Single Auditory Image Research Articles

Low-Intensity Steady Background Noise Enhances Pitch Fusion Across the Ears in Normal-Hearing Listeners.

Binaural pitch fusion is the perceptual integration of stimuli that evoke different pitches between the ears into a single auditory image. This study was designed to investigate how steady background noise can influence binaural pitch fusion. The binaural fusion ranges, the frequency ranges over which binaural pitch fusion occurred, were measured with three signal-to-noise ratios (+15, +5, and −5dB SNR) of the pink noise and compared with those measured in quiet. The preliminary results show that addition of an appropriate amount of noise can reduce binaural fusion ranges, an effect called stochastic resonance. This finding increases the understanding of how specific noise levels can sharpen binaural pitch fusion in normal hearing individuals. Furthermore, it elicits more pathways for research to explore how this benefit can practically be used to help improve binaural auditory perception.

Attribute capture underlying the precedence effect in rats

In a reverberant environment, humans with normal hearing can perceptually fuse the soundwave from a source with its reflections off nearby surfaces into a single auditory image, whose location appears to be around the source. This phenomenon is called the precedence effect, which is based on the perceptual capture of the reflected (lagging) sounds' attributes by the direct wave from the source. Using the paradigm of attentional modulation of the prepulse inhibition (PPI) of the startle reflex, with both the prepulse-feature specificity and the perceived-prepulse-location specificity, this study was to examine whether the perceptual attribute capture underlying the precedence effect occurs in rats. One broadband continuous noise was delivered by each of two spatially separated left and right loudspeakers with a 1-ms inter-loudspeaker delay. A silent gap was embedded in one of the two noises as the prepulse stimulus. The results showed that regardless of whether the gap was physically in the leading or lagging noise when the leading noise was either the left or right one, fear conditioning the gap enhanced PPI only when the leading noise was delivered from the loudspeaker that was the leading but not the lagging loudspeaker during the conditioning, indicating that due to the spatial specificity (either left or right) in the attentional enhancement of PPI, the perceived location of the conditioned gap was always on the leading side even though the gap was physically on the lagging side. Thus, rats have the same perceptual ability of attribute capture, thereby experiencing the auditory precedence effect as humans.

Binaural Pitch Fusion in Children With Normal Hearing, Hearing Aids, and Cochlear Implants.

Binaural pitch fusion is the perceptual integration of stimuli that evoke different pitches between the ears into a single auditory image. Adults who use hearing aids (HAs) or cochlear implants (CIs) often experience abnormally broad binaural pitch fusion, such that sounds differing in pitch by as much as 3 to 4 octaves are fused across ears, leading to spectral averaging and speech perception interference. The main goal of this study was to measure binaural pitch fusion in children with different hearing device combinations and compare results across groups and with adults. A second goal was to examine the relationship of binaural pitch fusion to interaural pitch differences or pitch match range, a measure of sequential pitch discriminability. Binaural pitch fusion was measured in children between the ages of 6.1 and 11.1 years with bilateral HAs (n = 9), bimodal CI (n = 10), bilateral CIs (n = 17), as well as normal-hearing (NH) children (n = 21). Depending on device combination, stimuli were pure tones or electric pulse trains delivered to individual electrodes. Fusion ranges were measured using simultaneous, dichotic presentation of reference and comparison stimuli in opposite ears, and varying the comparison stimulus to find the range that fused with the reference stimulus. Interaural pitch match functions were measured using sequential presentation of reference and comparison stimuli, and varying the comparison stimulus to find the pitch match center and range. Children with bilateral HAs had significantly broader binaural pitch fusion than children with NH, bimodal CI, or bilateral CIs. Children with NH and bilateral HAs, but not children with bimodal or bilateral CIs, had significantly broader fusion than adults with the same hearing status and device configuration. In children with bilateral CIs, fusion range was correlated with several variables that were also correlated with each other: pure-tone average in the second implanted ear before CI, and duration of prior bilateral HA, bimodal CI, or bilateral CI experience. No relationship was observed between fusion range and pitch match differences or range. The findings suggest that binaural pitch fusion is still developing in this age range and depends on hearing device combination but not on interaural pitch differences or discriminability.

Binaural Pitch Fusion in Bilateral Cochlear Implant Users.

