12.1: Overview of Brain Mechanisms in Intelligence, Language, and Cognition

Learning Objectives

1. Discuss the general relationship across species between brain size, body size, and intelligence, including trends in corticalization and cortical folding
2. Discriminate the functions of glial cells
3. Distinguish the locations and functions of each of the lobes of the cerebral cortex
4. Describe lateralization and the role of split-brain research
5. Describe neuroplasticity and briefly discuss examples of it

Overview

We begin this chapter with a general review of the brain areas involved in complex cognitive and psychological functions, focusing on the cerebral cortex, including studies of the split brain. This foundational neuroanatomical information is critical in order to fully appreciate the neural mechanisms of human cognition.

The Cerebral Cortex Creates Intelligence, Language, and Thinking: Overview of the Basics

All animals have adapted to their environments by developing abilities that help them survive. Some animals have hard shells, others run extremely fast, and some have acute hearing. Human beings do not have any of these particular characteristics, but we do have one big advantage over other animals—we are very, very smart.

You might think that we should be able to determine the intelligence of an animal by looking at the ratio of the animal’s brain weight to the weight of its entire body. But this does not really work. The elephant’s brain is one thousandth of its weight, but the whale’s brain is only one ten-thousandth of its body weight. On the other hand, although the human brain is one 60th of its body weight, the mouse’s brain represents one fortieth of its body weight. Despite these comparisons, elephants do not seem 10 times smarter than whales, and humans definitely seem smarter than mice.

The key to the advanced intelligence of humans is not found in the size of our brains. What sets humans apart from other animals is our larger cerebral cortex—the outer bark-like layer of our brain that allows us to so successfully use language, acquire complex skills, create tools, and live in social groups (Gibson, 2002). In humans, the cerebral cortex is wrinkled and folded, rather than smooth as it is in most other animals. This creates a much greater surface area and size, and allows increased capacities for learning, remembering, and thinking. The increasing amount of cortex in mammals involving folding of the cerebral cortex is referred to as corticalization.

Although the cortex is only about one tenth of an inch thick, it makes up more than 80% of the brain’s weight. The human cortex contains about 20 billion nerve cells (the entire human brain has somewhere between 100 and 200 billion neurons) and at least 300 trillion synaptic connections (de Courten-Myers, 1999). Supporting all these neurons are billions more glial cells (glia), cells that surround and link to the neurons, protecting them, providing them with nutrients, and absorbing unused neurotransmitters. The glia are essential partners of neurons, without which the neurons could not survive or function (Miller, 2005).

As you recall from the chapter on anatomy of the nervous system, the cerebral cortex is divided into two hemispheres, and each hemisphere is divided into four lobes: frontal, parietal, occipital, and temporal.

If we look at the cortex starting at the front of the brain and moving over the top (see Figure \(\PageIndex{1}\)), we see first the frontal lobe (behind the forehead), which is responsible primarily for thinking, planning, memory, and judgment. Following the frontal lobe is the parietal lobe, which extends from the middle to the back of the skull and which is responsible primarily for processing information about touch and spatial perception. Other parts appear to be involved in visualization. Spatial perception is one component of intelligence measured on IQ tests. Then comes the occipital lobe, at the very back of the skull, which processes visual information. Surrounding the primary visual cortex is visual association cortex which is involved in higher order, more complex visual processing and perception. Finally, in front of the occipital lobe (near the temples and ears) is the temporal lobe, responsible primarily for hearing and language. Also located in the temporal lobe is the Inferotemporal (IT) cortex which is involved in visual recognition.

Figure \(\PageIndex{1}\): The brain is divided into two hemispheres (right and left), each of which has four lobes (temporal, frontal, occipital, and parietal). Furthermore, there are specific cortical areas within the lobes that control different processes. From Brain lobes by Laura Guerin, CC BY-NC 3.0 via CK-12

A relatively large portion of the cortex in each lobe is made up of association areas in which sensory and motor information is combined and associated with our stored knowledge. These association areas of cortex are the places in the brain that are responsible for most of the things that make human beings seem human. The association areas are involved in higher mental functions, such as learning, thinking, planning, judging, moral reflecting, figuring, and spatial reasoning.

