Dichotic Sounds Research Articles

Duplex perception reveals brainstem auditory representations are modulated by listeners' ongoing percept for speech.

So-called duplex speech stimuli with perceptually ambiguous spectral cues to one ear and isolated low- versus high-frequency third formant "chirp" to the opposite ear yield a coherent percept supporting their phonetic categorization. Critically, such dichotic sounds are only perceived categorically upon binaural integration. Here, we used frequency-following responses (FFRs), scalp-recorded potentials reflecting phase-locked subcortical activity, to investigate brainstem responses to fused speech percepts and to determine whether FFRs reflect binaurally integrated category-level representations. We recorded FFRs to diotic and dichotic stop-consonants (/da/, /ga/) that either did or did not require binaural fusion to properly label along with perceptually ambiguous sounds without clear phonetic identity. Behaviorally, listeners showed clear categorization of dichotic speech tokens confirming they were heard with a fused, phonetic percept. Neurally, we found FFRs were stronger for categorically perceived speech relative to category-ambiguous tokens but also differentiated phonetic categories for both diotically and dichotically presented speech sounds. Correlations between neural and behavioral data further showed FFR latency predicted the degree to which listeners labeled tokens as "da" versus "ga." The presence of binaurally integrated, category-level information in FFRs suggests human brainstem processing reflects a surprisingly abstract level of the speech code typically circumscribed to much later cortical processing.

P526: Study of binaural interaction using synchronous/asynchronous averaging for dichotic sounds

P526: Study of binaural interaction using synchronous/asynchronous averaging for dichotic sounds

The loudness of sounds whose spectra differ at the two ears

Moore and Glasberg [(2007). J. Acoust. Soc. Am. 121, 1604-1612] developed a model for predicting the loudness of dichotic sounds. The model gave accurate predictions of data in the literature, except for an experiment of Zwicker and Zwicker [(1991). J. Acoust. Soc. Am. 89, 756-764], in which sounds with non-overlapping spectra were presented to the two ears. The input signal was noise with the same intensity in each critical band (bark). This noise was filtered into 24 bands each 1 bark wide. The bands were then grouped into wider composite bands (consisting of 1, 2, 4, or 12 successive sub-bands) and each composite band was presented either to one ear or the other. Loudness estimates obtained using a scaling procedure decreased somewhat as the number of composite bands increased (and their width decreased), but the predictions of the model showed the opposite pattern. This experiment was similar to that of Zwicker and Zwicker, except that the widths of the bands were based on the ERB(N)-number scale, and a loudness-matching procedure was used. The pattern of the results was consistent with the predictions of the model, showing an increase in loudness as the number of composite bands increased and their spacing decreased.

Effect of reverberation on loudness perception

Stimuli with a slow‐attack/fast‐decay (S‐F) envelope are judged to be louder than stimuli with a temporally‐reversed, equal‐energy version of the same envelope (fast‐attack/slow‐decay or F‐S). It may be that the energy in the tails of F‐S stimuli is discounted from loudness judgments because it is perceptually attributed to room reverberation. Here we examine the characteristics of real‐room reverberation that affect loudness perception, particularly the way that reverberant tails tend to be de‐correlated at the two ears. Our experiments used reverberation from real‐room measurements of binaural room impulse responses (BRIRs). We ask if the perceived reduction in loudness of F‐S stimuli relative to S‐F stimuli is still apparent with dichotic sounds processed by the BRIRs.

The effect of eye position on the orientation of sound lateralization

Conclusions. The data suggest that sound lateralization sensitivity during interaural time difference (ITD) discrimination is altered by eccentric gaze and that sound lateralization shifts toward the direction of the gaze. Objective. Using dichotic sounds, the effect of eye position on the orientation of sound lateralization was investigated in humans. Subjects and methods. The subjects were studied by testing ITD discrimination under different conditions of visual fixation. Results: The amplitudes obtained during the ITD discrimination tests during eccentric fixation were significantly greater than those obtained while gazing straight ahead (p<0.05). The median line of amplitude obtained during the ITD discrimination test shifted toward the direction of the gaze.

A loudness model in dichotic conditions

Existing loudness models are specified only to diotic sounds in spite of the fact that normal human beings hear dichotic sounds. The arithmetic mean of loudness values of both ear signals has been suggested for the approximated value of the resultant perceived loudness. In this study, the dependence of overall loudness perception on the interaural level differences was investigated by the subjective tests. It was found that the larger the interaural level difference, the louder the perception than the mean of calculated loudness values at both ears and the lower the critical band rate or the reference level, the louder the perception than the mean value. A modified loudness model was proposed for the application to dichotic sounds by using the equivalent diotic levels.

Comparisons of auditory cortical imaging for the various dichotic listening

Aim: The study was designed to estimate the auditory laterality of the human brain that was based on the dichotic listening (DL) technique. Non-invasive detection to know the ear advantage is useful for the hearing aid fitting for mono-aural wearing. Subject: Ten normal hearing adult volunteers were employed in this study. Methods: 31ch-Auditory-evoked potentials (AEP) for the dichotic sounds were recorded electrically from the scalp. A 60dBSPL of vowel “/a/” was represented mono-aurally with a 50dBSPL of the contra-lateral masking noise, white noise, and multi-talker noise. Brain functions, especially for the cortical response, within the dichotic listening were visualized non-invasively using the scalp potential-based Laplacian mapping technique. Results: Right ear advantage for vowel sound within the left-brain dominancy can be observed in six subjects. The remaining four subjects could not be categorized in the term of ear advantage or brain laterality. Conclusion: It is observed that the auditory processing for the dichotic listening behavior differed from the mono-aural listening. Objective and non-invasive assessment for the brain laterality within the auditory dominancy can be applied for the hearing aids fitting technique.

Dichotic loudness of tone-noise complexes

The overall loudness of dichotic sounds composed of a tone to one ear and a low- or high-pass noise to the other ear, was determined by absolute magnitude estimation for groups of ten listeners with normal hearing. Within a 30-dB range above 70 dB, the sound pressure levels (SPL) of the tone and noise were independently varied in 5-dB steps. Results showed that total loudness was generally a nonmonotonic function of the overall SPL of the complex. Moreover, the obtained loudness functions were steeper than those typically measured separately for pure tones and broadband noise. Instead of the standard slope of 0.60 (log loudness as a function of SPL), overall slopes ranged from 0.75 to 0.86. For a given complex, the rate of loudness growth was dependent on the relative loudness of the component stimuli. Loudness increased more rapidly with overall SPL when the tone and noise were nearly equal in loudness than when they were markedly unequal. In contrast to earlier results for the same stimuli presented together binaurally [R. P. Hellman, J. Acoust. Soc. Am. 75, 209–218 (1984)], the current findings suggest that dichotic loudness is mainly based on the summed output from each ear without the effects of intraaural interactions. [Work supported by NIH.]

Spectral versus temporal features in dichotic listening

Ear advantage for the processing of dichotic speech sounds can be separated into two components. One of these components is an ear advantage for those phonetic features that are based on spectral acoustic cues. This ear advantage follows the direction of a given individual's ear dominance for the processing of spectral information in dichotic sounds, whether speech or nonspeech. The other factor represents a right-ear advantage for the processing of temporal information in dichotic sounds, whether speech or nonspeech. The present experiments were successful in dissociating these two factors. Since the results clearly show that ear advantage for speech is influenced by ear dominance for spectral information, a full understanding of the asymmetry in the perceptual salience of speech sounds in any individual will not be possible without knowing his ear dominance.