The Two Hemispheres

The surface of the brain, known as the cerebral cortex, is very uneven, characterized by a distinctive pattern of folds or bumps, known as gyri (singular: gyrus), and grooves, known as sulci (singular: sulcus), shown in Figure 3.15. These gyri and sulci form important landmarks that allow us to separate the brain into functional centers. The most prominent sulcus, known as the longitudinal fissure, is the deep groove that separates the brain into two halves or hemispheres: the left hemisphere and the right hemisphere.

Figure 3.15 The surface of the brain is covered with gyri and sulci. A deep sulcus is called a fissure, such as the longitudinal fissure that divides the brain into left and right hemispheres. (credit: modification of work by Bruce Blaus)

There is evidence of specialization of function—referred to as lateralization—in each hemisphere, mainly regarding differences in language functions. The left hemisphere controls the right half of the body, and the right hemisphere controls the left half of the body. Decades of research on lateralization of function by Michael Gazzaniga and his colleagues suggest that a variety of functions ranging from cause-and-effect reasoning to self-recognition may follow patterns that suggest some degree of hemispheric dominance (Gazzaniga, 2005). For example, the left hemisphere has been shown to be superior for forming associations in memory, selective attention, and positive emotions. The right hemisphere, on the other hand, has been shown to be superior in pitch perception, arousal, and negative emotions (Ehret, 2006). However, it should be pointed out that research on which hemisphere is dominant in a variety of different behaviors has produced inconsistent results, and therefore, it is probably better to think of how the two hemispheres interact to produce a given behavior rather than attributing certain behaviors to one hemisphere versus the other (Banich & Heller, 1998).

The two hemispheres are connected by a thick band of neural fibers known as the corpus callosum, consisting of about 200 million axons. The corpus callosum allows the two hemispheres to communicate with each other and allows for information being processed on one side of the brain to be shared with the other side.

Normally, we are not aware of the different roles that our two hemispheres play in day-to-day functions, but there are people who come to know the capabilities and functions of their two hemispheres quite well. In some cases of severe epilepsy, doctors elect to sever the corpus callosum as a means of controlling the spread of seizures (Figure 3.16). While this is an effective treatment option, it results in individuals who have "split brains." After surgery, these split-brain patients show a variety of interesting behaviors. For instance, a split-brain patient is unable to name a picture that is shown in the patient’s left visual field because the information is only available in the largely nonverbal right hemisphere. However, they are able to recreate the picture with their left hand, which is also controlled by the right hemisphere. When the more verbal left hemisphere sees the picture that the hand drew, the patient is able to name it (assuming the left hemisphere can interpret what was drawn by the left hand).

The outermost part of the brain is a thick piece of nervous system tissue called the cerebral cortex, which is folded into hills called gyri (singular: gyrus) and valleys called sulci (singular: sulcus). The cortex is made up of two hemispheres—right and left—which are separated by a large sulcus. A thick fiber bundle called the corpus callosum (Latin: “tough body”) connects the two hemispheres and allows information to be passed from one side to the other. Although there are some brain functions that are localized more to one hemisphere than the other, the functions of the two hemispheres are largely redundant. In fact, sometimes (very rarely) an entire hemisphere is removed to treat severe epilepsy. While patients do suffer some deficits following the surgery, they can have surprisingly few problems, especially when the surgery is performed on children who have very immature nervous systems.

Figure 35.20 These illustrations show the (a) coronal and (b) sagittal sections of the human brain.

In other surgeries to treat severe epilepsy, the corpus callosum is cut instead of removing an entire hemisphere. This causes a condition called split-brain, which gives insights into unique functions of the two hemispheres. For example, when an object is presented to patients’ left visual field, they may be unable to verbally name the object (and may claim to not have seen an object at all). This is because the visual input from the left visual field crosses and enters the right hemisphere and cannot then signal to the speech center, which generally is found in the left side of the brain. Remarkably, if a split-brain patient is asked to pick up a specific object out of a group of objects with the left hand, the patient will be able to do so but will still be unable to vocally identify it.

