Cortical control of whole-arm motor tasks

Making an arbitrary motor response to a sensory signal would appear to require at least two sequential steps--planning the appropriate response and generating a motor command to implement it. However, neuronal correlates of these two putative steps do not occur in strict serial order, nor are they subserved by separate cortical regions. Instead, they are distributed in a continuous, overlapping and non-uniform manner across the cerebral cortex, including primary motor, premotor and parietal regions. These processes take the form of temporal and spatial gradients of cell activity that are distributed within and across cortical regions. Instead of two serial steps, these neuronal events may be better described in terms of two parallel functions--action specification and action selection. These processes occur continuously, both before and during movement. Recent studies show that the activity of single cells in the caudal part of the primary motor cortex is strongly modulated by arm geometry and by task dynamics during whole-arm isometric and reaching tasks. This indicates that these cells contribute to the transformation between neural representations of the global attributes of motor actions and of the mechanical details of their implementation.