Scientific Understanding of Consciousness
Neural Origin of Willed Actions
Science 8 May 2009: Vol. 324. no. 5928, pp. 811 - 813
Movement Intention After Parietal Cortex Stimulation in Humans
Michel Desmurget,1,2 Karen T. Reilly,1,2 Nathalie Richard,1,2 Alexandru Szathmari,3 Carmine Mottolese,3 Angela Sirigu1,2,
1 Centre de Neuroscience Cognitive, CNRS, UMR 5229, 69500 Bron, France.
Parietal and premotor cortex regions are serious contenders for bringing motor intentions and motor responses into awareness. We used electrical stimulation in seven patients undergoing awake brain surgery. Stimulating the right inferior parietal regions triggered a strong intention and desire to move the contralateral hand, arm, or foot, whereas stimulating the left inferior parietal region provoked the intention to move the lips and to talk. 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.
A central question in the study of human behavior concerns the origin of willed actions. Where in the brain are intentions formed? How do we become aware of these intentions?
We used direct electrical stimulation (DES) in seven individuals with brain tumors located anteriorly or posteriorly to the central sulcus. Patients were operated under local anesthesia by using DES as a functional mapping technique. DES was delivered with a bipolar electrode using standard increasing intensities (2, 5, and 8 mA) and durations (1, 2, and 4 s). Up to four replications were performed for each stimulation site. Replications were delivered nonconsecutively to avoid provoking seizures. Throughout the experiment, electromyographic (EMG) signals were collected in the contralesional hemibody in 12 muscles covering the face, hand, wrist, elbow, knee, and foot. Stimulation sites were localized with high resolution on individual magnetic resonance (MR) images by using a peri-operative neuronavigation system and reconstructed offline.
Fifty-seven sites were stimulated in the frontal, parietal, and temporal regions. Posterior parietal stimulations were performed in Brodmann areas (BAs) 7, 39, and 40. Premotor stimulations were performed in the dorsal sector of BA 6, excluding the convexity and mesial structures involving the supplementary motor area (SMA).
We report two main contrasting findings: (i) Stimulation of the posterior parietal cortex caused human participants to intend to move and to report having moved, even in the absence of actual motor responses. (ii) Stimulation of the premotor cortex triggered limb and mouth movements that were not consciously detected by the patients.
It is possible that both the parietal cortex and the SMA are linked to motor intentions but that intentions processed in these two regions correspond to different stages of movement planning: Intentions in the parietal lobe may be processed in relation to sensory predictions, whereas in the SMA intentions may be more closely related to motor commands.
Our study suggests that motor intention and awareness are emerging consequences of increased parietal activity before movement execution. The subjective (and potentially illusory) feeling that we are executing a movement does not arise from movement itself, but it is generated by prior conscious intention and its predicted consequences.
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