Decoding Graded Grip-Force Intensity From fMRI Data Reveals a Transformation From Abstract to Effector- and Movement-Specific Codes Prior to Execution.

Motor planning entails a progressive transformation of neural representations - from abstract motor goals, which represent intended action-outcomes independent of any particular effector (i.e., the body part executing the action), to effector-specific movement plans. Functional MRI (fMRI) studies have shown that parametric variations in parietal activity patterns reflect the encoding of intended force-intensities in effector-specific regions, even before detailed movement parameters are specified. However, how these intended force-intensities are initially represented in an abstract, effector-independent format and subsequently transformed into effector- and movement-specific plans remains unclear. To address this, human-participants performed a delayed grip-force task during fMRI. They first prepared two of four possible force-intensities, then received a cue indicating which hand should apply which force, and finally executed both grips simultaneously. Using time-resolved support vector regression (SVR) combined with a searchlight approach, we identified brain-regions that parametrically code grip-force intensities across two 6-s delay periods. During the first delay, above-chance decoding was observed in a medial section of the posterior parietal cortex spanning the precuneus and superior parietal lobule (PCu/SPL), whereas during the second delay it emerged in effector-specific regions, including the contralateral intraparietal sulcus (pIPS/aIPS), primary somatosensory cortex (S1), dorsal premotor cortex (PMd), and supplementary motor area (SMA). Cross-decoding confirmed effector-independent coding in the PCu/SPL, while cross-temporal generalization revealed stable representations in the contralateral IPS and PMd from the second delay through execution. Together, these findings indicate a progressive transformation from abstract representations of intended force-intensity in the PCu/SPL to effector- and movement-specific plans in the IPS and PMd.