Brain Laterality

Up, Right, Forward

Kenneth M. Heilman, Kenneth Heilman

• Publisher: Routledge
• Year: 2021
• Print ISBN: 9781032073927
• eBook ISBN: 9781000465402
• Topic: Psychology
• Subtopic: Cognitive Psychology & Cognition
• Index: Psychology

Brain Laterality answers one of the major questions we ask ourselves every day: where? The book provides a thematic, comprehensive overview of the brain mechanisms that influence whether we go to the left or right, on which side we stand, and which hand we use.

Covering a broad range of topics, including handedness, apraxia, and motor control, alongside theories of emotion, creativity, and genetics, the book condenses a vast amount of research from multiple fields into a concise and entertaining read.

Featuring anecdotes from the author's own illustrious research and clinical career, this book is a must-read for psychology students, neuropsychologists, neurologists, and anyone interested in the brain's role in handedness, directional movement, intention, action, and posturing.

1

RIGHT AND LEFT

DOI: 10.4324/9781003206682-2

Knowing Left from Right

One of the most frequent decisions we make during our entire life is choosing between left and right. We make this decision when we use our hands and arms. We also make this decision when we are looking for something, or going somewhere when walking or driving. All humans and all animals interact with other objects in space as well as their own body and other people’s body. The first step in any interaction is the need to allocate attention to that object, by directing spatial attention and then perceiving the object and knowing its meaning. When needed or desired, we can then perform an action upon or with this object.

The direction of attention and movements in space can only take place in three body- centered directions, lateral-horizontal (right versus left), vertical (up versus down), and radial (forward versus backward). In this chapter, we are focusing on lateral, right and left. Right–left laterality can be both egocentric (body-head centered) and allocentric (object centered). For example, the letter “C” can appear at the beginning or end of a sentence. If the center of the book is directly in front of the reader, and the C is at the beginning of the first sentence in this book, it will be egocentrically (body-head centered) on the left side. However, independent of location in the sentence, the opening in the letter C is allocentrically (object centered) on the right. Many of the spatial behaviors and cognitive activities performed by animals and humans are right versus left lateralized, and the selection of right- versus left-lateralized actions is determined by neuronal networks in our brain. As will be discussed, different portions of the brain are important for mediating allocentric (object- centered) and egocentric (body-centered) attention.

While eating a lobster, after I finished eating the tail, I looked over the claws and was getting ready to crush the shell and start eating the meat in the claws, which has sweeter meat than does the tail; I noticed that the two claws were different in size and shape. The larger of the two claws is called the “crusher claw” and the smaller claw is called the “pincer.” The larger crusher claw was on this lobster’s right side. I wondered if most lobsters have their crusher claw on their right side. I looked this up and read that 50% have the crusher on the right and 50% have their crusher claw on their left.

Although lobsters are unlike humans, who do have a strong hand asymmetry, lobsters as well as almost all animals have a body and/or action asymmetry. Why do we and other animals have this asymmetry?

If you build a wall that is symmetrical and has two holes with one hole on the left side and the other on the right side and these holes have the same size and shape, and while an animal is watching, you put food into one of these holes, what will this animal do? Even though the food cannot be seen from the outside, almost any hungry animal will have no trouble going to the correct hole, finding, and then eating this food. Since the wall and holes are symmetrical, how can the animal know if the food is in the hole on the right versus left side?

I do not know about lobsters, but almost all mammals can find this hole, because all animals like humans have asymmetries. These asymmetries of the self can help the human and animal localize the right and left sides of their body as well as the left and right sides of space. For example, many animals have a paw preference, and this may allow them to know one side from the other. In humans, it is often handedness that allows us to correctly navigate and know right from left. I recall when I was a young boy, a teacher asked me, “Kenny, in which hand am I holding the chalk.” She was holding the chalk in her left hand. I answered, “I think you are holding the chalk in your left hand.” But how did I know this. My brother Fred would play ball with me and he know that I threw the ball much better with my right than left hand and so he told me, “Kenny just throw with your right hand. You are right-handed.” This was fine with me because it felt so much more natural to throw with my right hand. Therefore, when the teacher asked me this question, I threw a ball in my mind and used my right hand. I therefore knew which hand was my right hand. Since she was facing me, I knew that she was holding this chalk in her left hand. I finally answered one of her questions correctly and she said, “That is right. I mean correct. I hope you were not guessing.”

