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Zusammenfassung:
Involvement of the left posterior parietal cortex (lPPC) in online motor control has been demonstrated in recent years using fMRI (Culham et al, 2006). However, the human homologue to macaque parietal reach region, or even more detailed functional anatomy of processes involved in motor control, is still controversial (lacoboni, 2006). The main challenge is the spatial co-localization of functions that are also involved in motor execution, e.g. attention, saccades, and motor planning (Astafiev et al, 2003). TMS offers the possibility to disentangle these functions due to its high temporal resolution, and can also discriminate necessary from co-activated brain areas. Desmurget et al (1999) showed that online correction for reaching to an altered target can be disturbed using TMS over the lPPC. In this study, we developed an fMRI localizer to assess lPPC sub-regions that are involved in online motor control. Subsequently, we tested these sites with event-related TMS. Using closed-loop reaching (with visual hand feedback) allows investigating the processes involved in body's effectors representation in addition to environment representation during goal-directed reaching, by introducing different visual perturbations.
The fMRI localizer consists of blocks for fixation, saccades, and reaching with an MR-compatible joystick. Within the reaching blocks, different visual perturbations (including none) were randomized in a fast event-related design. The different perturbation conditions were contrasted against the unperturbed reaching to assess online-correction activation. Nine right-handed subjects were tested. On group level, different visual perturbations resulted in spatial different activation patterns in the lPPC. In addition, we observed pronounced inter-individual differences in activation. Maxima from the group analyses and the individuals own maxima were used as stimulation sites for the subsequent TMS study.
Four of the subjects were tested so far using event-related TMS on target alteration. Despite huge inter-individual differences in BOLD activation, we could demonstrate a closer match of TMS effect localization with subject's individual activation than with group activation (fig. 1).
This finding shows that TMS is capable of investigating sub-regions of the lPPC. Furthermore, it stresses the importance of individual analyses when investigating functions located there. The next step is to map the other visual perturbations.
