Transcranial Alternating Current Stimulation (tACS) differentially modulates speech perception in young and older adults

•The study investigates the effect of transcranial alternating current stimulation (tACS) on auditory phoneme processing in normally aging older adults and young adults.•While 40 Hz tACS applied over bilateral auditory cortex regions hampered phoneme categorization in young adults, older adults benefitted from this stimulation resulting in enhanced task accuracy.•The results causally confirm the notion on altered gamma oscillations as crucial underlying neurophysiological mechanism leading to impaired auditory temporal resolution in older age.•Results are discussed with respect to an inverted U-shaped relationship between gamma oscillations in the auditory cortex and phoneme processing.•Our findings are promising in the context of future clinical applications for subjects with impaired speech processing.

BackgroundNormal aging is accompanied by a functional decline in processing temporal features of spoken language, such as voice onset time (VOT). On an electrophysiological level, this finding is paralleled by altered patterns of gamma oscillations.ObjectiveUsing 40 Hz transcranial alternating current stimulation (tACS) over the bilateral auditory cortex, this study aims to compare the effect of tACS to modulate VOT-processing in samples of healthy young and older adults.MethodsTwenty-five healthy young (age 20-35 years) and 20 older adults (age 60-75 years) participated in this study. Presented with an auditory phoneme categorization task participants received 40 Hz and 6 Hz tACS on two consecutive sessions.ResultsWhile 40 Hz tACS diminished task accuracy in young adults, older adults benefitted from this stimulation resulting in a more precise phonetic categorization.ConclusionThe results of the study are discussed with respect to the non-linear relationship between gamma oscillations in the vicinity of the auditory cortex and VOT-processing. The present findings are promising in the context of an intervention for subjects with impaired ability to process temporal acoustic features in the speech signal.