Supplementary motor area and anterior intraparietal area integrate fine-graded timing and force control during precision grip

We investigated the neuronal processing of the physiologically particularly important precision grip (opposition of index finger and thumb) by the combination of functional magnetic resonance imaging (fMRI) and an MR-compatible haptic interface. Ten healthy subjects performed isometric precision grip force generation with visual task instruction and real-time visual feedback in a block design. In a 2 x 2 two-factorial design, both the timing and force could be either constant or varying (identical average timing and force). As we expected only small changes in the fMRI response for the different fine-graded motor control conditions, we maximized the sensitivity of the data analysis and implemented a volumes of interest (VOI) restricted general linear model analysis including non-explanatory force regressors to eliminate directly force-related low-level activations. The VOIs were defined based on previous studies. We found significant associations: timing variation (variable vs. constant) and primary motor area (M1) and dorsal premotor area (PMd); force variation (variable vs. constant) and primary somatosensory area (S1), anterior intraparietal area (AIP) and PMd; interaction of timing and force and supplementary motor area (SMA) and AIP. We conclude that SMA and AIP integrate fine-graded higher-level timing and force control during precision grip. M1, S1 and PMd process lower-level timing and force control, yet not their integration. These results are the basis for a detailed assessment of manual motor control in a variety of motor diseases. The detailed behavioural assessment by our MR-compatible haptic interface is particularly valuable in patients due to expected larger inter-individual variation in motor performance.