Fiehler, K. | Search bioRxiv

bioRxiv2019

HD-tDCS over mIPS causally modulates online reach correction

Brain lesion and stimulation studies have suggested posterior parietal cortex and the medial intraparietal sulcus in particular as a crucial hub for online movement error corrections. However, causal evidence for this is still sparse. Indeed, lesion studies are potentially confounded by compensatory reorganization mechanisms while brain stimulation studies have produced heterogeneous results when employing transcranial magnetic stimulation. Here we designed a new complementary paradigm using fMRI-guided high-definition transcranial direct current stimulation (HD-tDCS) of the left medial intraparietal sulcus (mIPS) together with regression-based mediation analysis to re-examine the causal role of mIPS in online reach corrections to jumping targets. We obtained two independent measures of stimulation-induced changes in brain activity by modeling current flow in the brain and through EEG recordings before and after HD-tDCS stimulation. Third, to quantify behavioral effects of HD-tDCS we computed movement curvature as a measure of online correction. We demonstrate that both of our measurements of brain activity were consistent with a polarity-specific modulation of the online correction for targets jumping to the contralateral side of the stimulation. Importantly, using a mediation analysis of the relationship between stimulation current and movement curvature suggests that the induced current modifies brain activity, which then leads to the observed behavioral changes. This unique combination of methods and analysis thus provides complementary evidence for the crucial role of the posterior parietal cortex in online error correction, while at the same time setting a new methodological standard with respect to the causal influence of transcranial direct current stimulation. New & NoteworthyTranscranial direct current stimulation (tDCS) is an interesting and potentially useful tool for asking causal scientific questions and design clinical treatments. With our unique combination of highly accurate fMRI guided stimulation, current forward modeling, EEG recordings before and after the stimulation and behavioral changes we could unravel the causal structure of tDCS. Our approach naturally deals with the variability of tDCS results, increasing its potential usefulness as a tool for research and clinical applications.

neuroscience↗

bioRxiv2019

Reaching around obstacles accounts for uncertainty in coordinate transformations

When reaching to a visual target, humans need to transform the spatial target representation into the coordinate system of their moving arm. It has been shown that increased uncertainty in such coordinate transformations, for instance when the head is rolled toward one shoulder, leads to higher movement variability and influence movement decisions. However, it is unknown whether the brain incorporates such added variability in planning and executing movements. We designed an obstacle avoidance task in which participants had to reach with or without visual feedback of the hand to a visual target while avoiding collisions with an obstacle. We varied coordinate transformation uncertainty by varying head roll (straight, 30{degrees} clockwise and 30{degrees} counterclockwise). In agreement with previous studies, we observed that the reaching variability increased when the head was tilted. Indeed, head roll did not influence the number of collisions during reaching compared to the head straight condition, but it did systematically change the obstacle avoidance behavior. Participants changed the preferred direction of passing the obstacle and increased the safety margins indicated by stronger movement curvature. These results suggest that the brain takes the added movement variability during head roll into account and compensates for it by adjusting the reaching trajectories.

neuroscience↗