Studying how people form a conscious intention to move is troublesome for at least two reasons. First, as soon as you instruct a participant that now is the time for them to move freely, of their own volition, you’ve already undermined the idea that they’re making up their own minds. Second, there’s no room in materialist science for a conscious will, separate from the electro-chemical workings of brain. However, according to human movement expert Patrick Haggard, these two obstacles are partly overcome by a new study that follows in the steps of celebrated neurosurgeon Wilder Penfield – directly stimulating patients’ brains with electrodes and observing the effects on behaviour and sensation.
Michel Desmurget and colleagues used this approach with seven patients undergoing neurosurgery for the removal of brain tumours (see images and further related info). Stimulation of the premotor cortex at the front of the brains of four of these patients led them to perform limb and mouth movements that they were unaware of. By contrast, stimulation of the parietal cortex at the rear of the brains of three of the patients led them to experience a powerful desire to move. Even higher power stimulation in this region provoked an erroneous belief that they had in fact moved when really they hadn’t.
This new research is the first to link the parietal cortex directly with the feeling of a desire to move. Whereas frontal brain regions are associated with actual movement execution, the parietal cortex is known to be involved in predicting the sensory consequences of our own actions. This new research suggests this predictive activity may play a role in the feeling of having made a decision to move.
The story doesn’t end there. Past research has shown that stimulation of a frontal area – supplementary motor cortex – is also linked with an urge to move. This region is involved in actual motor command planning. So the complete picture, so far, appears to be that the feeling of a desire to move arises from frontal areas associated with movement execution and from parietal areas associated with predicting the sensory consequences of moving. “Just how the frontal, motor aspect of this experience differs from the parietal, sensory aspect, is the next question,” said Haggard in his comment piece.
M Desmurget, K Reilly, N Richard, A Szathmari, C Mottolese, & A Sirigu (2009). Movement intention after parietal cortex stimulation in humans. Science, 324, 811-813.