Following the surgical removal of a body part, amputees often report sensations which seem to originate from the missing limb. This is thought to occur because the brain’s model of the body (referred to as the body image) still contains a representation of the limb, and this leads to the experience that the missing limb is still attached to their body. Occasionally, amputees say that they cannot move their phantom limbs – they are perceived to be frozen in space, apparently because they cannot be seen.
Yet, research shows that the body image is malleable and easily manipulated. And according to a new paper published in the Proceedings of the National Academy of Sciences, phantom limbs can be altered by internal brain mechanisms alone. The study shows that some amputees can make their phantom limbs defy the anatomical constraints of the physical body, using visual imagery to make them perform movements which could not possibly be performed by a real limb.
Lorimer Moseley and Peter Brugger recruited seven arm amputees, all of whom had experienced vivid and enduring phantom limb sensations, and encouraged them to use visual imagery to contort their phantoms into configurations which would be normally be impossible because of the biomechanical constraints of the body. Initially, all the participants reported that it felt as if they were observing somebody else’s arm performing the movements, or that the phantom they visualized did not feel like their own.
But with training, four of them reported that they had successfully learned how to perform the impossible movement, by visualizing their phantom wrist spinning the wrong way. After learning, these participants also reported feeling a change in the structure of their phantom limbs, involving the development of a new phantom wrist joint which could rotate freely on the forearm, which permitted the limb to perform the impossible movement. One also reported that his phantom now included a new axis which protruded into the hand, and drew a picture to illustrate its structure (below left).
These subjective reports were corroborated by the participants’ performance on two motor imagery tasks which provided an objective and quantifiable measure of implicit motor behaviour. In the first of these tasks, the participants were shown a series of images of human hands and asked to judge, as quickly as possible, whether each was of a left or a right hand. Included in the task were pictures of left and right hands in two different postures at either end of the impossible movement which had been practiced earlier, and some of the trials required the participants to hold their phantom hand in one of these postures.
Correct judgement on this task is known to require mental rotation, whereby one imagines rotating one’s hand until it reaches the position depicted in the image. The response time is related to the extent of the mental manoeuvres required to imagine one’s own hand rotating from its resting position to one which matches the hand in the picture. For healthy participants, reaction times on the task are shorter when the posture of the real hand matches that in the image. Earlier studies have also shown that amputees take longer than non-amputees to correctly identify an image corresponding to their missing limb.
At the outset of the experiment, the performance of all the participants was consistent with these earlier findings. Their reaction times were shorter when their hand posture matched that depicted in the presented image, and reaction times for pictures corresponding to the missing limb were longer than for those of the intact limb. After training, however, some differences were observed in the four participants who had reported successful learning of the impossible movement. In these participants, but not in the other three, reaction times for pictures of the missing limb decreased, and were the same as those for pictures of the unaffected limb. Their performance also improved on trials in which the position of their phantom limb was opposite to that of the hand shown in the image.
In the second task, the participants were shown two alternating images, depicting a hand at either end of the impossible movement, and asked to report the apparent path of motion required to move their phantom hand between one posture and the other. This involves either a long rotation of the wrist or a shorter, but anatomically impossible, rotation in the opposite direction (above, middle and right). Normally, non-amputees perceive the hand moving through the longer motion path when the interval between the two flashed images is equal to, or slower than, the time taken to actually move their hand in that way (0.7 – 1 seconds). The shorter path is only reported when the interval is outside this range.
Again, the participants initally reported perceiving the long rotational movement, but only when the alternating images were separated by intervals of greater than 0.7 seconds. After training, however, the four participants who claimed to have learnt the new movement successfully perceived the shorter motion path regardless of the time interval between the alternating hand images. In all four participants, this was found to be true only for images which corresponded to the amputated limb. In trials involving images of the unaffacted hand, manipulations of the time interval caused no change in performance. And there was no change the responses of the three participants who were unable to learn the new movement.
These results provide good evidence that the four successful participants had modified the neural representation of their phantom limbs, to incorporate a new joint which enabled the phantoms to perform impossible movements of which they were previously incapable. Furthermore, they had also generated novel motor commands with which the movement can be executed. Remarkably, the newly configured phantom limbs were apparently still constrained by the laws of physics. Two of the successful participants reported that the new wrist joint had made it more difficult for them to move their phantom hands from side to side. It seems that the brain’s representations of body parts are constrained by the relationship between structure and function, even when the representations are not based on a physical substrate.
Although the body image has a genetic basis, it is also thought to be continuously updated by sensory feedback from the body. Manipulating the information which flows into the brain through the senses can profoundly alter the way we perceive our bodies. For example, conflict between visual and tactile information can give rise to out-of-body experiences, or to the illusion that the sensations one feels are originating from someone else’s body and not one’s own. But as this study shows, the body image can also be altered by internally generated mechanisms, in the complete absence of feedback from the body. This suggests that visual imagery could be used to re-adjust the body image distortions that occur in a variety of neurological and psychiatric conditions.
- The body swap illusion
- Voluntary amputation and extra phantom limbs
- Distorting the body image affects perception of pain
- Phantom feelings exorcized by changes in body position
Moseley, G. L. & Brugger, P. (2009). Interdependence of movement and anatomy persists when amputees learn a physiologically impossible movement of their phantom limb. Proc. Nat. Acad. Sci. DOI: 10.1073/pnas.0907151106.