Authors: Dean Buonomano
The ability of the cortex to adapt and reorganize is among its most powerful features. Cortical plasticity is why practice makes perfect, why radiologists see pneumonia in x-rays that look like out-of-focus blotches to the rest of us, and why Braille readers have more sensitive fingertips. Cortical plasticity is also the reason why a child born in China ends up with a brain well suited to decipher the tonal sounds of Mandarin, which sound indistinguishable to the average English speaker. However, cortical plasticity is also responsible for some of the brain bugs that emerge in response to mild or serious injury. The pain of a phantom limb is the brain’s own fault, produced by a glitch in the brain’s attempt to adapt to the missing limb. The brain’s extraordinary ability to reorganize can be maladaptive.
Glitches in brain plasticity may also underlie a much more common medical condition: tinnitus. Roughly 1 to 3 percent of the general population experiences the annoying and incessant buzzing or “ringing” that characterizes tinnitus. It is the number-one disability reported among Iraq war veterans.
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The consequences of tinnitus can be serious, and include the inability to concentrate, loss of sleep, and depression.
Sound is detected by hair cells located in the sensory organ of the ear, the cochlea. Tiny “hairs” or cilia on top of each of these cells respond to minute changes in air pressure; their movement results in the generation of action potentials in the auditory nerve that conveys information to the brain. Different groups of hair cells tend to be activated by specific frequencies. Like the keyboard of a piano, the cochlea represents low frequencies at one end and high frequencies at the other. In the same manner that the somatosensory cortex contains a topographic map of the body, the primary auditory cortex contains a
tonotopic
map of the cochlea. If a neurosurgeon stimulated your auditory cortex, instead of feeling that someone touched you, you would hear a sound, and depending on the exact location of the stimulation it would be low or high in pitch.
One might be inclined to speculate that the grating ringing that sufferers of tinnitus experience is produced by overactive sound detectors in the ear; that for some reason some of the hair cells in the cochlea are continuously active, generating the illusion of a never-ending sound. Despite its plausibility, this hypothesis is not consistent with much of the evidence. Tinnitus is generally accompanied by a decrease in activity in the cochlea and auditory nerve, and associated with the death of hair cells.
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The loss of these cells can be produced by certain drugs, chronic or acute exposure to very loud sounds, and normal aging. The ear is particularly sensitive to environmental hazards and the aging process because we are born with precious few hair cells—each cochlea only contains around 3500 of the most important type of hair cell: inner hair cells (in contrast to the 100 million photoreceptors in each retina, for example). Damage to hair cells that respond to high frequencies, of course, results in impaired hearing of high-pitched sounds. The ringing people experience usually corresponds to the same pitch in which people suffer their hearing loss.
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That is, loss of the hair cells at the base of the cochlea, which respond to high-frequency sounds, may result in a continuous high-frequency ringing. At this point, the parallel with phantom limbs should be clear: both tinnitus and phantom limbs are associated with the damage to or absence of normal sensory inputs. Tinnitus is the auditory equivalent of a phantom limb—a phantom sound.
As is the case with phantom limbs, maladaptive cortical plasticity seems to be one of the causes of tinnitus.
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The hypothesis is that if a specific part of the cochlea is lesioned, the corresponding location in the auditory cortex is deprived of its normal source of input. This area might then be “captured” by the neighboring regions of the auditory cortex. The phantom sound may be generated by the neurons in the auditory cortex (or other stations in the auditory pathway) that lost their original input source, and came to be permanently driven by inputs from neighboring neurons. The causes of both phantom limbs and tinnitus are, however, not fully understood, and each is likely to have more than one underlying cause. Nevertheless, brain plasticity gone awry contributes to both syndromes.
GRACEFUL DEGRADATION VERSUS CATASTROPHIC BREAKDOWNS
The great majority of the brain’s 90 billion neurons,
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close to 70 billion, are quite frankly, simpleton neurons called
granule cells
, which reside in the cerebellum (a structure that among other things plays an important role in motor coordination). If you had to part ways with a few billion neurons, these are the ones to choose. Whereas your average cortical neuron receives thousands of synapses, a granule cell receives fewer than 10.
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But granule cells make up for their rather narrow view of the world with sheer numbers. Of the remaining 20 billion or so neurons, most reside in the cortex. This number is not quite as impressive as it sounds. Today, a single computer chip routinely possesses billions of transistors, so some parallel computers have more transistors than the brain has cortical neurons. I’m not implying that one should think of a transistor as being in any way the computational peer of a neuron (even a granule cell), but in terms of component computational units, your average desktop currently exceeds the number of neurons in the brains of many animals, including mice.
Until the 1990s the reigning dogma was that all mammals were born with their maximal complement of neurons; no new neurons were made after birth. We now know that this is not the case. Some neurons continue to be generated throughout life, but mostly in restricted areas of the brain (the olfactory bulb and part of the hippocampus).
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But, truth be told, the contribution of these neurons to the total number count is probably not significant. If it were these structures would have to grow throughout our lifespan, which they don’t. So in terms of absolute numbers it’s a downhill voyage from cradle to grave. It has been estimated that we lose 85,000 cortical neurons a day, and that total gray matter volume progressively decreases by about 20 percent over our adult life.
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The amazing thing about these facts is how little impact they have on our day-to-day lives. Despite the constant circuit remodeling, cell death, brain shrinkage, and the inevitable whacks to the head, each of us remains as we have always been. For the most part we retain our important memories, core personality traits, and cognitive abilities. Scientists and computer scientists refer to systems that can absorb significant amounts of change and damage without dramatic effects on performance as exhibiting
graceful degradation,
but brains and computers differ considerably in their ability to degrade gracefully.
