The effects of transcranial alternating current stimulation (tACS) at individual alpha peak frequency (iAPF) on motor cortex excitability in young and elderly adults

Abstract

Transcranial alternating current stimulation (tACS) can modulate brain oscillations, cortical excitability and behaviour. In aging, the decrease in EEG alpha activity (8-12 Hz) in the parieto-occipital and mu rhythm in the motor cortex are correlated with the decline in cognitive and motor functions, respectively. Increasing alpha activity using tACS might therefore improve cognitive and motor function in the elderly. The present study explored the influence of tACS on cortical excitability in young and old healthy adults. We applied tACS at individual alpha peak frequency for 10 min (1.5 mA) to the left motor cortex. Transcranial magnetic stimulation was used to assess the changes in cortical excitability as measured by motor-evoked potentials at rest, before and after stimulation. TACS increased cortical excitability in both groups. However, our results also suggest that the mechanism behind the effects was different, as we observed an increase and decrease in intracortical inhibition in the old group and young group, respectively. Our results indicate that both groups profited similarly from the stimulation. There was no indication that tACS was more effective in conditions of low alpha power, that is, in the elderly.

Keywords: Aging; Alpha oscillation; Electroencephalogram; Neuronal entrainment; Transcranial alternating current stimulation; Transcranial magnetic stimulation.

Fig. 1

Course of the experiment. First, baseline TMS measures of corticospinal (single-pulse TMS and I/O curves) and intracortical excitability (SICI/ICF) were recorded. Immediately after, a 3-min, eyes-open and eyes-closed spontaneous EEG measurement was obtained. EEG data during the eyes-open condition was analyzed to identify the iAPF. Then tACS was administered for 10 min using the participant’s iAPF. This was followed by the TMS measurements. SI 1 mV stable 1 mV threshold for single-pulse TMS, I/O input/output, SICI short interval intracortical inhibition, ICF intracortical facilitation, EEG electroencephalogram, iAPF individual alpha peak frequency

Fig. 2

The effect of tACS at iAPF on corticospinal excitability measured by single pulse TMS. The x-axis displays the time points before and after stimulation (Pre = before stimulation). The y-axis displays the non-normalized MEP amplitudes (mean ± SEM as error bars) in µV. After tACS stimulation, corticospinal excitability increased in both groups. Filled symbols indicate MEP amplitudes in the tACS stimulation conditions

Fig. 3

The effect of tACS at iAPF on corticospinal excitability measured by recruitment curves. The x-axis displays the RMT (%) for each time point (in min) before and after stimulation (Pre = before stimulation). The y-axis displays the MEP amplitude (mean ± SEM as error bars) in µV. a Young group: significant effect of tACS stimulation compared to sham 60 min (130% RMT) and 120 min (130 and 150% RMT) after stimulation. b Old group: There were no significant changes in the MEP amplitudes between tACS and sham stimulation. Filled symbols indicate MEP amplitudes in the tACS stimulation conditions

Fig. 4

The effect of tACS at iAPF on intracortical excitability. The x-axis displays the short interval intracortical inhibition (SICI) on the left and short interval intracortical facilitation (ICF) on the right for each time point (in min) before and after stimulation (Pre = before stimulation). The y-axis displays the MEP amplitude (mean ± SEM) normalized to the test pulse (SI 1 mV). a Young group: tACS stimulation decreased SICI. b Old group: tACS increased SICI post stimulation. Filled symbols = tACS stimulation, open symbols = sham conditions