of the body didn’t light up, so to speak, on the scans. But touching the unaffected limb did. If we can replicate this finding with a larger number of patients, our theory will be well supported.
One curious aspect of apotemnophilia that is unexplained by our model is the associated sexual inclinations in some subjects: desire for intimacy with another amputee. These sexual overtones are probably what misled people to propose a Freudian view of the disorder.
Let me suggest something different. Perhaps one’s sexual “aesthetic preference” for certain body morphology is dictated in part by the shape of the body image as represented—and hardwired—in the right SPL and possibly insular cortex. This would explain why ostriches prefer ostriches as mates (presumably even when smell cues are eliminated) and why pigs prefer porcine shapes over humans.
Expanding on this, I suggest that there is a genetically specified mechanism that allows a template of one’s body image (in the SPL) to become transcribed into limbic circuitry, thereby determining aesthetic visual preference. If this idea is right, then someone whose body image was congenitally armless or legless would be attracted to people missing the same limb. Consistent with this view, people who wish to have their leg amputated are almost always attracted to leg amputees, not arm amputees.
SOMATOPARAPHRENIA: DOCTOR, THIS IS MY MOTHER’S ARM
Distortion of body-part ownership also occurs in one of the strangest syndromes in neurology, which has the tongue-twisting name “somatoparaphrenia.” Patients with a left-hemisphere stroke have damage to the band of fibers issuing from the cortex down into the spinal cord. Because the left side of the brain controls the right side of the body (and vice versa), this leaves the right side of their bodies paralyzed. They complain about their paralysis, asking the doctor whether the arm will ever recover, and not surprisingly they are often depressed.
When the stroke is in the right hemisphere, the paralysis is on the left. The majority of such patients are troubled by the paralysis as expected, but a small minority deny the paralysis (anosognosia), and an even smaller subset actually deny ownership of the left arm, ascribing it to the examining physician or to a spouse, sibling, or parent. (Why a particular person is chosen isn’t clear, but it reminds me of the manner in which the Capgras delusion often also involves a specific individual.)
In this subset of patients there is usually damage to the body maps in S1 and S2. In addition to this, the stroke has destroyed the corresponding body-image representation in the right SPL, which would ordinarily receive input from S1 and S2. Sometimes there is also additional damage to the right insula—which receives input the directly from S2 and also contributes to the construction of the person’s body image. The net result of this combination of lesions—S1, S2, SPL, and insula—is a complete sense of
Why is somatoparaphrenia only seen when the right parietal is damaged but not when the left one is? To understand this we have to invoke the idea of division of labor between the two hemispheres (hemispheric specialization), a topic I will consider in some detail later in this chapter. Rudiments of such specialization probably exist even in the great apes, but in humans it is much more pronounced and may be yet another factor contributing to our uniqueness.
TRANSSEXUALITY: DOCTOR, I’M TRAPPED IN THE WRONG KIND OF BODY!
The self also has a sex: You think of yourself as male or female and expect others to treat you as such. It is such an ingrained aspect of your self-identity that you hardly ever pause to think about it—until things go awry, at least by the standards of a conservative, conformist society. The result is the “disorder” called transsexuality.
As with somatoparaphrenia, distortions or mismatches in the SPL can also explain the symptoms of transsexuals. Many male-to-female transsexuals report feeling that their penis seems to be redundant or, again, overpresent and intrusive. Many female-to-male transsexuals report feeling like a man in a woman’s body, and a majority of them have had a phantom penis since early childhood. Many of these women also report having phantom erections.8 In both kinds of transsexuals the discrepancy between internally specified sexual body image—which, surprisingly, includes details of sexual anatomy—and external anatomy leads to an intense discomfort and, again, a yearning to reduce the mismatch.
Scientists have shown that during fetal development, different aspects of sexuality are set in motion in parallel: sexual morphology (external anatomy), sexual identity (what you see yourself as), sexual orientation (what sex you are attracted to), and sexual body image (your brain’s internal representation of your body parts). Normally these harmonize during physical and social development to culminate in normal sexuality, but they can become uncoupled, leading to deviations that shift the individual toward one or the other end of the spectrum of normal distribution.
I am using the words “normal” and “deviation” here only in the statistical sense relative to the overall human population. I do not mean to imply that these ways of being are undesirable or perverse. Many transsexuals have told me that they would rather have surgery than be “cured” of their desire. If this seems strange, think of intense but unrequited romantic love. Would you request that your desire be removed? There is no simple answer.
Privacy
In Chapter 4, I explained the role of the mirror-neuron system in viewing the world from another person’s point of view, both spatially and (perhaps) metaphorically. In humans this system may have turned inward, enabling a representation of one’s
Although mirror neurons allow you to tentatively adopt another person’s vantage point, they don’t result in an out-of-body experience. You don’t literally float out to where that other vantage point is, nor do you lose your identity as a person. Similarly, when you watch another person being touched, your “touch” neurons fire, but even though you empathize, you don’t actually feel the touch. It turns out that in both cases, your frontal lobes inhibit the activated mirror neurons at least enough to stop all this from happening so you remain anchored in your own body. Additionally, “touch” neurons in your skin send a null signal to your mirror neurons, saying, “Hey, you are not being touched” to ensure that you don’t literally feel the other guy being touched. Thus in the normal brain a dynamic interplay of three sets of signals (mirror neurons, frontal lobes, and sensory receptors) is responsible for preserving both the individuality of your own mind and body, and your mind’s reciprocity with others—a paradoxical state of affairs unique to humans. Disturbances in this system, we shall see, would lead to a dissolution of interpersonal boundaries, personal identity, and body image—allowing us to explain a wide spectrum of seemingly incomprehensible symptoms seen in psychiatry. For example, derangements in frontal inhibition of mirror-neuron system may lead to a disturbing out-of-body experience—as though you were really watching yourself from above. Such syndromes reveal how blurred the boundary between reality and illusion can become under certain circumstances.
MIRROR NEURONS AND “EXOTIC” SYNDROMES