Binaural pitch fusion is the fusion of stimuli that evoke different pitches between the ears into a single auditory image. Individuals who use hearing aids or bimodal cochlear implants (CIs) experience abnormally broad binaural pitch fusion, such that sounds differing in pitch by as much as 3-4 octaves are fused across ears, leading to spectral averaging and speech perception interference. The goal of this study was to determine if adult bilateral CI users also experience broad binaural pitch fusion. Stimuli were pulse trains delivered to individual electrodes. Fusion ranges were measured using simultaneous, dichotic presentation of reference and comparison stimuli in opposite ears, and varying the comparison stimulus to find the range that fused with the reference stimulus. Bilateral CI listeners had binaural pitch fusion ranges varying from 0 to 12 mm (average 6.1 ± 3.9 mm), where 12 mm indicates fusion over all electrodes in the array. No significant correlations of fusion range were observed with any subject factors related to age, hearing loss history, or hearing device history, or with any electrode factors including interaural electrode pitch mismatch, pitch match bandwidth, or within-ear electrode discrimination abilities. Bilateral CI listeners have abnormally broad fusion, similar to hearing aid and bimodal CI listeners. This broad fusion may explain the variability of binaural benefits for speech perception in quiet and in noise in bilateral CI users.

Similar Impacts of the Interaural Delay and Interaural Correlation on Binaural Gap Detection

The subjective representation of the sounds delivered to the two ears of a human listener is closely associated with the interaural delay and correlation of these two-ear sounds. When the two-ear sounds, e.g., arbitrary noises, arrive simultaneously, the single auditory image of the binaurally identical noises becomes increasingly diffuse, and eventually separates into two auditory images as the interaural correlation decreases. When the interaural delay increases from zero to several milliseconds, the auditory image of the binaurally identical noises also changes from a single image to two distinct images. However, measuring the effect of these two factors on an identical group of participants has not been investigated. This study examined the impacts of interaural correlation and delay on detecting a binaurally uncorrelated fragment (interaural correlation = 0) embedded in the binaurally correlated noises (i.e., binaural gap or break in interaural correlation). We found that the minimum duration of the binaural gap for its detection (i.e., duration threshold) increased exponentially as the interaural delay between the binaurally identical noises increased linearly from 0 to 8 ms. When no interaural delay was introduced, the duration threshold also increased exponentially as the interaural correlation of the binaurally correlated noises decreased linearly from 1 to 0.4. A linear relationship between the effect of interaural delay and that of interaural correlation was described for listeners participating in this study: a 1 ms increase in interaural delay appeared to correspond to a 0.07 decrease in interaural correlation specific to raising the duration threshold. Our results imply that a tradeoff may exist between the impacts of interaural correlation and interaural delay on the subjective representation of sounds delivered to two human ears.

Asynchronous Preparation of Tonally Fused Intervals in Polyphonic Music

An analysis of a sample of polyphonic keyboard works by J.S. Bach shows that synchronous note onsets are avoided for those harmonic intervals that most promote tonal fusion (such as unison, fifths and octaves). This pattern is consistent with perceptual research showing an interaction between onset synchrony and tonal fusion in the formation of auditory streams (e.g., Vos, 1995). The results provide further support for the notion that polyphonic music is organized so as to facilitate the perceptual independence of the concurrent parts. IN the perception of auditory scenes, a number of factors are known to influence the formation of auditory streams (Bregman, 1990; McAdams & Bregman, 1979; van Noorden, 1975). Both time domain and frequency domain factors contribute to the perception of either separate or integrated sound images. In the case of concurrently sounding tones, two important factors are tonal fusion and onset synchrony. Tonal fusion is the tendency for two or more harmonically related tones to fuse and form a single auditory image. Onset synchrony is the tendency for two or more tones having coincident onsets to evoke a single auditory image. Onset asynchronies as small as 10 ms have been shown to facilitate the segregation of two tones so they form independent sounds (Rasch, 1978). However, Vos (1995) has noted that the experimental paradigms used in such experiments employ cyclic (repeated) stimuli that make it easier to segregate the component sounds. Consequently, such experimental measures ought to be regarded as ideal threshold values rather than values typical for source separation in common listening situations. Using an experimental procedure that better approximates the uncertainties attending naturally-occurring sounds, Vos found that for non-cyclic stimuli, asynchronies of even 20 ms are insufficient to evoke segregation of the component tones. Larger amounts of asynchrony are necessary: Vos has suggested that for significant effects on perceptual separation to occur, asynchronies greater than 50 ms may be needed (Vos, 1995; p. 414). Beyond 50 ms, larger asynchronies may well continue to facilitate the segregation of auditory sources. It is plausible that the ceiling effect for source segregation due to onset asynchrony may not appear until the inter-onset interval is 100 ms or longer. This latter value is more typical of the asynchronies implied in musical scores where notated asynchronies between two parts are rarely less than a sixteenth or thirty-second note in duration. At 96 beats per minute, for example, two notes whose onsets differ by a sixteenth-note duration would have a nominal asynchrony of 156 ms. In the case of polyphonic music, musical samples have already been shown to be consistent with the pursuit of asynchronous onsets between concurrent parts. Rasch (1981) carried out an analysis of vocal works by Praetorius, and showed that onset asynchrony between the notated parts increases as the number of voices is increased. Experiments by Huron (1989a) and Parncutt (1989) have shown that listeners' abilities to track auditory streams become degraded as the number of concurrent voices increases. Hence, the observed increase in asynchrony might be expected to compensate for the increased difficulty of parsing auditory scenes that contain many parts. Even in the case of two-part polyphonic music, Huron (1989b, 1993) showed that, compared with a meter-controlled distribution, Bach's 15 two-part Inventions exhibit a systematic tendency to avoid synchronous onsets between the parts. In the case of tonal fusion, evidence also indicates that polyphonic music is organized so as to avoid tonally-fused intervals. Using data on tonal fusion collected by Stumpf (1890) and by DeWitt and Crowder (1987), Huron (1991a) showed that, compared with a pitch-proximity-controlled distribution of

