limb at some point—the persistent and unmistakable impression that
the lost limb is still present, still part of their body. These phantom
limbs feel somewhat less real than the rest of the body, a bit “ghostly.”
Silas Weir Mitchell, the American neurologist who introduced the concept
of phantom limbs in 1871, spoke of “ghostly members” haunting
people like “unseen ghosts of the lost part.” Often, the phantom recedes
gradually and finally disappears; in some cases, however, phantom limbs can persist for months or even years. Patients often have painful
sensations in their phantom limb. Sometimes, as in the now-classic case
soon to be described, the phantom is “paralyzed,” creating the subjective
impression that the absent limb is frozen in a fixed position and cannot
be moved.
In a set of experiments involving a patient with a paralyzed phantom
limb, V. S. Ramachandran and his UCSD colleagues demonstrated the
virtuality of the bodily self-model. They constructed a “virtual-reality
box” to show to what extent the content of the self-model depends on
perceptual-context information. Their idea was that by manipulating
the perceptual-context information—which in turn constrains the information-
processing activity in the brain—the content of the bodily selfmodel
can be changed.
Their virtual-reality box was quite simple. A mirror was placed vertically
inside a cardboard box open at the top, and two holes were cut in
the front of the box, to either side of the mirror. The experimenters
asked Philip, a patient who had been suffering from a paralyzed phantom
limb for many years, to insert both of his arms—that is, his right
arm and his left “phantom arm”—through the holes in the box. Then he
was told to observe the reflection of his real hand in the mirror. The
mirror image of his right hand was used to create the visual illusion that
he did actually have two hands. Next, he was asked to make symmetrical
movements with both his real arm and his phantom arm.
What would happen to the content of Philip’s self-model if the imagined
movements of his phantom arm were simultaneously matched with
visual input? What would happen to his paralyzed phantom if he could
see the movements of a hand in the mirror? Ramachandran described
the outcome:
I asked Philip to place his right hand on the right side of the
mirror in the box and imagine that his left hand (the phantom)
was on the left side. “I want you to move your right and left arm
simultaneously,” I instructed.
“Oh, I can’t do that,” said Philip. “I can move my right arm,
but my left arm is frozen. Every morning when I get up, I try to move my phantom because it’s in this funny position and I feel
that moving it might help relieve the pain. But,” he said, looking
down at his invisible arm, “I never have been able to generate a
flicker of movement in it.”
“Okay, Philip, but try anyway.”
Philip rotated his body, shifting his shoulder, to “insert” his
lifeless phantom into the box. Then he put his right hand on the
other side of the mirror and attempted to make synchronous
movements. As he gazed into the mirror, he gasped and then
cried out, “Oh, my God! Oh, my God, doctor! This is unbelievable.
It’s mind-boggling!” He was jumping up and down like a
kid. “My left arm is plugged in again. It’s as if I’m in the past. All
these memories from years ago are flooding back into my mind.
I can move my arm again. I can feel my elbow moving, my wrist
moving. It’s all moving again.”
After he calmed down a little I said, “Okay, Philip, now close
your eyes.” “Oh, my,” he said, clearly disappointed. “It’s frozen again. I
feel my right hand moving, but there’s no movement in the
phantom.”
“Open your eyes.”
“Oh, yes. Now it’s moving again.”
The phantom movement in this experiment is the content of the conscious
self-model. In the real world, there is no limb that can be felt or
controlled. In his moving phantom limb, Philip experiences—and controls—
a part of his bodily self that does not exist. Just as in the rubberhand
illusion, the experiential property of ownership seamlessly spreads
into the hallucinated part of the bodily self: The moving phantom limb
is owned, just as the rubber hand is owned. On the intellectual level,
Philip understands perfectly well that the phantom limb does not exist.
(This fact is cognitively available to him, as a philosopher might say.)
But the subjective experience of his phantom arm actually moving is robust
and realistic. And, as opposed to the rubber-hand illusion, there is
an additional quality—namely, the phenomenal experience of agency. A
full-blown bodily Ego is in place.
In order to survive, biological organisms must not only successfully
predict what is going to happen next in their immediate environment
but also be able to predict accurately their behavior and bodily movements
along with their consequences. The self-model is a real-time predictor.
