A new brain correlate of intention.

We have another installment in the story of how our conscious intention to move involves brain processes parallel to those that actually command the movements. It has been known for some time that stimulation of the presupplementary motor area in clinical exploration before brain surgery can produce an 'urge to move.' Desmurget et al. now find that stimulating areas in the inferior posterior parietal cortex (usually presumed to be involved in sensory motor coordination, not volition) can cause an urge to move specific body parts. When stimulation intensity was increased in parietal areas, participants believed they had really performed these movements, although no electromyographic activity was detected. Stimulation of the premotor region triggered overt mouth and contralateral limb movements. Yet, patients firmly denied that they had moved. Conscious intention and motor awareness thus arise from increased parietal activity before movement execution. Here is a figure from the review of this work by Haggard.

Voluntary action. (Top) The premotor cortex prepares commands for voluntary actions triggered by external stimuli, whereas the presupplementary motor area prepares commands for internally generated "intentional" actions, which are then executed by the primary motor cortex. Signals containing copies of prepared motor commands are also sent to the parietal cortex, where they are used to predict sensory consequences of movement. (Bottom) The preparation of motor commands for voluntary movement by the presupplementary motor area causes a sense of urge. The inferior part of the posterior parietal cortex generates sensory representations of the predicted consequences of the movement.