to the feeling of being someone. Your body image is surprisingly flexible.
Expert skiers, for example, can extend their consciously experienced
body image to the tips of their skis. Race-car drivers can expand it to include
the boundaries of the car; they do not have to judge visually
whether they can squeeze through a narrow opening or avoid an obstacle—
they simply feel it. Have you ever tried to walk with your eyes
closed, or in the dark, tapping ahead with a stick as a blind person does?
If so, you’ve probably noticed that you suddenly start to feel a tactile
sensation at the end of the stick. All these are examples of what philosophers
call the sense of ownership, which is a specific aspect of conscious
experience—a form of automatic self-attribution that integrates a certain
kind of conscious content into what is experienced as one’s self.
Neuroimaging studies have given us a good first idea of what happens
in the brain when the sense of ownership, as illustrated by the
rubber-hand experiment discussed in the Introduction, is transferred
from a subject’s real arm to the rubber hand: Figure 2 shows areas of increased
activity in the premotor cortex. It is plausible to assume that at
the moment you consciously experience the rubber hand as part of your body, a fusion of the tactile and visual receptive fields takes place
and is reflected by the activation of neurons in the premotor cortex.The rubber-hand illusion helps us understand the interplay among
vision, touch, and proprioception, the sense of body posture and balance
originating in your vestibular system. Your bodily self-model is created
by a process of multisensory integration, based on a simple
statistical correlation your brain has discovered. The phenomenal incorporation
of the rubber hand into your self-model results from correlated
tactile and visual inputs. As the brain detects the synchronicity underlying
this correlation, it automatically forms a new, coherent representation.
The consciously experienced sense of ownership follows.
In Matthew Botvinick and Jonathan Cohen’s study, subjects were
asked to close their eyes and point to their concealed left hand; they
tended to point in the direction of the rubber one, with the degree of
mispointing dependent on the reported duration of the illusion. In a
similar experiment, conducted by K. C. Armel and V. S. Ramachandran
at UCSD’s Brain and Perception Laboratory, if one of the rubber fingers was bent backward into a physiologically impossible position, subjects
not only experienced their phenomenal finger as being bent but also exhibited
a significant skin-conductance reaction, indicating that unconscious
autonomous mechanisms, which cannot be controlled at will,
were also reacting to the assumption that the rubber hand was part of
the self. Only two out of one hundred and twenty subjects reported feeling
actual pain, but many pulled back their real hands and widened their
eyes in alarm or laughed nervously.The beauty of the rubber-hand illusion is that you can try it at home.
It clearly shows that the consciously experienced sense of ownership is
directly determined by representational processes in the brain. Note
how, in your subjective experience, the transition from shoulder to rubber
hand is seamless. Subjectively, they are both part of one and the
same bodily self; the quality of “ownership” is continuous and distributed
evenly between them. You don’t need to do anything to achieve this
effect. It seems to be the result of complex, dynamic self-organization in
the brain. The emergence of the bodily self-model—the conscious image
of the body as a whole—is based on a subpersonal, automatic
process of binding different features together—of achieving coherence.
This coherent structure is what you experience as your own body and
your own limbs.
There are a number of intriguing further facts—such as the finding
that subjects will mislocate their real hand only when the rubber one is
in a physiologically realistic position. This indicates that “top-down”
processes, such as expectations about body shape, play an important
role. For example, a principle of “body constancy” seems to be at work,
keeping the number of arms at two. The rubber hand displaces the real
hand rather than merely being mistaken for it. Recently, psychometric
studies have shown that the feeling of having a body is made up of various
subcomponents—the three most important being ownership,
agency, and location—which can be dissociated. “Me-ness” cannot be
reduced to “here-ness,” and, more important, agency (that is, the performance
of an action) and ownership are distinct, identifiable, and separable
aspects of subjective experience. Gut feelings (“interoceptive
body perception”) and background emotions are another important cluster anchoring the conscious self, but it is becoming obvious that
ownership is closest to the core of our target property of selfhood. Nevertheless,
the experience of being an embodied self is a holistic construct,
characterized by part-whole relationships and stemming from
many different sources.Phenomenal ownership is not only at the heart of conscious selfexperience;
it also has unconscious precursors. Classical neurology hypothesized
about a body schema, an unconscious but constantly
updated brain map of limb positions, body shape, and posture. Recent
research shows that Japanese macaques can be trained to use tools even
though they only rarely exhibit tool use in their natural environment.During successful tool use, changes occur in specific neural networks in
their brains, a finding suggesting that the tools are temporarily integrated
into their body schemata. When a food pellet is dispensed beyond
their reach and they use a rake to bring it closer, a change is
observed in their bodily self-model in the brain. In fact, it looks as
though their model of their hand and of the space around it is extended
to the tip of the tool; that is, on the level of the monkey’s model of reality,
properties of the hand are transferred to the tool’s tip. Certain visual
receptive fields now extend from a region just beyond the
fingertips to the tip of the rake the monkey is holding, because the parietal
lobe in its brain has temporarily incorporated the rake into the
body model. In human beings, repeated practice can turn the tip of a
tool into a part of the hand, and the tool can be used as sensitively and
as skillfully as the fingers.
