Abstract
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.
Highlights
Choice reaction time (RT) tasks require participants to react to one of multiple possible stimuli with corresponding responses
Post-hoc Student’s t-tests performed on premotor RT collapsed across hands between pre- and post-Transcranial direct current stimulation (tDCS) blocks confirmed that tDCS with the anode over the left motor cortex led to a significant, t(11) = 4.453, p = .001, r = .744, decrease in premotor RT (-13 ms, SD = 10), and tDCS applied with the anode over the right motor cortex led to a significant, t(11) = 3.091, p = .01, r = .611, decrease in premotor RT (-7 ms, SD = 8), whereas no difference (p = .696) was noted between pre- and post-tDCS blocks in the sham conditions (-1 ms, SD = 12)
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 (Fig 1), irrespective of whether the anode was placed over the left or the right motor cortex
Summary
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 [1]. These RT differences have been suggested to result from the additional processing required to select and execute the proper response in a choice-RT task [2]. Bi-hemispheric tDCS over M1 reduces choice reaction time in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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