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 1 mA of tDCS_anodal (current density at the electrodes of 0.04 mA/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 40 min after a 20 min 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.

Introduction

Somatosensory input is crucial for skillful motor control and for learning new motor skills (Johansson and Westling, 1984, Pause et al., 1989, Hermsdorfer et al., 2004). In animal experiments, surgically abolished sensation in one of the forelimbs in monkeys leads to a decrease of voluntary motor activity (Taub, 1977). More recently, the same phenomenon has been studied in detail in human subjects. For example, a reduction of somatosensory input by local anesthesia impairs motor control in normal volunteers (Monzee et al., 2003, Duque et al., 2005). Conversely, it has been shown that increased somatosensory input by means of peripheral nerve stimulation modulates cortico-motor excitability in healthy volunteers and enhances transiently motor function in chronic stroke patients (Ridding et al., 2000, Kaelin-Lang et al., 2002, Conforto et al., 2007). Additionally, previous studies demonstrated that it is possible to modulate somatosensory (Tegenthoff et al., 2005, Dieckhofer et al., 2006, Pleger et al., 2006) and motor (Hummel et al., 2005) function by noninvasive brain stimulation. These techniques might therefore play an adjuvant role in rehabilitative treatments of neurological and psychiatric disorders (for review see (Kobayashi and Pascual-Leone, 2003, Ward and Cohen, 2004, Cooke and Bliss, 2006)).

Transcranial direct current stimulation (tDCS), a particular form of noninvasive brain stimulation, is a procedure used to polarize brain regions through the application of weak direct currents (Nitsche and Paulus, 2000, Wassermann and Grafman, 2005). For example, anodal tDCS (tDCS_anodal) applied over the primary motor cortex (M1) transiently increases cortical excitability beyond the period of stimulation, while cathodal stimulation often decreases it (Priori et al., 1998, Nitsche and Paulus, 2001). Furthermore, tDCS-induced excitability changes seem to be associated with effects on performance of motor as well as non-motor tasks (Hummel et al., 2005, Hummel and Cohen, 2006).

In the somatosensory cortex, comparable polarity-specific differential effects of tDCS on the high and low frequency components or amplitudes of somatosensory evoked potentials (SEPs) have also been demonstrated (Matsunaga et al., 2004, Dieckhofer et al., 2006). However, it is still unknown if tDCS_anodal application over primary somatosensory cortex (S1) is capable to induce changes in tactile spatial acuity in human subjects.

Here, we intended to clarify if application of tDCS_anodal centered over the left S1 influences tactile discrimination skills on the index-finger (IF) contralateral to stimulation in healthy volunteers. As a behavioral outcome measure, we choose the grating orientation task (GOT), an accurate and commonly accepted measure of tactile spatial acuity in human subjects (Johnson and Phillips, 1981, Van Boven and Johnson, 1994).

Since tDCS elicits lesser perceptual phenomena than transcranial magnetic stimulation (TMS) or theta burst stimulation (TBS) such as acoustic noise or sudden scalp sensation, tDCS has possibly a greater potential for sham-controlled studies and clinical applications. Furthermore, one technical advantage of tDCS application is that performance changes can be measured during the time of stimulation. Therefore, tDCS could be a useful adjuvant tool when combined with performance of complex cognitive tasks or rehabilitative treatments after brain lesions (Gandiga et al., 2006).