Physiological studies of the precedence effect in the inferior colliculus of the cat. I. Correlates of psychophysics.

The precedence effect (PE) is experienced when two spatially separated sounds are presented with such a brief delay that only a single auditory image at or toward the location of the leading source is perceived. The responses of neurons in the central nucleus of the inferior colliculus (ICC) of cats were studied using stimuli that are known to elicit the PE, focusing on the effects of changes in stimulus conditions that a listener might encounter in a natural situation. Experiments were conducted under both free-field (anechoic chamber) and dichotic (headphones) conditions. In free field, the PE was simulated by presenting two sounds from different loudspeakers with one sound delayed relative to the other. Either click or noise stimuli (2- to 10-ms duration) were used. Dichotically, the same conditions were simulated by presenting two click or noise pairs separated by an interstimulus delay (ISD) with interaural time differences (ITDs) imposed separately for each pair. At long ISDs, all neurons responded to both leading and lagging sources as if they were delivered alone. As the ISDs were shortened, the lagging response became suppressed. The ISD of half-maximal suppression varied considerably within the population of neurons studied, ranging from 2 to 100 ms, with means of 35 and 38 ms for free field and dichotic conditions, respectively. Several correlates of psychophysical findings were observed in ICC neurons: suppression was usually stronger with lower overall stimulus level and longer duration stimuli. Suppression also was compared along the azimuth and elevation in free field by placing the lagging source at (0 degree,0 degree), which is common to both axes, and the leading sources at locations along either plane that generated similar discharge rates. All neurons that showed suppression along the azimuth also did so in the elevation. In addition, there was a high correlation in the ISD of half-maximal suppression along the two planes (r = 0.87). These findings suggest that interaural difference cues, which are robust along the horizontal axis but minimal in the median plane, are not necessary for neural correlates of the PE to be manifested. Finally, single-neuron responses did not demonstrate a correlate of build-up of suppression, a phenomenon whereby echo suppression accumulates with ongoing stimulation. This finding adds credibility to theories about the PE that argue for a "higher order" component of the PE.

Spectral fissioning in phonemic transformations.

Listeners presented with a repeated sequence of brief (30- to 100-msec) steady-state vowels hear phonemic transformations--they cannot identify the vowels, but they perceive two simultaneous utterances that differ in both phonemic content and timbre (Warren, Bashford & Gardner, 1990). These utterances consist of either English words or syllables that occur in English words. In the present study, we attempted to determine whether the two percepts represent alternative interpretations of the same formant structures, or whether different portions of the vowels are used for each verbal organization. It was found that separate spectral regions are employed for each verbal form: Components below 1500 Hz were generally used for one form, and components above 1500 Hz for the other. Hypotheses are offered concerning the processes responsible for the verbal organization of the vowel sequences and for the splitting into two spectrally limited forms. It appears that the tendency to organize spectral regions separately competes with, and can overcome, the tendency to integrate the different spectral components of speech into a single auditory image. A contralateral induction paradigm was used in a procedure designed to quantitatively evaluate these opposing forces of spectral fission and fusion.

Dichotic verbal transformations: evidence of separate neural processors for identical stimuli

When a word or phrase heard monaurally or diotically is repeated over and over without pause, perceptual organization becomes unstable, and verbal transformations (VTs) to other words or phrases occur [see Warren, Psychol. Bull. 70, 261–270 (1968)]. We have extended study of this illusion to dichotic conditions. When each ear is presented with a different repeated word, CTs were found to occur on each side in a seemingly independent fashion. What if a single stimulus were presented dichotically with a delay of a few hundred milliseconds separating the arrival of the identical input at the two ears and preventing fusion into a single auditory image? Successive utilization of the same neural units for processing would produce the same VTs on each side, but independent lateral changes would indicate different processing centers for identical stimuli. Independent CTs were found for all 20 subjects: for example, the stimulus word “tress” might be heard correctly on one side, while the transformed word “tramp” was heard clearly on the other. Comparison of the rate and kinds of VTs for the right and left sides has produced information concerning the relative behavior of the separate analyzers.