The laminar pattern of proprioceptive activation in human primary motor cortex

The primary motor cortex (M1) has traditionally been viewed as a motor output generator. However, its vital role in proprioceptive somatosensation is increasingly being recognized. Yet, our understanding of proprioceptive somatosensation in M1 at the laminar scale is limited, largely due to methodological challenges. Empirical findings in primates and rodents suggest a pronounced role of superficial cortical layers, but the involvement of deep layers has yet to be examined in humans. Submillimeter resolution fMRI has emerged in recent years, paving the way for the study of layer-dependent activity in humans (laminar fMRI). In the present study, laminar fMRI was employed to investigate the laminar pattern of proprioceptive somatosensation on M1 deep layer activation using passive finger movements as proprioceptive stimulation. Significant M1 deep layer activation was observed in response to proprioceptive stimulation across 10 healthy subjects using vascular space occupancy (VASO) at 7T. For further validation, two additional subjects were scanned using a balanced steady-state free precession (bSSFP) sequence with ultrahigh (0.3 mm) in-plane resolution, yielding converging results. These results were interpreted in the light of previous laminar fMRI studies and the active inference account of motor control. We suggest that a considerable proportion of M1 deep layer activation is due to proprioceptive influence and that deep layers of M1 constitute a key component in proprioceptive circuits.