Transcranial direct current stimulation (tDCS) of left parietal cortex facilitates gesture processing in healthy subjects

Abstract

Gesture processing deficits constitute a key symptom of apraxia, a disorder of motor cognition frequently observed after left-hemispheric stroke. The clinical relevance of apraxia stands in stark contrast to the paucity of therapeutic options available. Transcranial direct current stimulation (tDCS) is a promising tool for modulating disturbed network function after stroke. Here, we investigate the effect of parietal tDCS on gesture processing in healthy human subjects. Neuropsychological and imaging studies suggest that the imitation and matching of hand gestures involve the left inferior parietal lobe (IPL). Using neuronavigation based on cytoarchitectonically defined anatomical probability maps, tDCS was applied over left IPL-areas PF, PFm, or PG in healthy participants (n = 26). Before and after tDCS, subjects performed a gesture matching task and a person discrimination task for control. Changes in error rates and reaction times were analyzed for the effects of anodal and cathodal tDCS (compared with sham tDCS). Matching of hand gestures was specifically facilitated by anodal tDCS applied over the cytoarchitectonically defined IPL-area PFm, whereas tDCS over IPL-areas PF and PG did not elucidate significant effects. Taking into account tDCS electrode size and the central position of area PFm within IPL, it can be assumed that the observed effect is rather the result of a combined stimulation of the supramarginal and angular gyrus than an isolated PFm stimulation. Our data confirm the pivotal role of the left IPL in gesture processing. Furthermore, anatomically guided tDCS of the left IPL may constitute a promising approach to neurorehabilitation of apraxic patients with gesture processing deficits.

Figure 1.

A, Stimulus examples. Four example stimuli (each consisting of a pair of photographs) for the gesture matching and person discrimination tasks are shown. The photos were taken from three different perspectives to increase task difficulty. Small color-coded letters (red “G,” gesture matching task; green “P,” person discrimination task), displayed between the stimuli, aided participants to adhere to the current task, which had also been presented at the beginning of each block of trails (see Materials and Methods). The four stimulus examples depict the four possible combinations of the same or different gestures/ persons: left upper stimulus example, same gesture and same person; right upper stimulus example, same gesture, but different persons; left lower stimulus example, different gestures, but same person; right lower stimulus example, different gestures and persons. B, Study design. tDCS was applied on three different days. tDCS sessions were separated by at least 48 h. The type of tDCS (sham, anodal, and cathodal) applied in a given session and the order of the six stimulus sets before and after tDCS were pseudo-randomized. Participants practiced the two tasks at the beginning of each session on 64 stimuli (practice set), which were not part of the experimental stimulus sets.

Figure 2.

Anatomical location of the individual tDCS sites. All individual stimulation sites for tDCS (n = 26) are displayed on surface renderings of the MNI standard brain (A) and the individual subjects' brains (B–D). A, The individual location of the tDCS electrode's center normalized to MNI standard coordinates and projected onto the surface rendering of the MNI standard template brain are shown for all 26 subjects. The color coding indicates the cytoarchitectonically defined area to which the individual stimulation sites could be assigned using the MPM provided by the Anatomy toolbox in SPM: blue represents PF; red represents PFm; yellow represents PG. In addition, for three selected central stimulation sites within the areas PF, PFm, and PG, a square is depicted that represents the extent of the stimulation electrode used in the current study projected on the surface rendering of the standard brain with the help of the software package Brainsight (Rogue Research). B–D, The location of the individual tDCS electrode's center projected onto the surface rendering of each individual, non-normalized brain is depicted for the subjects whose stimulation site was located in the cytoarchitectonically defined area PF (B, blue, left panel, n = 8), area PFm (C, red, middle panel, n = 8), and area PG (D, yellow, right panel, n = 10).

Figure 3.

Changes in task performance resulting from tDCS. Changes in task performance induced by tDCS as indexed by changes of RTs for the person discrimination task (dark gray) and the gesture matching task (light gray). Depicted are the changes of RTs when RTs after tDCS are compared with RTs before tDCS for anodal tDCS, sham tDCS, and cathodal tDCS applied over the three cytoarchitectonically defined areas PF (blue, left part), PFm (red, middle part), and PG (yellow, right part) of the left IPL. Positive values indicate an improvement of task performance resulting from anodal tDCS, sham tDCS, or cathodal tDCS, whereas negative values would indicate a worsening of task performance resulting from tDCS. Further exploration of the significant three-way interaction between TASK, STIMULATION, and SITE by three two-way ANOVAs for each of the three stimulus sites separately (PF, PFm, and PG), and planned paired t tests revealed a differential improvement of the performance in the gesture matching task (compared with the person discrimination task) for anodal tDCS only, but not for sham tDCS or cathodal tDCS (highlighted by the asterisk). Error bars indicate the SEM.