Brain Buys (11 page)

Knowing that the sensation of touch, or of the feeling of one’s arm, can be achieved solely by the activation of neurons in the brain allows us to understand how phantom sensations might arise. One of the first scientific hypotheses of the cause of phantom limb sensations was that they were a result of the regrowth of the severed nerves at the site of the amputation. This is a logical hypothesis since the distal ends of cut nerve fibers can indeed sprout into the remaining part of the limb, referred to as the
stump
. In this manner the nerves that used to innervate the hand could innervate the stump and send signals to the central nervous system, which would continue to interpret these signals as if the lost limbs were still present. This hypothesis was behind one of the early treatments for phantom pain, which was to surgically cut the nerves in the stump or as they enter the spinal chord. This procedure was beneficial in some cases, but generally did not provide a permanent cure for phantom pain.

Today scientists agree that in many cases phantom sensations do not reflect an abnormal signal from the nerves that used to innervate the missing limb, but are caused by changes that take place within the brain. Specifically, as in the monkey experiment in which direct brain stimulation appeared to substitute for a real stimulus, neurons in the brain that would normally be activated by the arm continue to fire, driving the perception of a phantom limb.
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But a question remains: why would the neurons in the brain that are normally driven by the limb continue to be active even when the limb is long gone? The answer to this question provides important insights into how one of the most powerful features of the brain, its ability to adapt, can become a brain bug.

NEURONS ABHOR SILENCE

Like space on a computer chip, cortical real estate is an extremely valuable and limited resource. So how does the brain decide how much cortical area should be allocated to each part of the body? Does a square centimeter of skin in the small of your back deserve as much cortical computational power as the square centimeter of skin on the tip of your index finger?

One might guess that the amount of cortical area devoted to different body parts is genetically determined and, indeed, to some extent cortical allocation is hardwired. For instance, per square centimeter there are fewer sensory fibers innervating your back than your hand (your back is a low-resolution input device, while your fingertips have high input resolution). This is a function of our neural operating system as defined in the Introduction. But such a predestined strategy alone would be an overly rigid and ill-conceived evolutionary design. The elegant (and somewhat Darwinistic) solution to the cortical allocation problem is that different body parts have to fight it out: the most “important” parts of the body are awarded more cortical real estate.

If you close your eyes, and ask someone to touch a finger on your hand you can easily report which finger was touched. If this person is willing to repeat this experiment by touching one of your toes, you may find that you are not so sure which toe was touched, and may even get the answer wrong. This is in part because, in all likelihood, your brain devotes more of the somatosensory cortex to your fingers than to your toes. The amount of cortex allocated to each part of the body contributes to how precisely we can localize the point of contact, and how easily we can determine if we were touched by a pin, a pen, or someone’s finger. One can envision that a seamstress, surgeon, or a violinist, compared to a professor, lawyer, or a cymbalist, would benefit immensely from having a larger amount of somatosensory cortex allocated to processing information from the fingertips. Furthermore, if you were to try to learn Braille, it would be very convenient to perform an upgrade to the part of your somatosensory cortex devoted to your fingertips. It makes sense to be able to allocate different amounts of cortical surface to the fingertips on a case-by-case manner along the lifespan of an individual. It turns out that the brain can dynamically allocate cortical resources depending on computational need; that is, the cortical area representing different parts of the body can expand or contract as a result of experience.

For many decades, it was thought that the somatosensory maps observed in humans and other animals were rigid throughout adult life. But this view was overturned in the early eighties by a series of groundbreaking studies by the neuroscientist Michael Merzenich, at the University of California in San Francisco. Merzenich and his colleagues demonstrated that cortical maps were “plastic”—like sand dunes in the desert the cortex was constantly being resculpted.
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Merzenich first showed that after cutting one of the nerves innervating the hand, the somatosensory cortex of monkeys reorganized—the map changed. Neurons in cortical areas that originally responded to the affected hand initially became unresponsive, but over the course of weeks and months. The neurons that were deprived of their normal input as a result of the nerve transection “switched teams”—they became progressively more responsive to other parts of the body. More importantly, subsequent studies showed that when monkeys were trained to use a few of their fingers for tactile discrimination over the course of months, the areas of the somatosensory cortex that represented those fingers expanded. It is as if there was some manager in the brain that went around redistributing precious cortical space to the parts of the body that needed it the most.

While these studies were initially met with great skepticism, cortical plasticity is now accepted as one of the key principles of brain function. Studies in humans have further established the importance of the cortex’s ability to reorganize for many types of learning. For example, using noninvasive techniques, studies have compared the amount of somatosensory cortex devoted to the fingers of musicians and nonmusicians. People who started playing string instruments at an early age were found to have more cortical area devoted to their fingertips. Similarly, an expansion of the fingertip areas was observed in people who learned to read Braille when they were young.
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In the early days, computer programmers had to preallocate the amount of memory to be devoted to a given program. That is, they had to estimate how much memory would be used, and some early pieces of software had a limit on the amount of information they could handle. Over the decades, more sophisticated programming languages have been developed that allow the dynamic allocation of memory: as I type more and more words into a word processor, the amount of memory dedicated to the file is dynamically increased. In terms of the allocation of computational power, the brain has used this strategy for tens of millions of years—though the dynamic allocation of cortical areas is a gradual process takes place over weeks and months.

