Consciousness and Intention: Where in the brain are our intentions formed and how do we become aware of these intentions? Desmurget et al. (p. 811 ; see the Perspective by Haggard ) investigated the effect of direct cortical stimulation of parietal and premotor regions in patients undergoing brain surgery for tumor removal. Stimulation of the parietal lobe provoked the conscious experience of wanting to move the upper limb, lips, or tongue without any concomitant motor activity. When stimulation intensity was increased, patients believed that they had actually moved or talked, but again no muscle activity was detected. When, however, the premotor region of the frontal lobes was stimulated, real complex multijoint movements were induced. However, patients did not experience these movements as produced by a conscious internal act of will. Indeed, they were not even aware that they had moved. Increasing stimulation intensity increased the amplitude or complexity of the movement but never made it reach consciousness.

The loss of a limb leads to sensorimotor modifications that are frequently accompanied by the vivid experience that the missing limb is still present, and that it can be moved at will. Furthermore, amputees can clearly distinguish between phantom movements of the fingers and of more proximal joints, like movements of the elbow. This phenomenon raises the question of whether these specific phantom movement experiences are translated into differentiated activity within the remaining muscles. We recorded stump muscle activity when above-elbow amputees voluntarily moved their phantom limb. Voluntary movements of the phantom hand triggered specific patterns of stump muscle activity, which differed from activity recorded in the same muscle groups during movements of the proximal limb. This result indicates that the brain's motor areas can be differentially activated according to the phantom movement the patient intends to perform, and suggests that hand motor commands are preserved after amputation. To further understand the interaction between central commands and sensory feedback in the perception of phantom movement we also measured stump muscle EMG activity in an amputee experiencing a frozen phantom limb, and in three below-elbow amputees with vivid phantom movements after inducing an ischaemic block. Failed attempts to move the paralysed phantom limb always resulted in the same EMG pattern, no matter what type of phantom movement was attempted, while ischaemic nerve block reduced or eliminated the ability to voluntarily move the phantom limb and produced a dramatic reduction in the amplitude of stump muscle EMG activity. Our data suggest that the experience of phantom hand movement involves the activation of hand motor commands. We propose that preserved hand movement representations re-target the stump muscles to express themselves and that when these representations are voluntarily accessible they can instruct the remaining muscles to move in such a way as if the limb is still there.

Limb amputation results in plasticity of connections between the brain and muscles, with the cortical motor representation of the missing limb seemingly shrinking, to the presumed benefit of remaining body parts that have cortical representations adjacent to the now-missing limb. Surprisingly, the corresponding perceptual representation does not suffer a similar fate but instead persists as a phantom limb endowed with sensory and motor qualities. How can cortical reorganization after amputation be reconciled with the maintenance of a motor representation of the phantom limb in the brain? In an attempt to answer this question we explored the relationship between the cortical representation of the remaining arm muscles and that of phantom movements. Using transcranial magnetic stimulation (TMS) we systematically mapped phantom movement perceptions while simultaneously recording stump muscle activity in three above-elbow amputees. TMS elicited sensations of movement in the phantom hand when applied over the presumed hand area of the motor cortex. In one subject the amplitude of the perceived movement was positively correlated with the intensity of stimulation. Interestingly, phantom limb movements that the patient could not produce voluntarily were easily triggered by TMS, suggesting that the inability to voluntarily move the phantom is not equivalent to a loss of the corresponding movement representation. We suggest that hand movement representations survive in the reorganized motor area of amputees even when these cannot be directly accessed. The activation of these representations is probably necessary for the experience of phantom movement.

Amputees can move their phantom limb at will. These 'movements without movements' have generally been considered as motor imagery rather than motor execution, but amputees can in fact perform both executed and imagined movements with their phantom and they report distinct perceptions during each task. Behavioural evidence for this dual ability comes from the fact that executed movements are associated with stump muscle contractions whereas imagined movements are not, and that phantom executed movements are slower than intact hand executed movements whereas the speed of imagined movements is identical for both hands. Since neither execution nor imagination produces any visible movement, we hypothesized that the perceptual difference between these two motor tasks relies on the activation of distinct cerebral networks. Using functional magnetic resonance imaging and changes in functional connectivity (dynamic causal modelling), we examined the activity associated with imagined and executed movements of the intact and phantom hands of 14 upper-limb amputees. Distinct but partially overlapping cerebral networks were active during both executed and imagined phantom limb movements (both performed at the same speed). A region of interest analysis revealed a 'switch' between execution and imagination; during execution there was more activity in the primary somatosensory cortex, the primary motor cortex and the anterior lobe of the cerebellum, while during imagination there was more activity in the parietal and occipital lobes, and the posterior lobe of the cerebellum. In overlapping areas, task-related differences were detected in the location of activation peaks. The dynamic causal modelling analysis further confirmed the presence of a clear neurophysiological distinction between imagination and execution, as motor imagery and motor execution had opposite effects on the supplementary motor area-primary motor cortex network. This is the first imaging evidence that the neurophysiological network activated during phantom limb movements is similar to that of executed movements of intact limbs and differs from the phantom limb imagination network. The dual ability of amputees to execute and imagine movements of their phantom limb and the fact that these two tasks activate distinct cortical networks are important factors to consider when designing rehabilitation programmes for the treatment of phantom limb pain.