The authors have declared that no competing interests exist.
Increased reaction times (RT) during choice-RT tasks stem from a requirement for additional processing as well as reduced motor-specific preparatory activation. Transcranial direct current stimulation (tDCS) can modulate primary motor cortex excitability, increasing (anodal stimulation) or decreasing (cathodal stimulation) excitability in underlying cortical tissue. The present study investigated whether lateralized differences in choice-RT would result from the concurrent modulation of left and right motor cortices using bi-hemispheric tDCS. Participants completed a choice-RT task requiring either a left or right wrist extension. In forced-choice trials an illuminated target indicated the required response, whereas in free-choice trials participants freely selected either response upon illumination of a central fixation. Following a pre-test trial block, offline bi-hemispheric tDCS (1 mA) was applied over the left and right motor cortices for 10 minutes, which was followed by a post-tDCS block of RT trials. Twelve participants completed three experimental sessions, two with real tDCS (anode right, anode left), as well as a sham tDCS session. Post-tDCS results showed faster RTs for both right and left responses irrespective of tDCS polarity during forced-choice trials, while sham tDCS had no effect. In contrast, no stimulation-related RT or response selection differences were observed in free-choice trials. The present study shows evidence of an effector-independent speeding of response initiation in a forced-choice RT task following bi-hemispheric tDCS and yields novel information regarding the functional effect of bi-hemispheric tDCS.
Choice reaction time (RT) tasks require participants to react to one of multiple possible stimuli with corresponding responses. The time to initiate the appropriate response is typically found to be longer than in a simple RT task, in which a single required response is known in advance [
One method used to modulate cortical activation is transcranial direct current stimulation (tDCS). By applying a weak electrical current over the scalp, polarity-dependent changes have been shown whereby anodal stimulation increases, and cathodal stimulation decreases the excitability of underlying neural tissue (see [
Twelve neurologically healthy volunteers (6M; 27.7 years, SD = 11.2) who self-reported as right handed participated in three experimental sessions on three separate days involving real tDCS applied in two sessions and sham tDCS applied in one session. A power calculation using GPower 3.1.9 was used to determine the sample size required to detect an effect size (.556) similar to that previously reported for the effect of tDCS on a simple RT task [
Participants sat facing a computer monitor at eye level, approximately 1 m away. Both forearms were placed in custom manipulanda which allowed flexion/extension of the wrists in the transverse plane. Both hands were semi-pronated with the palms facing inward in a neutral position (neither flexed nor extended), and secured to a swivelling rest with the axis of rotation at the wrist. The shoulders were flexed and abducted approximately 15° with the arms secured in armrests using two Velcro straps located between the wrist and elbow.
Participants performed a choice-RT task requiring a 20° wrist extension with either the right or left wrist upon illumination of an associated stimulus. A black central fixation circle (1 cm diameter) was displayed on the computer screen, along with 3 x 3 cm black squares located 4.5 cm to the right and left of fixation. At the beginning of each trial, a tone (100 ms, 200 Hz, 82 dB) sounded and the words “Get Ready!” were presented for 2 s above the central fixation. This was followed by a 1500–2000 ms random foreperiod prior to the illumination of the imperative stimulus (IS), which involved one of the boxes turning bright green. Participants were instructed to initiate a right wrist extension upon illumination of the right box, left wrist extension upon illumination of the left box, and if the central fixation circle illuminated they were instructed they had free-choice of either movement (right or left extension). Prior to testing, participants were informed that during free-choice trials, it did not matter which response side was chosen, only to respond with one as quickly as possible. Instructions emphasized fast reactions on all trials and a points scheme was used to encourage fast RTs. After each trial, feedback was displayed for 3 s including RT and running total of points awarded which were scaled to individual RT performance. Participants were notified if their movement amplitude error was greater than 10° and were also verbally encouraged to react as fast and accurately as possible in response to the IS.