This is how our best current theories explain what happened to
Philip: In our brains, we have a body emulator that uses motor commands
to predict the likely proprioceptive and kinesthetic feedback that
results from moving our limbs in a certain way. For our actions to be
successfully controlled, we cannot wait for the actual feedback from our
arms and legs as we move through the world. We need an internal image
of our body as a whole that predicts the likely consequences of, say, an
attempt to move our left arm in a certain way. In order to be really efficient,
we need to know in advance what this would feel like. Furthermore,
by “taking it offline,” we can use our body emulator to create
motor imagery in our mind—to plan or imagine our body movements
without actually executing them. This body emulator, which constantly generates forward simulations,
is a fundamental part of the human self-model and the centerpiece of the
Ego Tunnel. Philip’s self-model had learned that whatever motor commands
he issued to his amputated arm, there would be no feedback
telling him about a changed limb position. To be sure, the image of his
arm was still there, burned into his brain. It had adapted to zero feedback
and was therefore frozen. Ramachandran’s ingenious idea was to use a
mirror as a source of virtual information, allowing the virtual emulator
to perform a successful update. When Philip tried to move both his real
arm and his phantom arm, the changes in the visual self-model perfectly
matched the motor commands fed into the body-state predictor in
Philip’s brain. The conscious experience that his missing left arm was
actually moving and under volitional control followed suit.
Now we can understand why our self-model is a virtual model.
Clearly, Philip’s moving left arm is just a simulation. It is an “as if” arm;
what has turned into a new possibility for the
brain is portrayed to Philip as a reality. If one
does not think about it but simply attends to
the experience itself, the moving phantom
limb can perhaps be experienced as just as realistic
as the rest of the body; they are both
parts of the same unified self, and they are
both under volitional control. But exactly how
real the parts of our self-model appear depends
on many different factors.
One interesting fact about phantom-limb
experiences is that they also happen to people
who were born missing certain limbs. A recent
case study conducted by Swiss neuroscientist
Peter Brugger and his colleagues of the University
Hospital in Zürich used a seven-point scale
to rate the subjectively perceived vividness of
phantom limbs. Interestingly, the ratings
showed highly consistent judgments across sessions
for their subject, “AZ,” a forty-four-yearold university-educated woman born without forearms and legs. For as
long as she could remember, she had experienced mental images of her
nonexistent forearms (including fingers) and legs (including feet and the
first and fifth toes). But, as the figure shows, these phantoms were not as
realistic as the content of her nonhallucinatory body model.
Moreover, she reported that “[a]wareness of her phantom limbs is
transiently disrupted only when some object or person invades their felt
position or when she sees herself in a mirror.” Functional magnetic resonance
imaging (fMRI) of her imagined phantom hand movements
showed no activation of primary sensorimotor areas, but did show activity
in the bilateral premotor and parietal cortex. Transcranial magnetic
stimulation of the sensorimotor cortex consistently elicited
sensations in the phantom fingers and hand, on the side opposite the
stimulation. Premotor and parietal stimulation evoked similar phantom
sensations, though without showing motor-evoked potentials in the
stump. Brugger’s data demonstrate that body parts that never physically
developed can be represented in sensory and motor areas of the cortex.
The fascinating question is this: Are AZ’s hallucinated forearms and
legs components of an innate body model—perhaps of a nucleus that
continues developing after birth? Or could they have been “mirrored
into” the patient’s self-model through the visual observation of the
movements of other human beings (see chapter 6 on the Empathic
Ego)? What exactly is it that you feel as your own body, right now, as you
are reading these words? At this point in our investigations into consciousness,
it seems obvious that we are never in direct contact with our
physical bodies but rather with a particular kind of representational
content. But what exactly is it that is represented in this layer of our conscious
self? In the second book of his famous work De anima, Aristotle
said that the soul is simply the form of the body and that it perishes at
death. Is that what we have newly rediscovered by studying phantom
limbs, the “inner form” of the body and the global model of its shape?
Spinoza said the soul is the idea that the body develops of itself because
“the object of our soul is the body as it exists, and nothing else.” Again,
it is intriguing to see how classical philosophical ideas contribute to a
deeper understanding of what it means to be an embodied self. Ramachandran’s and Brugger’s experiments demonstrate that the experiential
content of the bodily self-model is the content of an ongoing
simulation, part of a dynamic control mechanism. At any given time, the
content of bodily experience is the best hypothesis that the system has
about its current body state. The brain’s job is to simulate the body for
the body and to predict the consequences of the body’s movements, and
the instrument it uses is the self-model. This process takes place in the
real world, so it is time-consuming and necessarily generates a lag between
the actual state of the body and the self-model’s content.
Normally we’re unaware of this process, because nature engineered
it so efficiently that errors rarely occur. But the simple fact remains: You
are never in direct contact with your own body. What you feel in the
rubber-hand illusion, what AZ feels, or what Philip feels when his left
arm is “plugged in” is exactly the same as what you feel when you attend
to the sensation of your hands holding this book right now or to the feeling
of pressure and resistance when you lean back in your chair. What
you experience is not reality but virtual reality, a possibility. Strictly
speaking, and on the level of conscious experience alone, you live your
life in a virtual body and not in a real one. This point will become clearer
when we consider “offline states” in the chapter on dreaming and lucid
dreaming. But first, let us have a look at another essential feature of phenomenal
selfhood—the transition from ownership to agency.
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