14.2: Lateralization

Almost all mammals are bilaterally symmetrical, with a left half that is more or less a mirror image of the right half. The internal organs, however, often tell a different story. We have a single stomach, liver, and heart, none of which are symmetrical. Even paired organs like the lungs or kidneys, are slightly asymmetrical. The brain can most accurately be thought of as a pair of intimately-connected organs with subtle differences in function.

The brain’s two hemispheres are connected by white matter tracts which allow the two halves to communicate. The largest interhemispheric white matter tract is the corpus callosum, which is made up of 200-250 million axons. If you held a human brain and separated the two hemispheres dorsally along the longitudinal fissure, you would be able to see the fibers of the corpus callosum holding the two halves together. The corpus callosum is about 10 cm (~4 inches) long from anterior to posterior, and the middle part of the structure forms the dorsal-most roof of the lateral ventricles.

In addition to the corpus callosum, there are a handful of other white matter tracts that allow the hemispheres to communicate. The much-smaller anterior commissure is a tenth of the thickness of the corpus callosum, connects the two temporal lobes, and conveys important limbic information such as memory and emotion. The hippocampal commissure is one of the outputs of the hippocampus that connects the structures in the left and right hemispheres. These small white matter tracts are often used as points of reference in imaging studies or surgical dissection.

A pair of researchers, Drs. Ronald Myers and Roger Sperry, were very curious about these pathways of communication between the two hemispheres. In the 1950’s, they wanted to understand how information from one visual field gets conveyed into the opposite hemisphere of the brain. To answer the question of interhemispheric transfer, they conducted experiments in cats. One of their early experiments presented healthy cats with two different boxes, only one of which contained food. An eyepatch was placed over one of the cat’s eyes, and the cat was free to paw at one of the boxes, which if chosen correctly, would yield the food reward. At first, as expected, the cat would choose from the boxes at random, obtaining the food reward 50% of the time. Over multiple trials, as the cat began to learn which box held the food, the success rate rose to picking the rewarded box 100% of the time. When the eyepatch was then moved to the other eye, the cat performed the task correctly 100% of the time, reliably picking the box associated with food.

Then, Myers and Sperry performed two different surgical procedures on the cats. One severed the optic chiasm, which kept visual information in the ipsilateral hemisphere. This ensured that when wearing the eye patch, visual information does not cross into both hemispheres. The other procedure severed their corpus callosum, a process called a corpus callosotomy (or commissurotomy), which limited interhemispheric transfer after visual cortex processing. Between these two interventions, there were four groups of cats: Fully intact, optic chiasm cut, corpus callosum cut, and the experimental group with both optic chiasm and corpus callosum cut..

Figure 14.1 The corpus callosum, indicated in yellow with red arrow, in a coronal slice (left) and seen from the top when both hemispheres are gently pulled apart (right).

The box-selection behavioral experiment was then repeated. As with the intact cats, when the eyepatch was placed over one eye, the experimental cats (both chiasm and corpus callosum severed) initially guessed at the boxes, getting the reward 50% of the time. Again, as before, these animals improved their performance over repeated trials, eventually getting the reward every time. However, after the eyepatch was switched from one eye to the other, these cats essentially had to “start over” with their learning: they picked the rewarded box only 50% of the time, improving to 100% over trials. Because of the surgical procedures, the visual information and associated reward memory in one hemisphere never made it to the other half of the brain - a failure of interhemispheric transfer.

Figure 14.2 Myers and Sperry demonstrated that each hemisphere is capable of learning and storing memories independently.

The two other control groups immediately performed at 100% after the eyepatch was switched over, just as well as the fully intact cats. When the optic chiasm was severed with the corpus callosum intact, the visual information remained in the ipsilateral hemisphere, but after processing in V1, that information passed over the corpus callosum to the contralateral hemisphere. When the corpus callosum was severed with the optic chiasm intact, the visual information made their way into both hemispheres through the optic nerve.

Myers and Sperry then extended their research to humans. Sometimes, commissurotomy is suggested for younger patients with drug-resistant epilepsy. Grand mal seizures are often characterized by uncontrolled electrical activity in one hemisphere, which then crosses the corpus callosum to the other hemisphere before “bouncing back” to the original hemisphere. During the procedure, the surgeon cuts the corpus callosum, and in doing so, keeps the atypical electrical activity isolated in one hemisphere. Patients have significantly fewer and less severe seizures following recovery from the operation.

