Transcranial direct current stimulation over the primary motor cortex during fMRI.

Area of Science:

• Neuroscience
• Neuroimaging
• Motor Control

Background:

• Transcranial direct current stimulation (tDCS) modulates corticospinal excitability, with anodal and cathodal polarities inducing facilitatory and inhibitory effects, respectively, as measured by motor evoked potentials (MEPs).
• The relationship between these MEP-based excitability changes and concurrent brain activity, particularly hemodynamic responses measured by blood-oxygen-level-dependent (BOLD) functional magnetic resonance imaging (fMRI), remains incompletely understood.

Purpose of the Study:

• To investigate the online effects of short-duration anodal and cathodal tDCS on human brain activity and associated hemodynamics using concurrent BOLD fMRI.
• To compare the polarity-dependent effects of tDCS on corticospinal excitability (MEPs) with concurrent BOLD fMRI signals in the primary motor cortex (M1) and associated areas.

Main Methods:

• Concurrent BOLD fMRI at 3T was used to measure brain activity during 20-second periods of anodal or cathodal tDCS (1 mA) over the left M1, interspersed with 20-second rest periods.
• Stimulation was applied at rest or during a finger-tapping task. A control experiment involved electrode placement over the occipito-temporo-parietal junction.
• Motor evoked potentials (MEPs) were measured to assess corticospinal excitability, while BOLD signal changes were analyzed in various brain regions, including the supplementary motor area (SMA).

Main Results:

• Neither anodal nor cathodal tDCS over M1 induced detectable BOLD signal changes during rest or stimulation.
• Anodal tDCS during finger tapping led to a significant decrease in BOLD response in the SMA compared to finger tapping without stimulation.
• Cathodal tDCS showed a trend towards decreased SMA activity, but not statistically significant. Control stimulation over the occipito-temporo-parietal junction did not affect BOLD signals.

Conclusions:

• The polarity-dependent shifts in corticospinal excitability observed with MEPs following M1 stimulation are not directly paralleled by analogous changes in regional BOLD signals.
• This dissociation suggests that MEPs and BOLD fMRI reflect distinct physiological mechanisms: MEPs indicate transsynaptic excitability of pyramidal neurons, while BOLD reflects net synaptic activity across cortical neurons.
• Short-duration tDCS, while affecting MEPs, does not consistently alter BOLD signals, indicating a complex interplay between direct neuronal excitability and the hemodynamic response.