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. 2015 Oct 7:9:400.
doi: 10.3389/fncel.2015.00400. eCollection 2015.

Stimulating somatosensory psychophysics: a double-blind, sham-controlled study of the neurobiological mechanisms of tDCS

Claire J Hanley  1 , Mark Tommerdahl  2 , David J McGonigle  1
Affiliations

Stimulating somatosensory psychophysics: a double-blind, sham-controlled study of the neurobiological mechanisms of tDCS

Claire J Hanley et al. Front Cell Neurosci. .

Abstract

The neuromodulation technique transcranial direct current stimulation (tDCS) is thought to produce its effects on behavior by altering cortical excitability. Although the mechanisms underlying the observed effects are thought to rely on the balance of excitatory and inhibitory neurotransmission, the physiological principles of the technique are not completely understood. In this study, we examine the influence of tDCS on vibrotactile adaptation, using a simple amplitude discrimination paradigm that has been shown to exhibit modifications in performance due to changes in inhibitory neurotransmission. Double-blind tDCS (Anodal/Sham) of 1 mA was delivered for 600 s to electrodes positioned in a somatosensory/contralateral orbit montage. Stimulation was applied as part of a pre/post design, between blocks of the behavioral tasks. In accordance with previous work, results obtained before the application of tDCS indicated that amplitude discrimination thresholds were significantly worsened during adaptation trials, compared to those achieved at baseline. However, tDCS failed to modify amplitude discrimination performance. Using a Bayesian approach, this finding was revealed to constitute substantial evidence for the null hypothesis. The failure of DC stimulation to alter vibrotactile adaptation thresholds is discussed in the context of several factors that may have confounded the induction of changes in cortical plasticity.

Keywords: Bayesian statistics; GABA; NMDA; amplitude discrimination; neuromodulation; somatosensory; transcranial direct current stimulation; vibrotactile adaptation.

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Figures

FIGURE 1
FIGURE 1
Vibrotactile trials. (A) Trial stimulation: 25 Hz sinusoidal stimuli were delivered to D2 and D3 of the left hand. Adaptation trials consisted of a single pulse delivered to one digit (in this instance D3). During the test phase, stimuli were delivered simultaneously to D2 and D3. Subjects were required to determine which stimulus was of the highest amplitude. Baseline trials consisted only of the test phase. (B) Trial timing: Adaptation trials began with the presentation of a single pulse to the selected digit (A; 1000 ms), followed by an interval between the adaptor and test stimuli (1000 ms) before the standard and test stimuli were simultaneously delivered (S/T; 500 ms). Subjects were given an unrestricted response interval (RI) to indicate which digit they thought had received the stimulus of highest amplitude, after which an interval signaled the onset of the next trial (5000 ms; figure adapted from Tannan et al., 2007, with permission).
FIGURE 2
FIGURE 2
Electrode montage. Electrodes were positioned at locations CP4 (right hemisphere/contralateral to the stimulus, anode) and Fp1 (left hemisphere, cathode) of the 10–10 system.
FIGURE 3
FIGURE 3
Experimental design. Subjects initially completed one run of each of the vibrotactile tasks before receiving anodal or sham stimulation. This was followed by another two blocks of the tasks, post-stimulation. Subjects completed an adverse effects questionnaire between the post-stimulation task blocks.
FIGURE 4
FIGURE 4
Pre-tDCS amplitude discrimination thresholds. Average DL values for each task condition, obtained prior to DC stimulation (∗denotes significance, p < 0.05). Error bars represent ±1 SEM.
FIGURE 5
FIGURE 5
Post-tDCS amplitude discrimination thresholds. Average DL values obtained before and after tDCS, for each task condition in relation to the assessed stimulation modes (∗denotes significance, p < 0.05). Error bars represent ±1 SEM.
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