The Illusion of Conscious Will (8 page)

Waterman still feels a sense of effort when he moves (Cole 1986), and this suggests that the loss of muscle feedback that he experienced did not eliminate this indication of his feeling of doing. So, perhaps the experience of will can come from merely having messages sent from brain to muscles. It might also be that without muscle sense, the visual perception of own movement gives sufficient feedback so that a sense of effort can be achieved. This is not an entirely telling case, though, because Cole mentions that Waterman’s loss of muscle sense may not be total. There is evidence from certain cases of paralysis, though, that also supports the conclusion that the feeling of effort may be more dependent on efferent (brain to muscle) than afferent (muscle to brain) neurons. When a muscle becomes completely unmovable, the experience of effortfulness of its movement also goes away, even if the muscle’s position can be sensed.

This phenomenon was illustrated graphically in an account by Ernst Mach (1906) of his own experience of suffering a stroke:

I was in a railway train, when I suddenly observed, with no consciousness of any-thing else being wrong, that my right arm and leg were paralyzed; the paralysis was intermittent, so that from time to time I was able to move again in an apparently normal way. After some hours [as] it became continuous and permanent, there also set in an affection of the right facial muscle, which prevented me from speaking except in a low tone and with some difficulty. I can only describe my condition during the period of complete paralysis by saying that when I formed the intention of moving my limbs I felt no effort, but that it was absolutely impossible for me to bring my will to the point of executing the movement. On the other hand, during the phases of imperfect paralysis, and during the period of convalescence, my arm and leg seemed to me enormous burdens which I could only lift with the greatest effort. . . . The paralyzed limbs retained their sensibility completely . . . and thus I was enabled to be aware of their position and of their passive movements. (174-175)

The experience of muscle effort in this case, it seems, must depend on having some movement capacity. With reduced capacity, as in fatigue or weakness or partial paralysis, the feeling of effort increased, whereas with no capacity at all the feeling of effort dropped to zero. The odd fact in Mach’s case, and one that has been substantiated in other, more modern instances (Rode, Rossetti, and Boisson 1996), is that these variations in the experience of effort or heaviness can occur even when the patient has some afferent (muscle to brain) pathways intact (Gandevia 1982; 1987). Mach could feel where his paralyzed limbs were. In this sense, Mach was the opposite of Waterman, in that he lost efferent (brain to muscle) control while retaining afferent (muscle to brain) contact, and in Mach’s case, the feeling of effortful movement was eclipsed entirely when he could no longer move. But without efferent control, unfortunately, there was also nothing for Mach to do (just as in the case of people who can’t wiggle their ears), so it is not too surprising that no effort was experienced. This case, then, is also not entirely conclusive about the source of feelings of effort.

It is interesting to note, though, that Waterman’s ability to use his visual sense to substitute for muscle sense indicates that in normal people the two senses may often be combined to allow judgments of movement. Lajoie and colleagues (1992) tested another patient who, like Waterman, had lost muscle feedback and had learned to guide her movements visually. The researchers were able to test this patient on a mirror drawing task (“Please copy these letters by watching your hand in the mirror”) and found that she was notably better than neurologically normal subjects. The bizarre mirror drawings most people produce arise because they have trouble integrating the mixed signals they receive from sight and from muscle feedback. The two senses tell them different stories about where the pencil might be and they get mixed up as a result. With-out muscle feedback, and after learning to control action with vision alone, this patient was able to do something that baffles everyone else.

The studies of muscle sense seem to indicate that the feeling of effort that is part of the experience of conscious will may depend on outward signals from the brain to muscles and sometimes also on muscle sense, the returning signals from muscles to brain. It would be nice if we could sum things up at this point and head home for dinner and an evening of television, but we’re not done. There is a major additional mystery: The feeling of conscious will doesn’t always seem to go away when body parts go away.

Phantom Limbs

That’s right, people can sometimes feel they are willing movements that don’t even happen. Most people who have had an arm or leg amputated continue to sense the presence of the limb thereafter, what Mitchell (1872) called a “phantom limb.” Of some 300 amputees in prisoner-of-war camps during World War II studied by Henderson and Smyth (1948), for instance, fully 98 percent were found to experience a phantom limb, felt as a pleasant, tingling sensation that was not painful. Some people do experience such a limb as having pain, however, which makes it a particularly miserable burden.

Here’s the intriguing part. A phantom limb can often be perceived to move, either involuntarily (as when the stump of the limb is pushed by someone else) or voluntarily (as when the amputee tries to move it). The apparent voluntary movement is not merely a gross motion of the limb, either, because the person may very well feel separate parts moving and changing position in relation to each other. Fingers may be wiggled, elbows or knees bent, arms or legs twisted—all with nothing really there (Jones 1988). As a rule, the more distal parts of the limb (fingers, toes) are felt more strongly than the proximal parts (nearer the actual stump), apparently because the distal parts are represented more fully in the brain. Movement of the phantom limb becomes more difficult with time, and eventually the ability to “move” the digits may be lost even though the limb may still be perceived to exist. As the feeling of voluntary movement subsides over a period of months or years, the limb may “telescope” toward the body such that the last sensations the person may experience are only of the digits extending from the stump.

