Signal envelope and speech intelligibility differentially impact auditory motion perception

Abstract

Our acoustic environment contains a plethora of complex sounds that are often in motion. To gauge approaching danger and communicate effectively, listeners need to localize and identify sounds, which includes determining sound motion. This study addresses which acoustic cues impact listeners' ability to determine sound motion. Signal envelope (ENV) cues are implicated in both sound motion tracking and stimulus intelligibility, suggesting that these processes could be competing for sound processing resources. We created auditory chimaera from speech and noise stimuli and varied the number of frequency bands, effectively manipulating speech intelligibility. Normal-hearing adults were presented with stationary or moving chimaeras and reported perceived sound motion and content. Results show that sensitivity to sound motion is not affected by speech intelligibility, but shows a clear difference for original noise and speech stimuli. Further, acoustic chimaera with speech-like ENVs which had intelligible content induced a strong bias in listeners to report sounds as stationary. Increasing stimulus intelligibility systematically increased that bias and removing intelligible content reduced it, suggesting that sound content may be prioritized over sound motion. These findings suggest that sound motion processing in the auditory system can be biased by acoustic parameters related to speech intelligibility.

Figure 1

Speech intelligibility across conditions for each ENV type. Speech intelligibility (mean ± sem) was measured as a function of chimaera and control conditions, with stimulus ENV type indicated by color. As the number of frequency bands increased, intelligibility of sound content increased for chimaera with a speech-like ENV (blue), but decreased for chimaera with a noise-like ENV (red). During control conditions, listener’s performance on sound content intelligibility reached ceiling levels for original speech (OR_S), but floor levels for SMN (OR_SMN) and both reversed control stimuli (R_S/R_SMN).

Figure 2

Sensitivity to sound motion across conditions for each ENV type. Sensitivity scores (d’; mean ± sem) are plotted as a function of chimaera and control conditions, with ENV type indicated by color. Higher scores indicate better discriminability. Across chimaera conditions, listeners were significantly more sensitive to sound motion for stimuli with noise-like ENVs (red) compared to speech-like ENVs (blue). In control conditions, original stimuli did not differ from chimaera stimuli, but listeners were significantly less sensitive to sound motion when stimuli were speech (OR_S) compared to SMN (OR_SMN).

Figure 3

Response bias for sound motion across conditions for each ENV type. Response bias (c; mean ± sem) is plotted as a function of chimaera and control conditions, with ENV type indicated by color. Positive numbers indicate a bias towards judging sounds as moving, while negative numbers indicate a bias towards judging sounds as stationary (grey arrows). Statistical significance (see “Results”) is indicated by stars. Listeners showed a stationary perceptual bias for chimaera stimuli that had a speech-like ENV (blue) and at least six frequency bands. By contrast, chimaera stimuli with a noise-like ENV (red) and 4, 8 or 16 frequency bands induced a moving perceptual bias. Bias differed significantly for control conditions, in which listeners showed no bias for OR_SMN, while the strongest stationary bias was induced for OR_S. Removing sound content intelligibility, R_S, removed the stationary bias that was observed in listeners when the same sound’s content was intelligible, SC₁₆.