Age-Group Differences in the Temporal Structure of Speech and Forehead total-Hb Responses During a Phonemic Verbal Fluency Task: A Comparison of Adults in Their 40s and 70s

In populations with well-preserved cognitive function, cognitive screening total scores tend to cluster near the ceiling, making it difficult to characterize age-group differences from total scores alone. We compared the temporal structure of word production, acoustic features, and the magnitude and timing of forehead total-hemoglobin (total-Hb) responses during a phonemic verbal fluency task in adults in their 40s and 70s. A total of 254 healthy participants (115 in their 40s, 139 in their 70s) completed a 60-s phonemic verbal fluency task requiring words beginning with the Japanese syllable /ka/, administered as part of the Japanese version of the Montreal Cognitive Assessment (MoCA-J). We derived the total word count, word counts in 10-s bins, mean inter-word pause duration, speech offset time, smoothed cepstral peak prominence (CPPS), jitter, and shimmer. Area under the curve (AUC) and time-to-peak (TTP) were computed from forehead total-Hb signals recorded with a wearable single-wavelength near-infrared spectroscopy device. MoCA-J scores clustered near the ceiling in both groups, although the age-group difference was significant. The 70s group produced fewer words (14.00 ± 1.18 vs 17.09 ± 1.33) and showed longer inter-word pauses (1.60 ± 0.40 vs 0.72 ± 0.31 s). CPPS was lower, AUC was higher, and TTP was longer (32.97 ± 9.31 vs 15.63 ± 4.98 s) in the 70s group, whereas jitter did not differ. Word counts across 10-s bins showed an age group × time-bin interaction. Within each age group, participants who produced more words showed longer TTP. Age-group differences in TTP and AUC persisted after adjustment for speech offset time (proportions mediated, 6.7% and 0.5%) and in a subsample matched on speech offset time. Even when screening scores clustered at the ceiling, the temporal structure of word production and forehead total-Hb responses differed between age groups, and these two classes of measures dissociated. Because the sample was selectively recruited and single-wavelength total-Hb signals do not index localized neural activity, the findings are descriptive and motivate longitudinal, multi-axis characterization of speech in aging.