Motor programs stored in memory may have an impact on preserved abilities to use motor imagery even though paraplegic patients are no longer able to execute the same movement. To further investigate the role of memory, Nico and colleagues (2004) compared performance of upper limb amputees, patients with congenital deletion of the forearm, and patients with a non-functioning limb due to unilateral lesions of the brachial plexus. The patients saw line drawings of a right or left human hand in either a natural or unnatural posture, and they had to judge whether it is a right or a left hand.
Interestingly, some amputees reported a re-activation of extinguished phantom limb sensations while performing this task, suggesting that motor commands to the missing limb are elicited by the motor imagery required to make the laterality judgment. Dominant limb amputation resulted in much longer response times and the patient will use either a visual strategy (rotating the visual hand stimulus) or another motor strategy (rotation of the non-dominant hand). Interestingly, patients with congenital deletion were not affected when they viewed unnatural joint positions. They never had any experience with actual movements, and thus no prior memories could possibly influence mental imagery of movements. Prior experience is therefore crucial for motor imagery even though there is also scarce evidence showing that - to some extent - new movements can be trained via mental imagery (Mulder et al., 2004). In this context it is also of interest that we are currently studying the effects of motor imagery in stroke patients; preliminary results yield promising evidence for improved motor performance as a result of repeated motor imagery training (Grabherr et al., 2006). In the sample we studied, the gain in the imagery training group exceeded the gain in the execution training group, and this again suggests that neuroplastic changes are specific to the form of training (Nyberg et al., 2006).
Plenty of evidence suggests that motor areas associated with movement control and movement execution overlap with those used when we imagine the same movements. Interestingly, however, such motor activation is not restricted to mental imagery of movements. In the following section, we present findings showing that motor activations occur during imagery tasks without any obvious motor involvement.
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