Each cortical hemisphere contains regions called lobes that are involved in different functions. Scientists use various techniques to determine what brain areas are involved in different functions: they examine patients who have had injuries or diseases that affect specific areas and see how those areas are related to functional deficits. They also conduct animal studies where they stimulate brain areas and see if there are any behavioral changes. They use a technique called transcranial magnetic stimulation (TMS) to temporarily deactivate specific parts of the cortex using strong magnets placed outside the head; and they use functional magnetic resonance imaging (fMRI) to look at changes in oxygenated blood flow in particular brain regions that correlate with specific behavioral tasks. These techniques, and others, have given great insight into the functions of different brain regions but have also showed that any given brain area can be involved in more than one behavior or process, and any given behavior or process generally involves neurons in multiple brain areas. That being said, each hemisphere of the mammalian cerebral cortex can be broken down into four functionally and spatially defined lobes: frontal, parietal, temporal, and occipital. Figure 35.21 illustrates these four lobes of the human cerebral cortex.

Figure 35.21 The human cerebral cortex includes the frontal, parietal, temporal, and occipital lobes.

Agenesis of the corpus callosum (ACC) is a brain disorder in which the tissue that connects the left and right sides of the brain (its hemispheres) is partially or completely missing. It is caused by a disruption of brain cell migration during fetal development.

ACC can occur as an isolated condition or in combination with other cerebral abnormalities, including Chiari malformations, Dandy-Walker syndrome, schizencephaly, and holoprosencephaly. ACC can also be associated with malformations in other parts of the body, such as midline facial defects. The effects of the disorder range from subtle or mild to severe, depending on associated brain abnormalities. Children with the most severe brain malformations may have:

• Intellectual impairment
• Seizures
• Hydrocephalus
• Spasticity

People who have a disorder of the corpus callosum typically have:

• Delays in attaining developmental milestones such as walking, talking, or reading
• Clumsiness and poor motor coordination particularly with skills that require coordination of left and right hands and feet, such as swimming, bicycle riding, and driving
• Problems with mental and social processing
• Impairment in social interaction and communication that may overlap with autism spectrum disorder behaviors and typically become apparent between adolescence and adulthood

Females may have a gender-specific condition called Aicardi syndrome, which causes severe cognitive impairment and developmental delays, seizures, abnormalities in the vertebra of the spine, and lesions on the retina of the eye.

Other disorders of the corpus callosum include dysgenesis, in which the corpus callosum is developed in a malformed or incomplete way, and hypoplasia, in which the corpus callosum is thinner than usual. Individuals with these disorders have a higher risk of hearing problems and cardiac abnormalities.

Treatment for ACC usually involves management of symptoms and seizures. It is estimated that at least one in 4,000 individuals has a disorder of the corpus callosum.

Popular media often refer to right-brained and left-brained people, as if the brain were two independent halves that work differently for different people. This is a popular misinterpretation of an important neurological phenomenon. As an extreme measure to deal with a debilitating condition, the corpus callosum may be sectioned to overcome intractable epilepsy. When the connections between the two cerebral hemispheres are cut, interesting effects can be observed.

If a person with an intact corpus callosum is asked to put their hands in their pockets and describe what is there on the basis of what their hands feel, they might say that they have keys in their right pocket and loose change in the left. They may even be able to count the coins in their pocket and say if they can afford to buy a candy bar from the vending machine. If a person with a sectioned corpus callosum is given the same instructions, they will do something quite peculiar. They will only put their right hand in their pocket and say they have keys there. They will not even move their left hand, much less report that there is loose change in the left pocket.

The reason for this is that the language functions of the cerebral cortex are localized to the left hemisphere in 95 percent of the population. Additionally, the left hemisphere is connected to the right side of the body through the corticospinal tract and the ascending tracts of the spinal cord. Motor commands from the precentral gyrus control the opposite side of the body, whereas sensory information processed by the postcentral gyrus is received from the opposite side of the body. For a verbal command to initiate movement of the right arm and hand, the left side of the brain needs to be connected by the corpus callosum. Language is processed in the left side of the brain and directly influences the left brain and right arm motor functions, but is sent to influence the right brain and left arm motor functions through the corpus callosum. Likewise, the left-handed sensory perception of what is in the left pocket travels across the corpus callosum from the right brain, so no verbal report on those contents would be possible if the hand happened to be in the pocket.