A fascinating feature of phantom limb movement is that, at least on first analysis, it suggests that the intention to move can create the experience of conscious will
without any action at all
. For a number of researchers working in the late nineteenth century, this feature of phantom limb movement was taken as evidence that messages from the brain to the muscles could be perceived by the brain before they even left the brain to go to the muscles (Helmholtz 1867; Mitchell 1872). After all, there were no muscles out there, only a phantom. Phantom limb movements always occur consciously and are not spontaneously made (Jones 1988), and this also seems to substantiate the idea that there is some consciousness of a signal being sent to the absent limb.

Further research has found, however, that the sense of moving a phantom limb voluntarily depends on the continued functioning of sensory nerves and muscles in the stump. Henderson and Smyth (1948) observed that every voluntary movement of a phantom limb was accompanied by a contraction of the appropriate muscles in the stump, and that if the remaining muscles in the stump had lost their nerve connections, the ability to move the phantom was lost. If the brain has nowhere to send the movement commands, in other words, it no longer senses that the movement is occurring. The continued feeling of the
existence
of the phantom is not dependent on this nerve/muscle connection—just the
sensation of voluntary movement.
This finding suggests that there may be a role for information returning from the muscles in producing the sensation of phantom voluntary movement (Devor 1997; Jones 1988).

It turns out, however, that information coming from simply
looking
at a moving limb can create the sense of voluntary movement. An early hint that this might be possible appeared in a remarkable study by Nielson (1963), in which people with normal limbs were fooled into thinking that someone else’s hand was their own. People in this study were asked to don a glove, insert their hand in a viewing box, and then on a signal, draw a line down a piece of paper. Unbeknownst to them, the hand they saw in the box was actually a mirror reflection of another person’s hand, also gloved and holding a pen, which appeared in just the spot where they would expect their own hand to be (
fig. 2.3
). When the signal was given and this imposter hand started to draw a line that departed from the line the participant had been instructed to draw, participants typically adjusted their (own) arm to compensate for the observed arm’s mistaken trajectory (
fig. 2.4
). The visual feedback from the false hand was so compelling that participants briefly lost contact with their own movement.

Figure 2.3

Nielson’s (1963) mirror box had a subject (S) look down at a mirror (M
₂
), ostensibly to see his or her hand underneath, while actually viewing the hand of an assistant (A). Courtesy Scandinavian Psychological Association.

In a series of studies, V. S. Ramachandran and colleagues (1996; Ramachandran and Blakeslee 1998; Ramachandran and Rogers-Ramachandran 1996) used a different sort of mirror box to examine the experience of phantom limb movement. In one study, an individual with a phantom arm inserted both the phantom and his real other arm into the mirror box. The mirror in this case was placed so that a reflection of the person’s real arm appeared in the place where the phantom would be if it were real. The odd result of this was that when the person moved the real arm voluntarily, he experienced the phantom as moving voluntarily as well. The hand in the mirror seemed to extend from the person’s stump and was felt as if it were being flexed and moved on purpose. The visible hand guided the experience of the phantom.

There is the possibility here that the person experienced the movement through some brain pathway that produces symmetries in movements between the two hands. To check on this, in two cases, Ramachandran’s group arranged for an experimenter’s arm to appear in place of the phantom.

Figure 2.4

Nielson’s (1963) subjects each tried to trace the vertical line while the assistant’s hand wandered off on the dotted path to the right. The points on this graph show the final stopping places of the subjects—most of whom seem to have tried to compensatefor theapparentwanderingbymovingtotheleft.Courtesy Scandinavian Psychological Association.

They found that “the visual cue was sufficiently compelling that it created a vivid feeling of joint movements in the phantom whether or not the patient moved the contralateral hand (and even though no commands were sent to the phantom). One of the patients noted, however, that the joint sensations were less vivid when the experimenter’s hand was used than when he himself moved his fingers” (Ramachandran et al. 1996, 36).
⁴
The point to remember, in sum, is that willful movement can be experienced merely by watching
any
body move where one’s own body ought to be. This is not too surprising in view of the discovery (in monkeys) of the existence of
mirror neurons
—neurons that are activated both by own movement and by the perception of that movement in another (Rizzolatti et al. 1996).

4
. The tendency for the eyes to run off with the rest of the body is something you can experiment with at home. Botvinick and Cohen (1998) reported, for example, a nice demonstration of such “tactile ventriloquism” with a rubber hand. When a person’s own arm is stroked gently out of view in sync with the stroking of a rubber hand that is in view, it just takes a little while for the person to begin reporting that the rubber hand feels like their own. Ramachandran and Blakeslee (1998) report a similar example, in which a person watches a spot on the table being tapped in rhythm with taps on their own hand under the table. After a bit of this, the person reports feeling the taps on the table, and in fact, a sharp rap on the table yields a startle response that can be measured as an increase in the person’s skin conductance level.