Our results demonstrate that a short period of tDCSanodal applied over the human S1 enhances tactile spatial acuity in the contralateral hand relative to sham stimulation. This performance improvement lasts for at least 40 minutes after the end of the stimulation period and is not present in the hand ipsilateral to the stimulated S1.
Improvements in tactile performance after non-invasive brain stimulation have been reported in previous studies. For example, 5 Hz rTMS, paired-associative stimulation or TBS applied over S1 could improve tactile performance, as quantified by two-point discrimination (2PD) thresholds (Ragert et al., 2003
; Tegenthoff et al., 2005
; Litvak et al., 2007
; Ragert et al., 2007
). It has been hypothesized that these performance improvements might be associated with excitability changes within the stimulated body part representation in the somatosensory cortex (Tegenthoff et al., 2005
Of particular interest is that tDCSanodal
centered over M1 results in an amplitude increase of the P25/N33, N33/P40 (parietal) and P22/N30 (frontal) components of the somatosensory evoked potentials (SEPs) elicited by stimulation of the median nerve of the contralateral hand whereas tDCScathodal
lacks such an effect (Matsunaga et al., 2004
). It has been proposed that the areas most likely polarized by tDCS centered over M1 included the surface of the postcentral gyrus such as Brodmann areas (BA) 1 and 2 since earlier SEP components such as the P14/N20, N20/P25 and N18/N22 were not affected by tDCS (Matsunaga et al., 2004
). On the other hand, Dieckhofer and colleagues reported a longer lasting reduction of the N20 component after tDCScathodal
centered over SI with no effect after tDCSanodal
(Dieckhofer et al., 2006
). The results from Dieckhofer and colleagues are in accordance with a recent psychophysical study (Rogalewski et al., 2004
) indicating that tDCS-induced excitability changes within SI may be associated with a modulation of tactile performance in human subjects: 7 min of tDCScathodal
over S1 induced a prolonged decrease of tactile frequency discrimination while tDCSanodal
or sham had no effect (Rogalewski et al., 2004
Possible reasons for the different effects of tDCSanodal reported across these different studies might be the different target area of tDCS (M1 vs. S1) and the different size of the stimulation electrodes used. The electrode size in the present study was smaller than electrodes used in previous investigations (25 vs. 35 mm2). Consequently the current density in the present study was likely higher as well as relatively more focal perhaps leading to more overt beneficial effects of tDCSanodal on tactile spatial acuity. Additionally, the application of tDCS for 7 min in previous studies (as compared to 20 min in the present study) may have been insufficient to elicit performance improvements. Finally, the use of different behavioral outcome measures might also account for some of these differences.
The purpose of our investigation was to determine the effects of tDCSanodal
applied to the human S1 on tactile spatial acuity in the contralateral hand, as measured by the GOT task (Van Boven and Johnson, 1994
; Craig, 1999
). Afferent information required to accurately perform this task derives predominantly from rapidly adapting (RA) and slowly adapting type 1 (SA) afferents (for review see (Johnson, 2001
)). The task has been extensively used as psychophysical measurement of spatial tactile acuity and in clinical settings (Van Boven and Johnson, 1994
; Van Boven et al., 2000
; Goldreich and Kanics, 2003
; Molloy et al., 2003
) and is sensitive enough to unveil changes in performance in healthy subjects (Tremblay et al., 2002
; Werhahn et al., 2002
) and in patients with stroke (Voller et al., 2006
When compared to other brain stimulation protocols like TMS, tDCS application may have a relatively lower focality because of the relatively larger stimulation electrodes (Gandiga et al., 2006
) and the electrode montage. As a consequence, a relatively more widespread current flow is expected with tDCS than with TMS in the respective projection areas (Lang et al., 2005
; Wagner et al., 2007
). Despite this apparent drawback, tDCS is a promising tool in neurorehabilitation since it is easy to apply, easier to sham than TMS, and performance changes can be documented more easily during the time of stimulation (Hummel and Cohen, 2006
The magnitude of tDCS-induced improvements in our study (29.65 ± 5.01 %) as well as its duration (≥ 40 min) compares favorably with effects reported on performance of motor tasks such as the Jebson-Taylor Test (JTT; improvements of approximately 11.75 ± 3.61 % in chronic stroke patients or 10.96 ± 3.61 % in healthy controls) when applied over M1 (Hummel et al., 2005
; Hummel and Cohen, 2005
). Therefore, effects of tDCS appear to elicit qualitatively similar effects on different modalities (somatosensory and motor domain).
The mechanisms underlying these behavioral gains in tactile spatial acuity are still prospective. On one hand, it is possible that tDCS-induced excitability changes within S1 contributed to this effect (Matsunaga et al., 2004
). On the other hand, it is possible, while formal evidence to this effect is lacking, that tDCS application contributed to improve the signal-to-noise ratio for grating detection in somatosensory cortices, as previously reported for the effects of dopaminergic agents on motor performance (Floel et al., 2005
). Resolution of this issue would require additional experiments.
An alternative explanation that could explain the described behavioral gains is that tDCS affected directly attentional processes. It could be argued that the location of the cathode electrode over the contralateral forehead could have influenced attentional processing, contributing to the described behavioral gains, since attention is able to modify cortical finger representations within S1 (Buchner et al., 1999
). This is however unlikely for different reasons. First, previous investigations using a frontal cathode electrode failed to detect attentional effects (Gandiga et al., 2006
). Second, consistent with these results, reports of attention before and after tDCSanodal
in our study were comparable to sham. Third, changes in attention, if present, should have impacted to relatively similar extents tactile performance in both hands, and not only the hand contralateral to the stimulated S1, as we found. Finally, it should be kept in mind that, given the size of electrodes, the areas polarized might have expanded beyond the targeted S1.
In summary, these results provide novel evidence that tDCSanodal delivered over S1 results in improvement in tactile spatial acuity in the contralateral hand, an effect of potential application in future studies geared to improve somatosensory function in patients with diminished tactile perception.