Improvement of spatial tactile acuity by transcranial direct current stimulation

Abstract

Objective: Non-invasive brain stimulation such as transcranial direct current stimulation (tDCS) has been successfully used to induce polarity-specific excitability changes in the brain. However, it is still unknown if anodal tDCS (tDCS(anodal)) applied to the primary somatosensory cortex (S1) can lead to behavioral changes in performance of tactile discriminative tasks.

Methods: Using an accurate tactile discrimination task (grating orientation task: GOT) we tested the hypothesis that application of 1mA of tDCS(anodal) (current density at the electrodes of 0.04mA/cm2) over the left S1 can lead to an improved tactile spatial acuity in the contralateral index-finger (IF).

Results: Performance in the GOT task with the contralateral IF but not with the ipsilateral IF was enhanced for about 40min after a 20min application of tDCS(anodal) in the absence of changes with sham stimulation.

Conclusions: These results provide the first evidence that tDCS(anodal) over S1 improves performance in a complex somatosensory task beyond the period of stimulation.

Significance: The ability to induce performance improvement in the somatosensory domain with tDCS applied over S1 could be used to promote functional recovery in patients with diminished tactile perception.

Figures

Figure 1. Experimental design

In order to obtain a stable baseline performance for the right IF, GOT thresholds were measured on 4 consecutive sessions (GOT1-4) before tDCSanodal/sham was applied. The left IF served as controls to assess spatial specificity of perceptual effects. GOT thresholds of the left IF were tested only immediately before (GOT1left) and after (GOT2left) the stimulation period because effects of task familiarization are known to generalize across fingers. After initial GOT testing on both fingers, tDCS was applied over the left S1. After tDCS, GOT thresholds on both index fingers (IFs) were measured. GOT7 served to assess the recovery of tDCS-induced effects on tactile GOT thresholds of the right IF.

Figure 2

Figure 2a. Effect of tDCSanodal (20 min) on right IF GOT thresholds. All subjects achieved a stable baseline performance prior to tDCS. Ten minutes after the onset of tDCSanodal, subjects showed a significant discrimination improvement that persisted for at least 40 minutes. The GOT thresholds on the left IF ipsilateral to tDCSanodal was not affected. Figure 2b. Effect of tDCSsham (20 min) on right IF GOT thresholds. In comparison to tDCSanodal, there was no change in GOT thresholds neither on the right nor on the left IF.

Figure 2

Figure 2a. Effect of tDCSanodal (20 min) on right IF GOT thresholds. All subjects achieved a stable baseline performance prior to tDCS. Ten minutes after the onset of tDCSanodal, subjects showed a significant discrimination improvement that persisted for at least 40 minutes. The GOT thresholds on the left IF ipsilateral to tDCSanodal was not affected. Figure 2b. Effect of tDCSsham (20 min) on right IF GOT thresholds. In comparison to tDCSanodal, there was no change in GOT thresholds neither on the right nor on the left IF.

Figure 3. Effect of tDCSanodal and tDCSsham on GOT thresholds

Please, note a decrease in GOT thresholds in the right IF with tDCSanodal in all but one subject.