The rhythm of life: Individualised rhythmic transcranial magnetic stimulation to auditory cortex entrains alpha oscillators in older adults during listening

Age-related declines in top-down mechanisms like auditory working memory and attentional control can have a significant impact on an older adult's ability to perceive speech. These declines may be reflected in oscillatory alpha-band activity, associated with attending to relevant speech and inhibiting distracting speech. As such, entraining alpha activity via transcranial magnetic stimulation (TMS) holds promise for supporting speech perception in older age, particularly in challenging listening environment. This study aimed to determine if TMS delivered at an individualised alpha frequency (alpha-TMS) can entrain alpha oscillators in older adults, and if this confers any behavioural benefit. Thirty-two younger adults (18-35 years old) and thirty-two older adults (60-80 years old) were asked to attend to and remember a sequence of spoken digits, whilst ignoring an irrelevant sentence that was either easy or difficult to ignore. Before the irrelevant sentences, participants received alpha-TMS to the vertex, parietal cortex and auditory cortex in separate conditions. Parietal and auditory alpha power during the attending and maintenance of the digits, were compared across conditions and age groups. The proportion of digits recalled in the correct order was also evaluated. Behavioural findings supported previous research suggesting that distracting speech high in acoustic detail has a negative impact on the ability to attend to and retain target speech. For the first time, we found that alpha-TMS preferentially entrained alpha oscillators in the auditory cortex of older adults. This was not observed in younger adults, or for parietal oscillators in either age group. No significant differences in auditory working memory performance between younger and older adults were observed. These data demonstrate the neurophysiological basis of a new approach for listening augmentation. Future work should explore whether alpha entrainment may benefit other aspects of speech perception in older adults.