Computers depend on the extraordinary reliability of transistors, each of which can perform the same operation trillions of times without making a single mistake or breaking. However, if a few of the transistors etched into a modern CPU chip did break, depending on their location on the chip, the consequences could be highly ungraceful. In sharp contrast, losing a few dozen neurons in your cortex, independent of location, would have no perceptible consequence. This is in part because neurons and synapses are surprisingly noisy and unreliable computational devices. In sharp contrast to a transistor, even in the well-controlled environ of a laboratory, a neuron in a dish can respond differently over multiple presentations of the same input. If two cortical neurons are connected by a synapse, and an action potential is elicited in the presynaptic neuron, the presynaptic component of the synapse will release neurotransmitter that will excite the postsynaptic neuron. The truth is, however, that there is a significant probability that the synapse between them will fail, and that the message will not make it across to the postsynaptic neuron. This so-called failure rate is dependent on many factors, and is generally around 15 percent but can be as high as 50 percent.
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The unreliability of cortical neurons should probably not be presented in an overly negative light, because this variability is in place by evolutionary design—synaptic transmission at some synapses outside the cortex can be vastly more reliable. Some neuroscientists believe that like someone trying to find the next piece of a puzzle by trial and error, the fallibility of cortical synapses helps networks of neurons explore different solutions to a computational problem and pick the best one. Furthermore, the unreliability of individual neurons and synapses may be one reason the brain exhibits graceful degradation, since it ensures that no single neuron is critical.
The brain’s graceful degradation is often looked at with some envy by computer scientists. But the envy is somewhat misplaced; in some cases the brain’s degradation is not at all graceful. True, only massive damage to the cortex (or of critical areas of the brainstem) can produce a system crash (coma or death), but small lesions can lead to stunning breakdowns in specific abilities.
One example of a fantastical syndrome that can arise when certain areas of the brain are injured is called
alien hand syndrome
. It is a very rare disorder that can have a number of different causes, including head injuries and strokes. Patients with alien hand syndrome experience a dissociation between themselves and one or more of their limbs. The limb isn’t paralyzed or incapable of fine motor movements, but rather, it is as if the limb has acquired a new master—one with some warped hidden agenda. Patients with alien hand syndrome have been known to be buttoning a shirt with their unaffected hand while the alien hand proceeds to unbutton it, or to simultaneously try to open a drawer with one hand and close it with the alien hand. The syndrome often results in perplexed and frustrated reports from patients: “I don’t know what my hands are doing. I’m no longer in control”; “The left hand wants to take over when I do something, it wants to get into the swim”; and as one patient with a wayward hand reported to the nurse, “if I could just figure out who’s pulling at my hair because that hurts.”
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Another syndrome that results in a catastrophic failure of a specific ability, rather than a graceful degradation of cognition in general, is characterized by the delusional belief that familiar people, often the patient’s parents, are impostors.
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This rare condition is known as
Capgras syndrome
. An individual with Capgras may acknowledge that his mother looks very much like his mother, but insist that she is actually someone pretending to be his mother. Some patients with Capgras may even maintain that the person in the mirror is an impostor. In some cases patients attack the alleged imposter, as they understandably become distressed over why someone is impersonating their family, and seek to find out where their loved ones really are.
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How is it that an organ widely known for its resilience and graceful degradation sometimes undergoes epic failures such as alien hand or Capgras syndrome? One reason is that many of the computations the brain performs are modular in nature—that is, different parts of the brain specialize in performing different types of computations.
In the late 1700s, Franz Joseph Gall, a preeminent neuroanatomist, proposed that the cortex was modular—a collection of different organs each dedicated to a specific task. Gall was also the father of the “science” of phrenology. He argued that one area of the cortex was responsible for love, another for pride, others for religion, the perception of time, wit, and so on, and that the size of these areas was directly proportional to how much of the given personality trait someone possessed. Gall further maintained that it was possible to tell how big each cortical area was based on bumps on the skull. Together, these fanciful assumptions provided a convenient means to determine the true nature of people by palpating their skulls. A big protrusion on the back of the skull, and you will be a loving parent; a large bump behind the ear, and you are secretive and cunning. People consulted with phrenologists to obtain insights into the psychological profile of others and of themselves, and to determine whether couples would be compatible. The lack of any scientific foundation, together with the subjectivity of confirming whether someone was secretive or witty, made phrenology a highly profitable field for charlatans and quacks.
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In a sense Gall was right—the brain is modular. But he made a mistake that often plagues scientists. He assumed that the categories we devise to describe things are something more than that. While love, pride, secretiveness, and wit may be distinct and important personality traits, there is no real reason to assume that each has its own little brain module. We may describe a car as having style, but nobody attributes this quality to any single component of the car. The tendency to assume that the categories we use to describe human behavior at the psychological level reveals something about how the brain is structured, is still in effect today among those who believe that complex personality traits such as intelligence, “novelty-seeking,” or spirituality can be attributed to single genes, or localized to a single brain area.
The division of labor in the brain is best understood in the light of evolution and the manner in which the brain performs computations. We have already seen that different parts of the brain are dedicated to processing sounds and touch. While there is not a single area responsible for language, specific areas do subserve different aspects of language such as speech comprehension and production. It is even the case that different parts of the visual system are preferentially devoted to recognizing places or faces. Similarly, there are areas that play an important, but slightly more intangible, role in human personality. This was famously illustrated by the case of Phineas Gage. After accidentally having a rod 1 meter long and 3 centimeters thick blasted through his skull, Phineas went from being the type of person you would enjoy hanging out with to the rude, unreliable, disrespectful type most of us would go out of our way to avoid.
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Phineas Gage’s lesion affected part of the ventromedial prefrontal cortex, an area important for inhibiting socially inappropriate behaviors, among other things.