Connectome (43 page)

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overall brain volume:
Steen et al. 2006; Vita et al. 2006. The difference exists even in patients receiving their first psychiatric treatment, so it does not appear to be a long-term effect of antipsychotic medications.

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lateral and third ventricles:
Steen et al. 2006.

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“graveyard”: Plum 1972.
Plum 1972.

 

2. Border Disputes

 

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infant brain grows rapidly:
Voigt and Pakkenberg 1983.

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philosophy of education:
Spurzheim was actually quite sophisticated for his time, acknowledging that other changes in the brain might take place besides growth: “The growth of the organs [brain regions], however, is not the only or even most important advantage to be derived from proper exercise. . . . [T]he size of the organ . . . will not augment in proportion to its being exercised, but its fibres will act with more facility” (Spurzheim 1833, pp. 131–132).

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through simple mazes:
The Hebb–Williams test of animal intelligence was a battery of twenty-four problems, each involving finding food in a simple maze. Donald Hebb pioneered this type of research on the effects of environmental enrichment. It's briefly noted in Hebb 1949, which is better known for its presentation of Hebb's theories of the cell assembly and synaptic plasticity (see Chapters 4 and 5 below).

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Mark Rosenzweig:
Rosenzweig 1996. The test of statistical significance was based on comparisons of siblings born in the same litter. The change in cortical size was not due to an overall change in brain size. In fact, the noncortical areas of the brain were slightly smaller. The change was not due to an increase in body size either. The environmentally enriched rats were actually somewhat lighter, owing to their increased activity.

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learning to juggle balls:
Draganski et al. 2004; Boyke et al. 2008.

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intensive study for exams: Draganski et al. 2006.
Draganski et al. 2006.

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Korbinian Brodmann:
His map spanned the neocortex, which is the predominant part of the cerebral cortex. Confusingly, the term
cortex
often serves as an abbreviation for the neocortex alone. Brodmann divided the cortex into forty-three areas (Brodmann 1909), but not all are visible in Figure 11, which includes only one view of the cerebrum. If you look closely, you'll notice that 52 is the map's largest number, not 43. That's because Brodmann skipped 12–16 and 48–51. He reserved these numbers for cortical areas in animals that appeared to have no analogues in the human cortex. Brodmann used a microscope to delineate the areas, as I'll describe in Chapter 10. However, the areas line up roughly with the cortical folds, so they can be located approximately even without a microscope.

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after three months:
Cramer 2008.

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after stroke:
Cramer 2008.

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removing one hemisphere:
Mathern 2010. The procedure is justified, for example, when MRI reveals a one-sided brain abnormality that is clearly the cause of the seizures.

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walk and even run:
Vining et al. 1997. For inspiring testimonials by patients, see
http://hemifoundation.intuitwebsites.com
.

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migrate to the right hemisphere:
Basser 1962 discusses very early childhood; Boatman et al. 1999, late childhood. The phenomenon was already noted by Broca in the nineteenth century.

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Miguel Nicolelis:
Nicolelis 2007.

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crude act of butchery:
Bagwell 2005. By medieval times the church had taken over the practice of medicine. A 1215 papal edict forbade the clergy from practicing surgery, because contact with blood or bodily fluids was considered contaminating. Surgery was left to barbers, who may have been more effective healers than the university-trained physicians.

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tie off large arteries:
Finger and Hustwit 2003.

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unremarked for so long:
The history of phantom limbs from Paré to Mitchell is surveyed in Finger and Hustwit 2003.

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phantom is not real:
Reilly and Sirigu 2008.

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irritated nerve endings:
This explanation is credited to Descartes by Finger and Hustwit 2003.

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this didn't help:
Ramachandran and Blakeslee 1999.

   
Wilder Penfield:
Penfield and Boldrey 1937.

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V. S. Ramachandran:
Ramachandran, Stewart, and Rogers-Ramachandran 1992. An entertaining and readable account of this research is provided in Ramachandran and Blakeslee 1999. Ramachandran's discoveries in humans were probably not surprising to Mike Merzenich and other neuroscientists who had already found similar phenomena in animals, as reviewed in Buonomano and Merzenich 1998.

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sensation of a phantom limb: This description may sound incomplete, because I've spoken only of functions and avoided inputs and pathways, which are discussed later in this book. It's more revealing to say that amputation deprives the lower-arm territory of inputs from sensory pathways. Remapping replaces these with sensory inputs from the face and upper arm.
This description may sound incomplete, because I've spoken only of functions and avoided inputs and pathways, which are discussed later in this book. It's more revealing to say that amputation deprives the lower-arm territory of inputs from sensory pathways. Remapping replaces these with sensory inputs from the face and upper arm.

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stroked the face of an amputee:
There was even a one-to-one mapping between facial locations and digits of the phantom hand (cheek to thumb, chin to pinkie, and so on).

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Functional MRI:
More precisely, fMRI measures the BOLD (blood oxygen level dependent) signal, which was discovered by the Japanese scientist Seiji Ogawa. This is defined as the ratio between the oxygenated and deoxygenated forms of hemoglobin, the molecule in the blood that ferries oxygen from the lungs to the rest of the body. Using a brain region has two opposing effects on the BOLD signal. First, the region burns more energy, which deoxygenates hemoglobin. Second, blood flow increases, which carries in more oxygenated hemoglobin. (Many believe that blood flow increases in response to use, because the brain precisely regulates blood flow to fulfill the energy needs of each region.) Since either of these effects can dominate, using a brain region can either increase or decrease the BOLD signal, which confuses the interpretation of fMRI. On a related note, since the BOLD signal reflects energy consumption, some people quip that using fMRI to understand the brain is like trying to understand the engine of a car by measuring where it gets the hottest.

