Is it possible to form and execute motor intentions without being aware of when those intentions were formed? Precisely this pattern was observed
by among (ha!) patients with parietal damage, as reported by Sirigu et al. They showed that patients with parietal damage are specifically impaired at estimating the time they formed the intention to commit a voluntary action, although they are unimpaired on other visual and temporal aspects to the task relative to healthy controls and to patients with cerebellar damage.
The authors argue that intentions formed in prefrontal cortex may be used by parietal areas to create a “foward model” of the anticipated action – matching the consequence of an action against the intention.
Sirigu et al. examined healthy subjects, parietal patients, and cerebellar patients in a simple paraigm: subjects observed a second hand spin around a clock, and were asked to press a button at any point after the first full revolution of the second hand. The paradigm had two conditions: in the “moved” condition, subjects were required to report the location of the second-hand at the time they initiated the movement, and in the “willed” condition, they were required to report the location of the second-hand at the time they formed an intention to move.
Previously this task has shown that intentions to move are estimated to occur some 200-300 miliseconds prior to the estimates of the movement. Estimates of the onset of movement are also quite accurate: they’re not significantly different from the time movements are actually made.
Whereas all subjects (healthy controls, and those with cerebellar or parietal damage) were able to estimate the timing of their movements very reliably, only the parietal patients estimated their movement intentions to have occurred at the moment the movement was made. A possible conclusion is that parietal damage leaves the intentions themselves intact (after all, subjects shouldn’t have responded at all, if they were unable to form intentions), but specifically impairs the maintenance of this intention over time, as indicated by a failure to reliably estimate its onset, and by other work indicating an important role for parietal cortex in motor intentions.
Perhaps parietal cortex is important for these maintenance and anticipatory aspects of movement- and intention-related processing. This clearly squares with the idea that parietal cortex is specialized for maintaining information over time, like the relationships between stimuli and responses and of occluded objects.
However, there are other studies indicating that the presupplementary motor area and the dorsolateral prefrontal cortex may be more involved in the intention-related aspect of motor movement. As discussed previously, Lau et al suggest that activity in the presupplementary motor area represents intentions, as shown by larger functional connectivity between prefrontal and pre-SMA relative to parietal and pre-SMA, but we might expect that simply based on the fact that pre-SMA and dlPFC are closer in cortex. Other work from the same group has shown that TMS to the preSMA distorted movement intention perception backwards in time, and shifted the perceived onset of movement forwards in time. This result seems to show a nonspecific role for preSMA in reporting motor-related activity (an unsurprising finding). Unfortunately, Lau & colleagues did not try TMS over the posterior parietal cortex, which according to the Sirigu et al theory should have selectively shifted the estimation of intentions but not perceptions of movement onset.