The brain, of course, has no supervisor to oversee the distribution of cortical real estate. So how does it figure out exactly how important different body surfaces are? It seems to use a rule of thumb. Since the degree of activity in a given zone of the somatosensory cortex roughly reflects how much the corresponding body part is used, the brain can assign importance based on activity.
¹⁰
Let’s consider what happens in the somatosensory cortex of a person who has the phantom sensation of an index finger that was lost in an accident. Normally, the neurons in the zone of the primary somatosensory cortex that represent the index finger would be driven by inputs originating in that finger. But, now deprived of their source of input, these cortical neurons should fire much less than they once did. For argument’s sake, let’s assume that the index finger neurons in the somatosensory cortex went totally silent after the accident. Neurons abhor silence. A neuron that never fires is mute; it barely deserves to be called a neuron since neurons are all about communication. So it is not surprising that neurons are programmed to do their best to avoid being mute for long periods of time. In the same manner that there are compensatory or
homeostatic
mechanisms in place to allow the body to regulate its temperature depending on whether it is warm or cold outside, neurons are able to homeostatically regulate their level of activity.

A neuron in the somatosensory cortex that is devoted to the index finger receives thousands of synapses. Many of these synapses convey information from the index finger, but some synapses originate from neighboring neurons in the cortex that represent other parts of the body. In this case, because of the topographic organization of the cortex, neurons surrounding the index finger neurons tend to be those representing the thumb and middle finger. These neighboring neurons should exhibit their normal levels of activity (or perhaps more, because someone who lost an index finger will start using the middle finger in its place). The silenced neurons of the index finger will amplify the inputs from the neighboring areas that are still active. This is what allows them to “change teams”—the ex-index finger neurons can become thumb or middle finger neurons by strengthening the synapses from neurons that already responded to the thumb or middle finger.

The exact mechanisms responsible for the amplification of previously weak inputs continues to be debated, but once again they seem to rely on the same synaptic and cellular mechanisms underlying learning and memory, including strengthening of existing synapses and the formation of new ones.
¹¹
Recall Hebb’s rule from Chapter 1:
neurons that fire together, wire together
. But suppose a neuron stops firing completely, as in our hypothetical example of the index finger neurons that went silent. Homeostatic forms of synaptic plasticity allow previously weak synapses to become strong even in the absence of much postsynaptic activity—essentially overriding Hebb’s rule.
¹²
If a strong signal is lost, the response to weak inputs can be amplified.

Now let’s return to our question of why neurons in the somatosensory cortex that have lost their original input continue to fire and mislead the brain into believing that the amputated limb, or finger, is still present. One hypothesis is that the neurons that used to be activated by the index finger are being driven by activity in the thumb and middle fingers, but in an exaggerated fashion. So even in the absence of their normal source of input the neurons in the primary somatosensory cortex that were previously responsible for conveying information about the index finger may still be active! Downstream or “higher-order” areas of the cortex continue to interpret this activity as evidence that the index finger is still in place. It is presumed that these higher-order areas somehow create the conscious experience of feeling one’s body, but no one knows how or where this comes about. Nevertheless, in people with a phantom limb it is clear that these areas never get the message that the body has changed—the master map is never updated. Just as a king who is never told that part of his empire has been captured might continue to “rule” over a territory he no longer controls, some part of the brain persists in generating an unaltered illusion of the body, blissfully unaware that some part of the body no longer exists.

THE FANTASTIC PLASTIC CORTEX

The discovery that the somatosensory cortex is continuously remodeled throughout life was seminal because it revealed a general feature of the cortex and the mechanisms of learning and memory: cortical plasticity is not restricted to the somatosensory cortex; it’s a general feature of the entire cortex. Many studies have demonstrated that the circuits in other cortical areas also undergo reorganization in response to experience.

Most of our knowledge of cortical plasticity comes from studies of the sensory areas, specifically the somatosensory, auditory, and visual cortices. Among these, vision is the notorious cortical hog. In primates, for example, the total amount of cortex devoted to visual processing far exceeds that of any other sensory modality. By some estimates close to half of the entire cortex may be devoted primarily to sight.
¹³
Thus, if these areas went permanently offline as a result of blindness, there would be billions of very bored neurons. Due to cortical plasticity, however, these visual areas can be put to work toward nonvisual tasks. For a person who lost her eyesight at an early age, the tactile task of determining whether the object in hand is a pen or pencil may activate “visual” areas (the part of the brain that would normally process sight). As proof of this, temporarily interfering with normal electrical activity in the “visual” cortex of blind people has been shown to degrade their ability to read Braille. The visual cortex is also more robustly activated by sounds in blind people.
¹⁴
In other words, a blind person may have more cortical hardware to devote to somatosensory and auditory processing, which likely contributes to superior performance on some somatosensory and auditory tasks.
¹⁵
The extent to which people can improve processing in sensory modalities, and use these to compensate for a lost modality such as vision, is illustrated in the extreme by the ability of some people to “see” using echolocation. Some animals, including bats and dolphins, can navigate their environment and discriminate objects in the absence of light. Dolphins can even perform a sonogram and “see” through some materials, which is why the U.S. Navy has trained dolphins to find mines hidden underneath layers of mud on the ocean floor.

Echolocation uses the same principles as sonar. Bats and dolphins emit sounds, wait for the echo of these sounds to return after they have rebounded off objects, and use their auditory system to interpret the scene rendered by these echoes. The delay between the sound emission and the returning echo is used to determine the distance of the object. Remarkably, some blind humans have learned how to echolocate. They emit clicklike sounds from their mouth (or use a walking cane to “tap”) and wait for the echo. One boy who lost both his eyes to cancer at the age of two was able to walk around and distinguish objects such as a car from a garbage can without touching them.
¹⁶
Although this ability has not been carefully studied, it likely relies on the brain’s ability to allocate cortical area according to an individual’s experience. It should be pointed out, however, that extraordinary sensory abilities are not simply a result of having more cortical space available to perform certain computations. They are also a product of intense practice and the immersive experience that comes with living in what amounts to an entirely different world.