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“spots on brains”:
These images give the misleading impression that a person uses a small fraction of the brain for any given task. However, each image is actually obtained by subtracting two images corresponding to two similar mental tasks. A “lit-up” region was used
more
in one task than the other. One should not conclude that all the other regions lay idle. Many of them were active, but the level of activity was similar in both tasks.

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the shift occurred:
Lotze et al. 2001 also demonstrated a similar remapping of area 4 in amputees, and measured brain activity caused by imagined movements of the phantom hand. Researchers also used fMRI to demonstrate remapping of area 4 in stroke patients. The hand representation moved up or down within area 4, depending on the location of brain damage. Further studies found that stroke can cause remapping on a larger scale, affecting distant areas on the same or the other side of the brain (Cramer 2008).

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left-hand representation:
Elbert et al. 1995 used magnetic source imaging rather than fMRI. They found a shift in the average location of the left-hand representation within area 3, which they interpreted as a change in area. But a direct measurement of the size of the representation showed no statistically significant change. They couldn't prove that the shift was caused by musical training, because of the possibility of selection bias. However, the size of the shift was correlated with the age at which musical training began. See Amunts et al. 1997 for a related study using MRI.

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crippling disorders:
Elbert and Rockstroh 2004.

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focal dystonias:
A famous example is the pianist Leon Fleisher, who lost the use of his right hand for thirty-five years but recently made a comeback with both hands after receiving treatment based on injections of Botox into his arm muscles.

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violin and Braille:
Sterr et al. 1998 not only showed an expanded hand representation but argued that the arrangement of the fingers in the representation was disorganized, which might distinguish Braille reading from violin playing.

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frontal lobe in schizophrenics:
Glahn et al. 2005.

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about brain disorders:
Kaiser et al. 2010 and Bosl et al. 2011 are two recent studies characterizing activity in the autistic brain.

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strength with a machine:
Actually the scientific studies use isometric measurements, meaning that the force is measured while the joint angle is held fixed. This is more controlled, because force depends on joint angle. Muscle size is quantified by cross-sectional area (CSA), which is expected to be roughly proportional to the number of fibers and hence to strength.

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correlation coefficients:
You might think it's silly to research this correlation, since common sense tells us it must be strong. Actually this has been surprisingly difficult to establish empirically. Maughan, Watson, and Weir 1983 reported lower correlation coefficients and took the contrarian view that “strength is not a useful predictive index of muscle cross-sectional area.” More recent studies like Bamman et al. 2000 and Fukunaga et al. 2001 appear to agree on stronger correlations, possibly thanks to improvements in measurement methods. Still, many interesting questions remain unanswered. For example, is the relationship between size and strength different for powerlifters and bodybuilders, or for elite athletes and regular people?

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boundaries of Brodmann's map:
Lashley and Clark 1946.

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cortical equipotentiality:
Lashley 1929.

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over 100 million:
The estimate that Brodmann area 17 contains over 100 million neurons is from Huttenlocher 1990.

 

3. No Neuron Is an Island

 

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Figure 13:
Although in the brain no neuron is an island, isolated neurons can be artificially cultured in a plastic dish, as shown in Figure 13. Even this neuron is not truly island-like, though, as its branches actually extend far outside the borders of the image, to form connections with other neurons in the dish. The image was obtained by scanning electron microscopy.

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one million:
If we don't restrict ourselves to the brain, neurites can be longer still. Some neurites travel from the brain to the spinal cord, and others connect the spinal cord to the toes and fingers. And let's not forget that giraffes and whales have neurites too.

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marked “ax” and “sp”:
“ax” marks an axon, and “sp” a dendritic spine, which sticks out of the dendrite like a thorn.

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do not really touch:
Invisible in the image in Figure 14 are various molecules that span the cleft between the membranes of the two neurons and bring them into direct contact. But the whole notion of “touching” starts to break down at even higher magnification. What we call matter consists mainly of empty space between its constituent particles.

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same small set of neurotransmitters:
Eccles et al. 1954 stated the principle that a neuron secretes a
single
neurotransmitter, and attributed it to Sir Henry Dale, who won a 1936 Nobel Prize for his studies of synaptic transmission. Eccles 1976 later revised Dale's Principle to allow for multiple neurotransmitters. Eccles himself shared a 1963 Nobel Prize for his work on synapses. More recently, researchers have found a further exception: neurons are capable of switching from one neurotransmitter to another.

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brain secretes thoughts:
The eighteenth-century French philosopher and physiologist Pierre Cabanis wrote that “the brain secretes thought as the liver secretes bile.”

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send them to specific targets:
In most biological contexts, chemical signaling relies upon the specificity of molecular binding (the lock-and-key mechanism). That's not sufficient to prevent crosstalk between synapses, because many synapses use exactly the same neurotransmitter.

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minimize crosstalk:
That's not to say there is
zero
crosstalk. Some spillover of neurotransmitter is known to occur, and appears important for brain function in certain cases.

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   “
most expensive loveseat”:
Russell 1978.

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67 miles of tangled wire:
Kolodzey 1981.

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insulating material:
Small amounts of crosstalk can still occur because of electrical fields, which penetrate the insulation.