Recent neuroscientific data indicate that any successful extension of
behavioral space is mirrored in the neural substrate of the body image in
the brain. The brain constructs an internalized image of the tool by assimilating
it into the existing body image. Of course, we do not know whether
monkeys actually have the conscious experience of ownership or only the
unconscious mechanism. But we do know of several similarities between
macaques and human beings that make plausible the assumption that the
macaques’ morphed and augmented bodily self is conscious.
One exciting aspect of these new data is that they shed light on the
evolution of tool use. A necessary precondition of expanding your space of action and your capabilities by using tools clearly seems to be
the ability to integrate them into a preexisting self-model. You can engage
in goal-directed and intelligent tool use only if your brain temporarily
represents the tools as part of your self. Intelligent tool use was
a major achievement in human evolution. One can plausibly assume
that some of the elementary building blocks of human tool-use abilities
existed in the brains of our ancestors, 25 million years ago. Then, due
to some not-yet-understood evolutionary pressure, they exploded into
what we see in humans today. The flexibility in the monkey’s body
schema strongly relies on properties of body maps in its parietal lobe.
The decisive step in human evolution might well have been making a
larger part of the body model globally available—that is, accessible to
conscious experience. As soon as you can consciously experience a
tool as integrated into your bodily self, you can also attend to this
process, optimize it, form concepts about it, and control it in a more fine-grained manner—performing what today we call acts of will. Conscious
self-experience clearly is a graded phenomenon; it increases in
strength as an organism becomes more and more sensitive to an internal
context and expands its capacities for self-control.
Monkeys also seem able to incorporate into their bodily self-model a
visual image of their hand as displayed on a computer monitor. If an image
of a snake or a spider approaches the image of the hand on the
screen, the animal retracts its real hand. Monkeys can even learn to control
a brain-machine interface that lets them grasp objects with a robot
arm controlled by certain parts of their brain. Perhaps most exciting
from a philosophical perspective is the idea that all of this may have
contributed to the evolutionary emergence of a quasi-Cartesian “metaself,”
the capacity to distance yourself from your bodily self—namely, by
beginning to see your own body as a tool. Clearly, the visual image of the robot arm, just as in the rubber-hand
illusion, is embedded in the dancing self-pattern in the macaque’s brain.
The integration of feedback from the robot arm into this self-model is
what allows the macaque to control the arm—to incorporate it functionally
into a behavioral repertoire. In order to develop intelligent tool use,
the macaque first had to embed this rake in its self-model; otherwise, it
could not have understood that it could use the rake as an extension of its
body. There is a link between selfhood and extending global control.
Human beings, too, treat virtual equivalents of their body parts as
seen on a video screen as extensions of their own bodies. Just think of
mouse pointers on computer desktops or controllable fantasy figures in
video games. This may explain the sense of “presence” we sometimes
have when playing these ultrarealistic games. Incorporation of artificial
actuators into widely distributed brain regions may someday allow human
patients successfully to operate advanced prostheses (which, for
example, send information from touch and position sensors to a brainimplanted,
multichannel recording device via a wireless link), while also
enjoying a robust conscious sense of ownership of such devices. All of
this gives us a deeper understanding of ownership. On higher levels,
ownership is not simply passive integration into a conscious self-model:
More often it has to do with functionally integrating something into a
feedback loop and then making it part of a control hierarchy. It now
looks as if even the evolution of language, culture, and abstract thought
might have been a process of “exaptation,” of using our body maps for
new challenges and purposes—a point to which I return in the chapter
on empathy and mirror neurons. Put simply, exaptation is a shift of
function for a certain trait in the process of evolution: Bird feathers are a
classic example, because initially these evolved “for” temperature regulation
but later were adapted for flight. Here, the idea is that having an
integrated bodily self-model was an extremely useful new trait because
it made a host of unexpected exaptations possible.
Clearly, a single general mechanism underlies the rubber-hand illusion,
the evolution of effortless tool use, the ability to experience bodily
presence in a virtual environment, and the ability to control artificial devices
with one’s brain. This mechanism is the self-model, an integrated representation of the organism as a whole in the brain. This representation
is an ongoing process: It is flexible, can be constantly updated, and
allows you to own parts of the world by integrating them into it. Its content
is the content of the Ego.
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