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

Abstract

Background: Normal 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. Objective: Using 40 Hz transcranial alternating current stimulation (tACS) over the bilateral auditory cortex, this study aims at compare the effect of tACS to modulate VOT-processing in samples of healthy young and older adults. Methods: 25 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. Results: While 40 Hz tACS diminished task accuracy in young adults, older adults benefitted from this stimulation resulting in a more precise phonetic categorization. Conclusion: The 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.