What Is Handedness?

The Merriam-Webster defines handedness as “a tendency to use one hand rather than the other” and the British Dictionary defines handedness as “a preference for using one hand as opposed to the other.” Thus, on the basis of hand use or hand preference, we divide humans into right-handers, left-handers, and people without a preference to use one hand more than another, called ambidextrous. As defined in these dictionaries, for the most part, people determine their handedness by the hand they most often select to perform tasks that require the use of one hand or arm or the hand that performs the most critical element of an action that requires both hands.

Most studies have revealed about 90% of people are right-handed and the remainder are left-handed or ambidextrous. However, as we will discuss, the hand a person selects to use is often dependent on the type of action they are performing. Although about 90% of people are right-handed, it is not entirely clear what produces this asymmetry. According to Darwin’s evolutionary hypothesis, survival of the fittest, there must have been some survival advantage to being right-handed, but it is not clear what this might have been. One speculative hypothesis has to do with spears and shields. If you were right-handed, you would carry the spear in your right hand and the shield in your left hand. In contrast, if you were left-handed, you would carry the spear in your left hand and shield in the right hand. The human heart is almost always on the left side of the chest. Therefore, carrying the shield with the left hand would be more likely to protect the heart, and during battle, it would be likely that left-handed fighters would be more likely to be killed than those who are right-handed. Unfortunately, this explanation would be very difficult to prove and has many flaws, but I know of no other valid and proven explanations.

There has been an enormous amount of research that has attempted to help us better understand the mechanisms that determine hand preference. However, the brain mechanisms that account for handedness are very complex and still not fully understood.

Crossed Control

The human brain has two hemispheres, a right and left. Each hemisphere is divided into four major lobes: frontal, temporal, parietal, and occipital (Figure 1.1). At the very back (posterior) portion of the frontal lobes, in each hemisphere, is an area called the motor cortex (Figure 1.2).

Figure 1.1The Four Major Lobes of Human’s Cerebral Cortex

The human cerebral cortex has four major sections called lobes. The most anterior (forward), not surprisingly, is called the frontal lobe. The lobe just behind the frontal lobe is called the parietal lobe. Below the lower portion of the posterior (back) portion of the frontal and the anterior portion of the parietal lobe is the temporal lobe. Behind the posterior sections of the parietal and temporal lobe is the occipital lobe.

Figure 1.2The Motor Cortex

The motor cortex, which is at the back end of the frontal lobes, contains the motor neurons that control our movements. These neurons send fibers to the spinal cord, and to the brain stem. The neurons going to the spinal cord are important for activating and controlling limb movements and those going to the brain stem control muscles in the face. The neurons in the motor cortex that control the legs are toward top, the arm and hand in the middle, and those that control the face are on the bottom closest the large fissure that separates the temporal lobe from the frontal and parietal lobes. This fissure is called the Sylvian fissure.

Figure 1.3The Corticospinal Tract

The neurons in the motor cortex (Figures 1.2) send their cables, called axons, down to the bottom of the brain. This part of the brain is called the pons and medulla. In the medulla, most of the cortical spinal tract crosses over to the other side and then goes down to the spinal cord. With this crossing, it is the motor cortex on the left side that controls the right arm and leg and the motor cortex on the right side that controls the left arm and leg.

When a person wants to move their hand, they activate the nerves cells in their motor cortex hand area (Figure 1.2). These nerve cells in this area send their axons (cables) down through the cerebral hemispheres, through the brain stem, and end in the spinal cord. The spinal cord has neurons that send their axons to the muscles that control our movement, and these corticospinal neurons activate these motor neurons in the spinal cord. For control of the upper limb, most of these spinal cord motor neurons are in the cervical (neck) portion of the spinal cord and they carry the neuronal messages to the muscles that control the arm, forearm, hand, and fingers. Before these corticospinal axons reach the spinal cord, in the brain stem (medulla), they cross to the opposite side (Figure 1.3). Therefore, the right hemisphere primarily controls the left arm and hand and the left hemisphere controls the right hand and arm.

The motor cortex is controlled and excited by areas immediately in front of the motor cortex. These areas are called the premotor cortex (Figure 1.4). The premotor cortex does receive some information from the opposite hemisphere by a large cable that connects the two hemispheres, called the corpus callosum (Figures 1.5 and 1.6). However, this premotor cortex in each hemisphere has the strongest connections with the networks in the same hemisphere. Thus, a person’s preference to use one hand rather than the other may strongly depend on the information stored in that hemisphere.