On each testing day participants first performed 16 practice trials (7 forced-right, 8 forced-left, 1 free-choice) which were followed by a pre-tDCS testing block consisting of 100 choice-RT trials (40 forced-left, 40 forced-right, and 20 free-choice trials). Trials were pseudo-randomized whereby free-choice trials were preceded an equal number of times by right and left forced-choice trials. Following the pre-tDCS block, tDCS (or sham tDCS) was administered, followed by an 8-minute rest interval of quiet sitting (see rationale below). A post-tDCS testing block of 100 trials was then performed. This sequence was repeated each testing day.
During the active stimulation (real tDCS) sessions two self-adhesive electrodes (small sponge electrode, 1.5cc, 7.8 cm2, Ionto+ Inc.) were placed bilaterally on the scalp over the left and right motor representations for wrist extensors. The location of the electrodes, 4.7 cm lateral and 1.1 cm anterior to the measured location of Cz (according to the International 10–20 system), corresponded to the approximate location of the right and left extensor carpi radials longus (ECR) representations on M1 [
Between pre- and post-tDCS testing blocks in each of the active stimulation testing sessions, tDCS was applied with either the anodal lead attached to the electrode over the right motor cortex and the cathodal lead attached to the electrode over left motor cortex, or vice versa. Stimulation was delivered using a Dupel iontophoresis constant current delivery device (Empi Inc.). Current was set at 1 mA and was delivered for 10 minutes (current density = 0.128 mA /cm2). Previous research has shown that using similar stimulation parameters, tDCS effects were greatest 10–25 minutes following stimulation [
Sham stimulation was administered with tDCS electrodes placed in a unilateral montage with one small sponge electrode (15cc, 7.8 cm2, Ionto+ Inc.) placed over either the right or left ECR motor representation and the other electrode (carbon-foam electrode, 39 cm2, Ionto+ Inc.) placed centrally on the forehead directly above the eyebrows. This different montage was used to ensure that participants would remain naïve to the purpose of the sham session, and electrode montage was not expected to have any direct bearing on the outcome. During sham stimulation, the stimulation device was only powered on while ramping up to 1 mA (<20 sec), then immediately shut off without the participant’s awareness. In six participants sham stimulation occurred during the first session, for the remaining six participants sham stimulation occurred in a third session. Sham stimulation side (right or left M1) was balanced between participants.
Surface electromyographic (EMG) data were collected from the muscle bellies of the left and right ECR and the left and right flexor carpi radialis (FCR) using bipolar preamplified (gain = 10) surface electrodes (Delsys Bagnoli DE-2.1) connected via shielded cabling to an external amplifier (Delsys Bagnoli-8). The recording sites were scrubbed and cleansed to decrease electrical impedance. Electrodes were placed parallel to the muscle fibres, and a reference electrode was placed on the right lateral epicondyle of participants. A potentiometer attached to the central axis of each manipulandum was used to collect wrist angular position data. On each trial, raw band-passed (20-450Hz) EMG and unfiltered position data were digitally sampled for 3 s at 1 kHz using a 16 bit analog to digital converter (National Instruments Inc.) and stored for offline analysis. Data collection was completed using a customized LabVIEW program (National Instruments Inc.)
Displacement onset was determined as the first point at which angular displacement of more than 0.2° occurred following the IS. If the incorrect wrist was used to respond, if a response was made in less than 100 ms or more than 500 ms following the go-signal, or if a response was made with both wrists in forced-choice trials, these responses were classified as “errors” and removed from analysis. In free-choice trials errors were noted only when a response was made with both wrists. This procedure resulted in the study-wide removal of 122/5760 of trials in the forced-choice conditions (2.1%), and 174/1440 trials in the free-choice conditions (12.1%).
EMG data were analyzed for differences in timing of burst onsets and offsets. Signals were rectified and filtered using a 25 Hz low pass elliptic filter, and displayed on a computer monitor using a customized LabVIEW program. Markers indicating EMG burst onsets in wrist extensors and flexors were then placed by the computer program on the EMG traces at the point in time at which EMG activity first reached a value 2 standard deviations above baseline levels (i.e., mean of 100 ms of EMG activity preceding the IS). Similarly, EMG offset markers were placed at the point in time when EMG activity first fell below 80% of peak activity and remained below for at least 25 ms. Activity between EMG onset and offset was defined as a distinct burst. EMG markers were manually adjusted (if necessary) to allow for correction of errors due to the strictness of the algorithm and these time points were recorded for analysis [
Kinematic variables included peak velocity, time to peak velocity, peak displacement, and time to peak displacement. Peak velocity was the maximum angular velocity achieved prior to reaching peak displacement and time to peak velocity was the time between movement onset and peak velocity. Peak displacement was the maximum angular displacement attained following movement onset, and time to peak displacement was the time between movement onset and this point.
