iTBS over the left hV6A enhances PPC-PPC functional connectivity during reaching tasks: an EEG study

Abstract

Introduction: The functional connectivity of the posterior parietal cortex-primary motor cortex (PPC-M1) is involved in goal-directed reaching actions and integrating visuomotor transformation. Human area V6A (hV6A), located in the medial PPC, is a critical node of the dorsomedial system that is involved in targeting during reaching movements. Here, we used Electroencephalography (EEG) to investigate functional connectivity and network efficiency during right-hand reaching tasks after inducing left hV6A activity with intermittent theta burst stimulation (iTBS).

Methods: Based on individualized MRI neural navigation, 23 healthy subjects were randomly accepted into either real left hV6A or sham iTBS on 2 days. Resting-state and goal-directed reaching task EEG were recorded at baseline and immediately after iTBS to assess the effects of iTBS on functional connectivity. Alongside the reaching task, an additional Stroop test was conducted to assess each participant's degree of attention.

Results: In the alpha band, medial posterior parietal cortical interhemispheric functional connectivity significantly increased during right-hand reaching tasks after hV6A iTBS (p = 0.008) but not after sham iTBS (p = 0.726). Alpha and beta bands small-worldness of right-hand reaching tasks significantly increased (p = 0.001 and 0.013, respectively) but not after sham iTBS (p = 0.915 and 0.511, respectively).

Discussions: Functional connectivity of the bilateral PPC and functional network efficiency increased after iTBS of the left hV6A during right-hand reaching tasks. These findings indicate that the left hV6A should be a potential target for iTBS modulation to improve the orienting movement function in space.

Keywords: V6A; functional neuroimaging; posterior parietal cortex; reaching; transcranial magnetic stimulation.

Figure 1

Experimental design and procedure of the goal-directed reaching tasks. The participants were randomly assigned to two iTBS protocols (blue for sham iTBS, red for real iTBS of the left hV6A) on different days. (A) Four target keyboards (black) were placed on a table around a center fixation (a black sticker) and a HOME keyboard (white). (B) After hearing the ‘direction’ from the voice prompt, the projectile’s range (straight line without stopping in the middle) to the corresponding target keyboard was arranged. The black keyboards from left to right represent ‘1’,’2’,’3’,and ‘4’. The keyboard was pressed and held until the ‘reset’ prompt indicated a return to the HOME keyboard, which ended this trial. (C) The resting-state EEG, reaching-task EEG, and Stroop test were successively recorded before and after modulation.

Figure 2

The location of the hV6A in the neuronavigational system. The red coil against the target left hV6A, which was located in the medial parieto-occipital cortex.

Figure 3

Alpha and beta bands functional connectivity measured at rest. (A–C) The alpha band LPPC-LM1, RPPC-LM1, and LPPC-RPPC functional connectivity, respectively. (D–F) The three types of beta band functional connectivity. The alpha and beta bands functional connectivity of LPPC-RPPC significantly increased after iTBS without stimulation protocol difference. * indicates a significant difference (p < 0.05). The blue line indicates sham iTBS and the red line indicates real iTBS over the left hV6A.

Figure 4

Alpha band functional connectivity measured during right-hand reaching tasks. (A) The functional connectivity situation of LPPC-LM1, RPPC-LM1, and LPPC-RPPC before real iTBS. (B) The same functional connectivity situation after real iTBS. The functional connectivity of LPPC-RPPC (C) increased after real iTBS over the left hV6A compared to the sham modulation (D). * indicates a significant difference (p < 0.05).

Figure 5

Small-worldness of right-hand reaching tasks. ‘sigma’ stands for small-worldness of alpha (A) and beta (D) bands. The network efficiency of these bands increased after real iTBS over the left hV6A compared to the sham. The area under the curve (AUC) represents the situation of alpha (B,C) and beta (E,F) bands sigma at different sparsity thresholds in the range 0.1–0.4. * indicates a significant difference (p < 0.05).