Pupillometry reveals effects of pitch manipulation within and across words on listening effort and short-term memory

Abstract

For individuals with hearing loss, even successful speech communication comes at a cost. Cochlear implants transmit degraded information, specifically for voice pitch, which demands extra and sustained listening effort. The current study hypothesized that abnormal pitch patterns contribute to the additional listening effort, even in non-tonal language native speaking normally hearing listeners. We manipulated the fundamental frequency (F0) within and across words, while participants listen and repeat (simple intelligibility task), or listen, repeat, and later recall (concurrent encoding task) the words. In both experiments, the F0 manipulations resulted in small changes in intelligibility but no difference in free recall or subjective effort ratings. Pupillary metrics were yet sensitive to these manipulations: pupil dilations were larger when words were monotonized (flat contour) or inverted (the natural contour flipped upside-down), and larger when successive words were organized into a melodic pattern. The most likely interpretation is that the natural or expected F0 contour of a word contributes to its identity and facilitate its matching and retrieval from the phonological representation stored in long-term memory. Consequently, degrading words' F0 contour can result in extra listening effort. Our results call for solutions to improve pitch saliency and naturalness in future development of cochlear implants' signal processing strategies, even for non-tonal languages.

Fig. 1

Behavioral data of Exp.1, in which the F0 within individual words was manipulated across 4 conditions: exag = exaggerated pitch contour; into = normal (intact) intonation; inve = inverted contour; mono = monotonized contour.

Fig. 2

Averaged pupil traces aligned at response onset, as a function of the pitch manipulation within a word: exag = exaggerated pitch contour; into = normal (intact) intonation; inve = inverted contour; mono = monotonized contour. Red traces are from the repeat & recall condition; gray traces are from the repeat-only condition. These traces are pooled across the 10 words of a list and three repetitions (with different lists), expressed in raw units to better appreciate the baseline diameter (top) or baseline-corrected to better appreciate the size and latency of the PPD (bottom). Lines represent the means and the areas reflect one standard error of the mean across subjects.

Fig. 3

Averaged pupil traces during word listening, aligned at response onset, as a function of the position of a word within a list. These traces are pooled across the 4 pitch manipulations, and expressed in raw units (top) or baseline-corrected (bottom). Red traces are from the repeat & recall condition; gray traces are from the repeat-only condition. Lines represent the means and the areas reflect one standard error of the mean across subjects.

Fig. 4

Three metrics extracted from the pupil data in Exp.1: baseline (top), PPD amplitude (middle, i.e., maximum pupil diameter relative to baseline), and PPD latency (bottom, i.e., time between peak dilation time point and the response onset), shown as a function of the experimental manipulation (left panels) or as a function of the word position within a list (right panels). Symbols represent the means and error bars are one standard error from the mean across subjects. The 4 F0 manipulations were: exag = exaggerated pitch contour; into = normal (intact) intonation; inve = inverted contour; mono = monotonized contour.

Fig. 5

Behavioral data of Exp.2, in which the F0 across individual words was manipulated to form a melody.

Fig. 6

Averaged pupil traces aligned at response onset, as a function of the pitch manipulation across words, to form either a steady pattern (fixed) or a melody (melodic) that was consistent throughout Experiment 2. Red traces are from the repeat & recall condition; gray traces are from the repeat-only condition. These traces are pooled across the 10 words of a list and three repetitions (with different lists), expressed in raw units to better appreciate the baseline diameter (top) or baseline-corrected to better appreciate the size and latency of the PPD (bottom). Lines represent the means and the areas reflect one standard error of the mean across subjects.

Fig. 7

Averaged pupil traces aligned at response onset, as a function of the position of a word within a list. Red traces are from the repeat & recall condition; gray traces are from the repeat-only condition. These traces are pooled across the 2 pitch manipulations, and expressed in raw units (top) or baseline-corrected (bottom). Lines represent the means and the areas reflect one standard error of the mean across subjects.

Fig. 8

Three metrics extracted from the pupil data in Exp.2: baseline (top), PPD amplitude (middle), and PPD latency (bottom), shown as a function of the pitch manipulation (left panels) or as a function of the word position within a list (right panels). Symbols represent the means and error bars are one standard error from the mean across subjects.

Fig. 9

Schematic illustration of the experimental procedure.