Pupillometry reveals effects of pitch manipulations both 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 acoustic, specifically pitch, information, 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 NH listeners. We manipulated the fundamental frequency 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 more sensitive to these manipulations: peak pupil dilations were larger when words were monotonized (flat pitch contour) or inverted (the natural contour flipped upside-down), and larger when successive words were organized into a melodic pattern. Therefore, natural or expected pitch contour of a word can participate to its identity and facilitate its matching and retrieval from the phonological representation stored in the long-term memory. Consequently, degrading words’ pitch 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.