Aperiodic 1/f brain activity is associated with corticospinal excitability

Electroencephalography (EEG) can be combined with transcranial magnetic stimulation (TMS) to perform brain-state-dependent stimulation. EEG-TMS studies have shown that corticospinal excitability, as measured via motor evoked potentials (MEPs), is associated with pre-stimulus periodic EEG features, such as sensorimotor mu-rhythm phase and power. However, the influence of aperiodic brain activity on corticospinal excitability is largely unexplored.

We evaluated the relationship between aperiodic 1/f and periodic mu-power, aperiodic exponent, and mu-phase on MEP amplitudes using EEG-TMS.

We applied 800 single TMS pulses to the left primary motor cortex in 78 healthy adults. We calculated aperiodic/periodic mu-power, aperiodic exponent, and mu-phase for each trial from the pre-stimulus C3-Hjorth transformed EEG. MEP amplitudes were extracted from the right first dorsal interosseous muscle. A linear mixed-effects model assessed relationships between MEP amplitudes and EEG features, with interactions between mu-phase and all other EEG features.

Aperiodic 1/f and periodic mu-power, aperiodic exponent, and mu-phase were significantly associated with MEP amplitude. Higher aperiodic/periodic mu-power was associated with larger MEP amplitudes. A weak and variable association was demonstrated between aperiodic exponent and MEP amplitudes. Notably, we found a significant interaction effect of aperiodic exponent and mu-phase on MEP amplitude. Aperiodic exponent was negatively associated with MEPs for trough, rising, falling phases, but positively associated with MEPs for peak phase.

Aperiodic 1/f and periodic features of brain activity are reflective of corticospinal excitability states. Future brain-state-dependent TMS interventions may include aperiodic EEG features, such as aperiodic mu-power and exponent, in addition to the well-established periodic features.