjects. Infant functions were adult-like for integration of multiple-pulse stimuli, suggesting that the traditional, long-term process of temporal summation is mature by 7 months of age. Differences between the present results and those previously obtained for 4-kHz stimuli appear to be consistent with the view that different mechanisms are involved in the detection of low- and high-frequency signals.     

Adaptation of central pitch-specific mechanisms

Three previous psychophysical studies have demonstrated that interaural time difference (ITD) coding mechanisms can undergo frequency-specific, selective adaptation. We sought to determine whether this phenomenon extends to the pitch domain, by employing the same psycho-physical paradigm as one used previously, but with harmonic tone complexes lacking energy at the fundamental frequency. Ten normal listeners participated in experiment 1. Psychometric functions for ITDs were obtained for harmonic tone complexes with fundamental frequencies of 110 Hz and 185 Hz, before and after selective adaptation with complexes of the same fundamental frequencies lateralised to opposite sides. In experiment 1, each subject was tested twice. On separate days, subjects were tested with 110 Hz and 185 Hz stimuli that were either partially resolvable complexes or unresolvable ones. Both partially resolved and unresolved stimuli supported adaptation, and at both fundamental frequencies. In experiment 2, which employed nine listeners, the adaptor tone complexes were presented in conjunction with a diotic noise background designed to mask difference tones generated by the adaptor stimuli. The use of the masker had little effect on the mean strength of the adaptation effected by the unresolved adaptor stimuli, and only slightly weakened the adaptation effect found with the partially resolved adaptor stimuli. Taken together, these data constitute the first demonstration of selective adaptation exerted on a central mechanism in the pitch domain.