Figure 1.4The Premotor Cortex

There are two areas in each hemisphere that are considered as the premotor cortex. The convexity premotor cortex is immediately in front of the motor cortex. The supplementary motor area is higher up in each hemisphere.

Figure 1.5The Corpus Callosum (Coronal-Vertical View)

The brain has two hemispheres. Each hemisphere stores different forms of knowledge and has different processing networks. The two hemispheres are connected by the corpus callosum. This large connection permits the interhemispheric transfer of information and can also excite or inhibit the functions of the other hemisphere. In this view (Figure 1.6), the brain is cut vertically, in the middle, from the left to right side. The corpus callosum, above the lateral ventricles, is in the center and is connecting the right and left hemispheres.

Figure 1.6The Corpus Callosum (Sagittal-Longitudinal View)

Figure 1.5 shows a coronal-vertical section, where the brain is cut in middle from left to right. This figure is a sagittal section, cut in the middle of the brain between the two hemispheres, from the front to the back. This figure shows the length of the corpus callosum. The middle of the corpus callosum is called the body; the front end is called the genu; and the back the splenium. Each section carries information from the portion of the hemisphere with which it is aligned. Therefore, the anterior (front) part of the corpus callosum carries information between the frontal lobes; the more posterior part or the body carries information from the parietal lobes; and the posterior portion carries information from the occipital lobes.

Hemispheric Laterality of Language and Hand Preference

Franz Joseph Gall (1758–1828), a productive neuroanatomist and a physiologist, is best known as the founder of phrenology. Gall put forth three major hypotheses about the brain’s function, organization, and anatomy including: 1) the human brain is organized in a modular fashion with different anatomic networks mediating different functions, and 2) the larger and better developed the module, the better this network can function. Gall’s third hypothesis was that the shape of the brain determines the shape of the skull. Therefore, Gall thought that measurements of the skull could determine a person’s attributes, which is the basic hypothesis of phrenology. Now we have several means of learning about the relationship of brain anatomy and function, using techniques such as functional magnetic resonance imaging (fMRI), positron emission tomography, and electroencephalography (EEG). Gall thought that skull measurement could reveal the size of portions of the brain. Unfortunately, phrenology became a pseudoscience, where many claims were made without the scientific evidence to support the claims. However, the shape of the brain does influence the shape of the skull, but phrenologists made claims that were not tested.

Areas of the brain that are important for mediating language, as well as many other factors, make the two hemispheres asymmetrical. With the advent of computed tomography (CT), which could reveal the shape of the brain and skull, LeMay (1977) revealed that there were asymmetries of the skull that were related to handedness and language laterality.

Paul Broca, who had a great interest in anthropology, attended a lecture by Aubertin, one of Gall’s disciples. In that lecture, Aubertin proposed that the frontal lobes were important for the production of speech. Paul Broca had a patient who was admitted to a hospital in Paris for a terrible infection, and who had previously had lost his ability to speak but maintained his ability to understand other people’s speech. After this patient died, Broca (1861) found on post mortem examination that this man had a cerebral lesion that was primarily located in his lower (inferior) part of the frontal lobe in his left hemisphere (Figure 1.7). In an important subsequent article, Broca (1865) reported eight right-handed patients who were aphasic and all these patients had injury to their left hemisphere. This report provided strong support for the postulate of modularity, that certain specific areas of the brain perform certain functions not performed by other parts of the brain and that the left hemisphere, in right-handed people, mediates language. In addition, since the left hemisphere controls the right hand and mediates language, based on this report and subsequent studies, it was posited that hand preference is related to hemispheric dominance for mediating language (Broca, 1865).

Figure 1.7Broca’s Area

The patient described by Paul Broca had a larger lesion that is shown in this figure. In addition to injuries of the left inferior frontal lobe, showed in this figure, Boca’s patient had injury to a portion of the superior temporal lobe and even the anterior portion of his parietal lobe.

Language is important not only for communication between people but also for hearing ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• CONTENTS
• INTRODUCTION
• 1 RIGHT AND LEFT
• 2 UP
• 3 APPROACH–STAY–LEAVE
• CONCLUSION
• REFERENCES
• INDEX