Entraining alpha oscillations to facilitate auditory working memory: A TMS-EEG study
Older adults often find listening to and remembering speech a challenging process, particularly in complex listening environments. Neural alpha oscillations may support working memory during speech perception. Specifically, alpha oscillations in parietal cortex may promote inhibition of distracting sounds, whereas alpha oscillations in temporal cortex may enhance attention to target sounds. Importantly, alpha activity during speech perception may be different for younger and older adults. We hypothesised that entraining alpha activity in these brain regions may facilitate speech perception. We investigated whether TMS delivered at an individualised alpha frequency (alpha-TMS) benefits auditory working memory, and how this may be affected by age.
Thirty younger adults (mean age = 20.9 years) and 15 older adults (mean age = 68.6) completed trials of an auditory working memory task. Participants attended to and recalled 9-digit sequences, whilst ignoring irrelevant sentences. Before the to-be-ignored sentences, participants received alpha-TMS. We investigated the effects of: (1) distractibility of irrelevant sentences (less vs. more distracting); (2) site of alpha-TMS (control vs. parietal vs. temporal); (3) age group (younger vs. older), on digital recall and alpha power. For alpha power, we also investigated differences across phases of the trial (attending vs. ignoring vs. recall).
Across all TMS conditions and age groups, digit recall was poorer in trials with more distracting to-be-ignored sentences [F(1,39)=13.19, p=.001]. Across all ages, sentences, and TMS sites, alpha power was highest during the digit attending phase and lowest during the recall phase [F(2,80)=15.54, p<.001]. There was also an interaction between trial phase and TMS site on alpha power [F(2.7,108.5)=5.19, p=.003]. Specifically, in the control and parietal TMS conditions, alpha power was highest during the attending phase, whereas, in the temporal TMS site condition, alpha power was highest during the ignoring phase. Alpha-TMS may modulate parietal and auditory alpha, which may influence independent inhibitory and attentional processes.