The Brain Is Flexible: Neuroplasticity

The control of some specific bodily functions, such as movement, vision, and hearing, is performed in specified areas of the cortex, and if these areas are damaged, the individual will likely lose the ability to perform the corresponding function. For instance, if an infant suffers damage to facial recognition areas in the temporal lobe (FFA), it is likely that he or she will never be able to recognize faces (Farah, Rabinowitz, Quinn, & Liu, 2000). On the other hand, the brain is not divided up in an entirely rigid way. The brain’s neurons have a remarkable capacity to reorganize and extend themselves to carry out particular functions in response to the needs of the organism, and to repair damage. As a result, the brain constantly creates new neural communication routes and rewires existing ones. Neuroplasticity refers to the brain’s ability to change its structure and function in response to experience or damage. Neuroplasticity enables us to learn and remember new things and adjust to new experiences as discussed in chapter 10 on learning and memory.

Our brains are the most “plastic” when we are young children, as it is during this time that we learn the most about our environment. On the other hand, neuroplasticity continues to be observed even in adults (Kolb & Fantie, 1989). The principles of neuroplasticity help us understand how our brains develop to reflect our experiences. For instance, accomplished musicians have a larger auditory cortex compared with the general population (Bengtsson et al., 2005) and also require less neural activity to move their fingers over the keys than do novices (Münte, Altenmüller, & Jäncke, 2002). This is because accomplished musicians through practice have eliminated synaptic connections associated with incorrect movements, making neural control more efficient.

Plasticity is also observed when there is damage to the brain or to parts of the body that are represented in the motor and sensory cortexes. When a tumor in the left hemisphere of the brain impairs language, the right hemisphere will begin to compensate to help the person recover the ability to speak (Thiel et al., 2006). And if a person loses a finger, the area of the sensory cortex that previously received information from the missing finger will begin to receive input from adjacent fingers, causing the remaining digits to become more sensitive to touch (Fox, 1984).

Although neurons cannot repair or regenerate themselves as skin or blood vessels can, new evidence suggests that the brain can engage in neurogenesis, the forming of new neurons (Van Praag, Zhao, Gage, & Gazzaniga, 2004). These new neurons originate deep in the brain and may then migrate to other brain areas where they form new connections with other neurons (Gould, 2007). This leaves open the possibility that someday scientists might be able to “rebuild” damaged brains by creating drugs that help grow neurons.

Research Focus: Identifying the Unique Functions of the Left and Right Hemispheres Using Split-Brain Patients

We have seen that the left hemisphere of the brain primarily senses and controls the motor movements on the right side of the body, and vice versa. This fact provides an interesting way to study brain lateralization—the idea that the left and the right hemispheres of the brain are specialized to perform different functions. Gazzaniga, Bogen, and Sperry (1965) studied a patient, known as W. J., who had undergone an operation to relieve severe seizures. In this surgery the region that normally connects the two halves of the brain and supports communication between the hemispheres, known as the corpus callosum, is severed. As a result, the patient essentially becomes a person with two separate brains. Because the left and right hemispheres are separated, each hemisphere develops a mind of its own, with its own sensations, concepts, and motivations (Gazzaniga, 2005).

In their research, Gazzaniga and his colleagues tested the ability of W. J. to recognize and respond to objects and written passages that were presented to only the left or to only the right brain hemispheres (see Figure \(\PageIndex{2}\)). The researchers had W. J. look straight ahead and then flashed, for a fraction of a second, a picture of a geometrical shape to the left of where he was looking. By doing so, they assured that—because the two hemispheres had been separated—the image of the shape was experienced only in the right brain hemisphere (remember that sensory input from the left side of the body is sent to the right side of the brain). Gazzaniga and his colleagues found that W. J. was able to identify what he had been shown when he was asked to pick the object from a series of shapes, using his left hand, but that he could not do this when the object was shown in the right visual field. On the other hand, W. J. could easily read written material presented in the right visual field (and thus experienced in the left hemisphere) but not when it was presented in the left visual field.