People who have had this surgery are sometimes called split-brain patients, a population of patients who were extensively studied by Dr. Michael Gazzaniga.. Overwhelmingly, split-brain patients are healthy with no significant changes in intelligence and no dramatic changes in personality. However, some of them do experience deficits in memory and concentration.

Clinical connection: Agenesis of the corpus callosum

In a handful of rare cases, people can be born without a corpus callosum, a condition called agenesis of the corpus callosum (ACC). Some people with ACC develop atypically, experiencing seizures and poor motor control or coordination. An estimated one-fourth of people diagnosed with ACC after birth have some intellectual disability, but most have typical levels of intelligence. They may have subtle abnormal developmental traits, such as a difficulty with processing common social cues (as seen in autism). Notably, the real-life savant who served as the inspiration for the movie Rain Man was born with ACC.

Among split-brain patients, very unique behavioral and cognitive deficits can be observed under specific experimental circumstances. The baseline test begins by briefly showing the patient some visual stimulus, such as a picture of a donut, only in their right visual field, which gets represented in the left visual cortex (refer back to chapter 7.2 for a reminder of the circuitry of the visual system). When asked what the patient had seen, they would report “a donut,” just as any typical person would (because the left hemisphere is highly involved in language and enables the person to report the object verbally).

In a second experiment, both of the patient’s hands are placed on a table hidden behind a screen. An object, such as an apple, is then placed in their right hand. As the patient feels that object, tactile information such as its hardness, diameter, and temperature, ascends contralaterally into the left somatosensory cortex (chapter 8). After the object is removed from their hand, the patient is asked to feel blindly through a collection of objects, all hidden behind the screen, and find a matching object. When doing this task with the right hand, they would be successful in selecting an apple (because the motor system is crossed). However, when the left hand was now tasked with reaching behind the screen to select a matching object, they would not be able to know which object to pick up because this information goes to the right somatosensory cortex (which has no knowledge of the apple). From these data, the researchers concluded that each hemisphere is independently capable of receiving their own sets of somatosensory inputs and storing their own memories. Without an intact corpus callosum, the two hemispheres are unable to share that knowledge, so the sensory and memory information that reaches the left hemisphere isn’t capable of reaching the right hemisphere, which controls the left hand - so the left hand is clueless to the object placed in the right hand.

Figure 14.3 Experimental setup for studying interhemispheric transfer of visual information in humans (left). After fixation on the dot in the center of the visual field, the stimuli (right) are flashed briefly, and the patient is asked the either name the item observed, or reach behind the screen and select a matching object.

Figure 14.4 In split-brain patients, when an apple is placed in the right hand, that information ascends contralaterally but cannot cross hemispheres.

In the next step of the experiment, a different visual stimulus, like a picture of a spoon, is presented to the left visual field, which is initially sent to the right half of the brain. When asked what they saw, they might say “nothing” or “I don’t know.” (because the right hemisphere is not specialized for language and the person is not able to report the object verbally). But, when the patient is asked to reach behind the screen with their left hand, they could successfully select a spoon! (Left hand is controlled by the right brain, which has knowledge of the spoon.) Their right hand, however, couldn’t correctly pick a matching object (since the left brain does not have the information about the spoon). Again, these results demonstrate that each hemisphere is capable of receiving their own contralateral sensory information and storing their own sets of memory.

For most people, who have their corpus callosum intact, information is transferred rapidly between hemispheres. So, when a spoon is shown to our right brain, the left brain learns that information as well, which is why we would be able to select a matching object with our right hand.

Myers and Sperry’s human studies noted an interesting difference in the ability of split-brain patients to respond verbally. When the stimulus was sent into the left hemisphere, either a visual stimulus in the right visual field or an object placed in the right hand, the patients were able to verbalize what they either saw or felt. But, when the stimulus was represented in the right hemisphere, they couldn’t. Their conclusion was that the left hemisphere is much better equipped for language-related functions compared to the right hemisphere. As it turns out, language comprehension and production is heavily lateralized to the left hemisphere. For his work regarding the “effects of disconnecting the cerebral hemispheres”, Dr. Sperry earned the 1981 Nobel Prize.