Neural Responses During Japanese Speech Listening at Different Playback Speeds: An Exploratory Functional MRI Study

Background/Objectives: The effects of changes in speech playback speed on brain activity provide important insights into the development of efficient information processing and learning methods. However, research has predominantly focused on Indo-European languages such as English. This study examined neural responses during Japanese speech listening at two playback speeds using functional magnetic resonance imaging (fMRI). Methods: Forty-two Japanese-speaking adults (mean age: 21.5 ± 1.8 years, 24 female) passively listened to synthesized readings from “Le Petit Prince” at 0.75× (slow) and 1.5× (fast) normal speed, followed by a comprehension test. Results: Both conditions elicited robust bilateral activation centered on the transverse temporal gyrus and superior temporal gyrus. The direct contrast between the conditions yielded no significant voxels, and region-of-interest analyses provided moderate evidence for the point-null model in five of six auditory regions (Bayes factors favoring the null: 3.7–6.0) but were inconclusive in the right Heschl’s gyrus (2.1); multiplicity-corrected tests excluded effects beyond the sensitivity-derived bounds in three of six regions, but classifications depended on motion handling. Responses were leftward lateralized under both conditions, but no significant difference in lateralization between speeds was detected. Nine-item comprehension accuracy was 61.4%; the fast–slow difference was −5.83 percentage points (95% CI [−17.28, 5.61]); 83.3% identified the faster run correctly. Playback speed was confounded with narrative content within participants; speed order was counterbalanced across participants. Conclusions: These findings provide an exploratory baseline for Japanese and indicate that, within this speed range, playback speed is not associated with detectable differences in the magnitude or lateralization of auditory cortical responses.