Figure \(\PageIndex{2}\): Visual and Verbal Processing in the Split-Brain Patient. From Introduction to Psychology, Open textbooks for Hong Kong. CC BY-SA 4.0

The information that is presented on the left side of our field of vision is transmitted to the right brain hemisphere, and vice versa. In split-brain patients, the severed corpus callosum does not permit information to be transferred between hemispheres (as it is in normal persons without brain damage), which allows researchers to learn about the functions of each hemisphere. In the perceptual test on the left, the split-brain patient could not choose which image had been previously presented because the left hemisphere cannot process visual information. In the test on the right side of Figure \(\PageIndex{2}\), the patient could not read the passage because the right brain hemisphere cannot process language.

This research, and many other studies following it, has demonstrated that the two brain hemispheres specialize in different abilities. In most people the ability to speak, write, and understand language is located in the left hemisphere. This is why W. J. could read passages that were presented on the right side and thus transmitted to the left hemisphere, but could not read passages that were only experienced in the right brain hemisphere. The left hemisphere is also better at math and at judging time and rhythm. It is also superior in coordinating the order of complex movements—for example, lip movements needed for speech. The right hemisphere, on the other hand, has only very limited verbal abilities, and yet it excels in perceptual skills. The right hemisphere is able to recognize objects, including faces, patterns, and melodies, and it can put a puzzle together or draw a picture. This is why W. J. could pick out the image when he saw it on the left, but not the right, visual field.

Although Gazzaniga’s research demonstrated that the brain is in fact lateralized, such that the two hemispheres specialize in different activities, this does not mean that when people behave in a certain way or perform a certain activity they are only using one hemisphere of their brains at a time. That would be drastically oversimplifying the concept of brain differences. We normally use both hemispheres at the same time, and the difference between the abilities of the two hemispheres is not absolute (Soroker et al., 2005).

Summary

The evolutionarily old brain—including the brain stem, medulla, pons, reticular formation, thalamus, cerebellum, amygdala, hypothalamus, and hippocampus—regulates basic survival functions, such as breathing, moving, resting, feeding, emotions, and memory.

The cerebral cortex, made up of billions of neurons and glial cells, is divided into the right and left hemispheres and into four lobes. The frontal lobe is primarily responsible for thinking, planning, memory, and judgment. The parietal lobe is primarily responsible for bodily sensations and touch. The temporal lobe is primarily responsible for hearing and language. The occipital lobe is primarily responsible for vision. Other areas of the cortex act as association areas, responsible for integrating information. The motor cortex controls voluntary movements. Body parts requiring the most control and dexterity take up the most space in the motor cortex. The sensory cortex receives and processes bodily sensations. Body parts that are the most sensitive occupy the greatest amount of space in the sensory cortex.

The brain changes as a function of experience and potential damage in a process known as plasticity. Neuroplasticity allows the brain to adapt and change as a function of experience or damage. The brain can generate new neurons through neurogenesis.

The severing of the corpus callosum, which connects the two hemispheres, creates a “split-brain patient,” with the effect of creating two separate minds operating in one person. Studies with split-brain patients as research participants have been used to study brain lateralization. The left cerebral hemisphere is primarily responsible for language and speech in most people, whereas the right hemisphere specializes in spatial and perceptual skills, visualization, and the recognition of patterns, faces, and melodies.

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Attributions

Content, including figures, adapted by Kenneth A. Koenigshofer, Ph.D., from Introduction to Psychology (Saylor Foundation), Chapter 3. (CC BY-NC-SA 3.0). Some material on functions of cortical areas was added by Kenneth A. Koenigshofer.

Brain lobes by Laura Guerin, CC BY-NC 3.0 via CK-12

Introduction to Psychology, Open textbooks for Hong Kong. CC BY-SA 4.0

This section was further edited by Alan Keys, Ph.D., Sacramento City College, Sacramento, CA.