Efficacy of Anodal Transcranial Direct Current Stimulation is Related to Sensitivity to Transcranial Magnetic Stimulation.
BACKGROUND
Transcranial direct current stimulation (tDCS) has become an important non-invasive brain stimulation tool for basic human brain physiology and cognitive neuroscience, with potential applications in cognitive and motor rehabilitation. To date, tDCS studies have employed a fixed stimulation level, without considering the impact of individual anatomy and physiology on the efficacy of the stimulation. This approach contrasts with the standard procedure for transcranial magnetic stimulation (TMS) where stimulation levels are usually tailored on an individual basis.


OBJECTIVE
The present study tests whether the efficacy of tDCS-induced changes in corticospinal excitability varies as a function of individual differences in sensitivity to TMS.


METHODS
We performed an archival review to examine the relationship between the TMS intensity required to induce 1 mV motor-evoked potentials (MEPs) and the efficacy of (fixed-intensity) tDCS over the primary motor cortex (M1). For the latter, we examined tDCS-induced changes in corticospinal excitability, operationalized by comparing MEPs before and after anodal or cathodal tDCS. For comparison, we performed a similar analysis on data sets in which MEPs had been obtained before and after paired associative stimulation (PAS), a non-invasive brain stimulation technique in which the stimulation intensity is adjusted on an individual basis.


RESULTS
MEPs were enhanced following anodal tDCS. This effect was larger in participants more sensitive to TMS as compared to those less sensitive to TMS, with sensitivity defined as the TMS intensity required to produce MEPs amplitudes of the size of 1 mV. While MEPs were attenuated following cathodal tDCS, the magnitude of this attenuation was not related to TMS sensitivity nor was there a relationship between TMS sensitivity and responsiveness to PAS.


CONCLUSIONS
Accounting for variation in individual sensitivity to non-invasive brain stimulation may enhance the utility of tDCS as a tool for understanding brain-behavior interactions and as a method for clinical interventions.
Department of Psychology, University of California, Berkeley, California, USA; Helen Wills Neuroscience Institute, University of California, Berkeley, California, USA. Electronic address: lulabrun@gmail.com.
Ludovica
L
Labruna
Department of Clinical Neurophysiology, University Medical Center, Georg-August-University, Goettingen, Germany.
Asif
A
Jamil
Department of Clinical Neurophysiology, University Medical Center, Georg-August-University, Goettingen, Germany.
Shane
S
Fresnoza
Department of Clinical Neurophysiology, University Medical Center, Georg-August-University, Goettingen, Germany.
Giorgi
G
Batsikadze
Department of Clinical Neurophysiology, University Medical Center, Georg-August-University, Goettingen, Germany.
Min-Fang
MF
Kuo
Department of Psychology, University of California, Berkeley, California, USA.
Benjamin
B
Vanderschelden
Department of Psychology, University of California, Berkeley, California, USA; Helen Wills Neuroscience Institute, University of California, Berkeley, California, USA.
Richard B
RB
Ivry
Department of Clinical Neurophysiology, University Medical Center, Georg-August-University, Goettingen, Germany; Leibniz Research Center for Working Environment and Human Factors, Dortmund, Germany; Department of Neurology, BG University Hospital Bergmannsheil, Ruhr-University Bochum, Bochum, Germany.
Michael A
MA
Nitsche
D000328:Adult; D004566:Electrodes; D019054:Evoked Potentials, Motor; D005260:Female; D006801:Humans; D008297:Male; D065908:Transcranial Direct Current Stimulation / Q000379:methods*; D050781:Transcranial Magnetic Stimulation / Q000379:methods*
Intensity; TMS; Variability; tDCS