Background: Rapid adaptation has been robustly shown in various kinds of degraded speech, such as fast, accented, or noise-vocoded speech. While perceptual adaptation to speech is associated with enhanced task performance, how it affects the cognitive resources recruited to process degraded speech signals remains unclear. The current study employed pupillometry and the subjective rating method to investigate how listening effort changes during rapid adaptation to noise-vocoded speech. Furthermore, the effect of divided attention on speech adaptation and listening effort was also examined.

Methods: Sixty participants [median age 21, IQR (19 – 23), 28% men] were recruited in a between-subject design (single-task vs. dual-task condition). All participants listened to 72 six-channel noise-vocoded IEEE sentences presented in four blocks. Each sentence contained two consecutive pure tones ranging from 100-300 Hz with a 15% pitch difference. In the single-task condition, participants focused solely on recognizing noise-vocoded sentences (full attention). In the dual-task condition, participants both recognized speech and judged the direction of pitch shifts (divided attention). Pupillary responses during stimulus presentation were recorded using an EyeLink 1000 Plus eye tracker. After each block, participants rated their self-perceived listening effort on a seven-point Likert scale.

Results and Conclusions: Both pupillary response measurements and subjective ratings demonstrated that rapid adaptation to noise-vocoded speech significantly reduces listening effort. Our investigation of the relationships between attention, adaptation, and effort revealed two key findings. First, perceptual learning may be largely automatic, as divided attention did not impair adaptation as indicated by speech task performance. Second, while divided attention did not affect adaptation rates, it influenced listening effort dynamics across trials. Specifically, divided attention led to a more pronounced reduction in listening effort over time, as evidenced by greater decreases in pupil dilation across trials. Overall, these results suggest that listeners with divided attention can achieve the same level of adaptation as those with undivided attention while simultaneously showing greater efficiency in managing listening effort during speech processing.