Constraints on Sensitivity to Auditory Modulation in the Perceptual Organization of Speech

Abstract

Background/study context: The perception of speech requires the integration of sensory details from a rapidly fading trace of a time-varying spectrum. This effortful cognitive function has been difficult to assess. New tests measuring intelligibility of sine-wave replicas of speech provided an assay of this critical function in normal-hearing young adults.

Methods: Four time-varying sinusoids replicated the frequency and amplitude variation of the natural resonances of spoken sentences. The temporal tolerance of perceptual integration of speech was measured by determining the effect on intelligibility of desynchronizing a single sine-wave component in each sentence. This method was applied in tests in which the sentences were temporally compressed or expanded over a 40% range.

Results: Desynchrony was harmful to perceptual integration over a narrow temporal range, indicating that modulation sensitivity is keyed to a rate of 20 Hz. No effect of variation in speech rate was observed on the intelligibility measure, whether rate was accelerated or decelerated relative to the natural rate.

Conclusion: Performance measures of desynchrony tolerance did not vary when speech rate was accelerated or decelerated, revealing constraints on integration that are arguably primitive, sensory, auditory, and fixed. Because these are not adaptable, they limit the potential for perceptual learning in this aspect of perceptual organization. Implications for describing the elderly listener are noted.

Figure 1

Schematic representation of the acoustic variations used in the present tests. Each panel depicts the phrase, “sew a button,” from the sentence, “Press the pants and sew a button on the vest.” Time ticks on the abscissa are 100 ms apart; frequency ticks on the ordinate mark kHz. A temporally veridical sine-wave version is shown (top center panel), in the next row the left panel depicts synthesis decelerated by 20%, and the right panel shows synthesis accelerated by 20%. In the bottom row, the left panel shows the pattern produced by desynchronizing the tone analog of the second formant, to make it lead the remaining tones by 50 ms. The 20% decelerated pattern is shown. The bottom right panel shows the pattern produced by desynchronizing the tone analog of the second formant to make it lag the remaining tones by 50 ms. The 20% accelerated pattern is shown.

Figure 2

The results of ten tests of the effects of synthesis rate on the perceptual tolerance of desynchrony. The ordinate is intelligibility (transcription performance). The abscissa indicates the temporal offset of the tone analog of the second formant relative to the other components of the sine-wave sentence. Intelligibility measures of the effects of a desynchronized tone component in temporally veridical sentences (from Remez et al., 2010) are shown in the round bullets.