Non-Invasive Brain Stimulation (TMS, tCS, FUS)

FUS (focused ultrasound stimulation)

applied transcranially, bone does not scatter sound waves. Long axis: orthogonal to scalp surface, so can penetrate at last a cm deep. Spatial resolution is a few millimeters, higher than other non-invasive stimulation methods in lateral directions. Typically guided by anatomical MRI. Low-intensity FUS leads to neural stimulation: non-thermal mechanical energy, small displacement of cells and several hypothesized effects on properties of receptors

Transcranial magnetic stimulation (TMS)

Based on Faraday’s principle: electric current in coil, generates transient magnetic field perpendicular to coil, induces second cent in brain that flows parallel but in opposite direction. Stimulates/depolarizes neurons. The current of the coil moves in the opposite direction in the brain. The figure of 8 coil design has a more focused magnetic field, more precise location of altered neural activity

Determinants properties transcranial magnetic stimulation

physical factors (distance to coil): effect gradually dissipates with long distance, restricts application to superficial structures | Biological factors: conductivity of tissue, orientation of axons

TMS variants

Repetitive TMS: multiple pulses, influences temporal resolution, stronger effect (also higher risk). Or Single/double pulse TMS. Sometimes combined, rTMS first to establish causal link, followed by single/double pulse TMS for better temporal resolution

Neuronavigation

using structural/anatomical MRI scan of participant

FMRI-guided TMS

using an fMRI localized task + neuronavigation

Potential of TMS

diagnostic value for testing connectivity between motor cortex and peripheral muscles (motor related disorders, after stroke) or as part of clinical treatments such as major depression: rTMS as alternative for electroconvulsive therapy or Migraine (single-pulse TMS over visual cortex)

TCS (transcranial current stimulation)

Principle similar to TMS but uses electrical (instead of magnetic) stimulation current flows from anode (depolarization of neuron, excitatory) to cathode (hyperpolarization of neuron, inhibitory). Side effects: skin sensation at start and end of stimulation

Potential of TCS

Small but robust effect on neural activity in particular circumstances, can make subthreshold stimulus perceivable, small effects accumulate over time, larger sample sized needed for accurate estimate of effect size.

Transcranial direct current stimulation (tDCS)

direct current (in one direction); location of electrodes determines flow current

Transcranial alternating current stimulation (tACS)

alternating current at fixed frequency (test whether particular frequency ranges are involved)

Transcranial random noise stimulation (tRNS)

alternating current based on random frequency spectrum)