For forced-choice and free-choice trials kinematic variables as well as premotor RT (IS to EMG onset) were each analyzed using separate 3 (Polarity: anode right, anode left, sham) x 2 (Time: pre-tDCS, post-tDCS) x 2 (Response limb: left, right) Repeated Measures Analysis of Variance (RM ANOVA). Additionally, for free-choice trials the proportion of responses made with the right hand were analyzed using a 3 (Polarity) x 2 (Time) RM ANOVA. Prior to analysis, proportion variables were corrected for normality using an arcsine square root transformation. Post-hoc Student’s t-tests were administered where appropriate to determine the locus of any differences. Partial eta squared (ηp2) is reported as an estimate of the proportion of the variance that can be attributed to the tested factor. Differences with a probability of less than .05 were considered significant.
Premotor RT during forced-choice trials pre- and post-stimulation is presented in
A) Left (L) and right (R) hand reaction times are shown in pre- and post-tDCS blocks for each stimulation polarity (anode left, anode right, and sham). Error bars denote standard error, * indicates significant differece (
Analysis of kinematic variables during forced-choice trials showed no significant main effects and no significant interactions between the variables (all
Analysis of the proportion of responses made with the right hand during free-choice trials revealed no significant main effects for Polarity,
A) Reaction times collapsed across hand are shown in pre- and post-tDCS blocks for each stimulation polarity (anode left, anode right, and sham). Error bars denote standard error. Note that when collapsed across stimulation polarity there is a significant difference between pre- and post-tDCS blocks (
Analysis of kinematic variables during free-choice trials showed a significant main effect of Response hand for time to peak velocity,
The purpose of the current study was to determine whether the application of bi-hemispheric tDCS would lead to lateralized RT changes in a choice-RT task including both forced-choice and free-choice trials. Contrary to expectations, results clearly show that applying tDCS in a bi-hemispheric montage led to a decrease in RT for both right and left hand forced-choice responses (
There are several distinct possibilities related to the bi-hemispheric electrode montage that may explain the observed polarity-independent effects on RTs during the forced-choice trials. The first possibility is that bi-hemispheric tDCS did
A second possible explanation for the polarity-independent RT effects observed in the current study is that regardless of polarity, the bi-hemispheric tDCS led to increased activation under both electrodes. There is some recent evidence to suggest that cathodal stimulation delivered with a high current density results in cortical excitability enhancement instead of inhibition [
A final possible explanation for the polarity-independent RT effects observed in the current study is that although previous research has shown that simple RT can be speeded or slowed using a unilateral tDCS montage [
In the free-choice conditions there appeared to be no effect of tDCS on the behavioural outcome. Specifically, there was no change in the proportion of responses made with the right hand following either real or sham stimulation. Yet, in the free-choice trials a significant reduction in RT was observed between the pre-tDCS and post-tDCS blocks for all stimulation sessions, including the sham session (
Although the precise mechanism and locus of neural excitability changes accompanying bi-hemispheric tDCS are unclear, the present study shows evidence of an effector-independent speeding of response initiation in a forced-choice RT task following bi-hemispheric tDCS delivered at 1 mA for 10 min with a current density of 0.128 mA/ cm2. It is self-evident, given the multiple explanations outlined above, that further investigation into the physiological effects of tDCS (including but not limited to the impact of stimulation parameters and electrode montages) is warranted. Moreover, inconsistencies in the amount of current applied (mA), duration of stimulation (minutes), and current density (mA / cm2) between tDCS studies adds to the difficulty in drawing firm conclusions. Here, the present work yields novel information regarding the functional effect of bi-hemispheric tDCS on motor processes, indicating that irrespective of current flow direction, forced-choice RT is facilitated following tDCS applied bi-laterally over the motor cortices.