Introduction For centuries, philosophers and scientists have pondered the question of the relationship between the brain and the mind. In the 1600s, the French philosopher René Descartes
proposed the dualistic theory, which is the belief that the mind and the body are different entities that work independently of each other but still interact. Descartes had problems, however, explaining how the invisible mind could influence a physical brain. He suggested that the mind and brain interact in the pineal gland, which is a small structure in the brain that releases the hormone melatonin.
Nearly all philosophers and neuroscientists today reject dualism. Their objection is that if the mind influences the brain, the mind must therefore be composed of matter or energy. The alternative description of the relationship between the brain and the mind is monism, which is the belief that the universe consists of only one kind of existence.
The identity position is a popular version of monism that most philosophers and scientists support. This view suggests that mental processes and brain processes are the same thing but are described in different terms. For example, if a man sees a speeding truck approaching him as he is crossing a street, fear might be his mental experience and running across the street to avoid the truck is his behavioral response. However, another description of the same experience could include how his brain records and interprets the threatening visual scene and then triggers other physical reactions, such as increases in heart rate and blood pressure and the release of stress hormones. These physical reactions then send a message back to the brain, which instructs the muscles to move, enabling the man to run.
With the help of brain imaging technology, such as magnetic resonance imaging (MRI) and positron emission tomography (PET), much more is known about how the brain and human experience relate to each other. An MRI is a method of imaging the living brain by using a magnetic field and a radio-frequency field to make certain atoms rotate in the same direction, then removing those fields and measuring the energy that the atoms release. The MRI allows neuroscientists to see the structure of the brain without damaging it. PET maps the activity of a living brain by recording the emission of radioactivity from injected chemicals. This allows neuroscientists to see what parts of the brain are active as the person is performing a particular task, such as solving math problems.
The human brain—composed of the hindbrain, the midbrain, and the forebrain—is an amazing structure. It is the only bodily organ with the ability to be aware of itself. This two- to three-pound gelatinous mass makes it possible for human beings to recognize and interpret sensory information, use complex systems of language, store and recall an infinite amount of factual information and a lifetime of experiences, create new ideas, and imagine the unseen.
Hindbrain and Midbrain The hindbrain is the most primitive part of the brain and includes the medulla, the pons, the reticular formation, and the cerebellum. The medulla is just above the spinal cord and controls vital reflexes, such as breathing, heart rate, vomiting, salivation, coughing, and sneezing. The pons and the reticular formation (a set of pathways) mediate alertness and arousal, increasing and decreasing the brain’s readiness to respond to stimuli. The cerebellum (from a Latin word meaning “little brain”) is a large hindbrain structure that helps with balance and the coordination of motor movement. People who have a damaged cerebellum stagger when they walk and often lose their balance.
The midbrain in adult mammals is small and surrounded by the forebrain. In lower animals, such as birds, reptiles, and fish, it is larger and more prominent. The midbrain includes part of the reticular formation as well as other pathways and provides a route for important sensory information to reach the forebrain quickly.
Forebrain The forebrain consists of the limbic system, the thalamus, and the cerebral cortex. The limbic system includes several structures that mediate emotions and primary motivations, such as hunger, thirst, sex drive, and memory. These structures include the amygdala, the hypothalamus, and the hippocampus.
The amygdala plays an important role in the experience of fear and anxiety. Both fear and anxiety are “escape emotions,” in that reacting to the emotion, for example by fleeing from a threatening stimulus such as a poisonous snake, causes the intensity of the emotion to diminish.
The hypothalamus is a pea-sized structure located near the base of the brain. It contains a number of distinct parts, some of which regulate the release of certain hormones. The hypothalamus also forms the biological basis of motivating behaviors that are crucial for survival, such as eating, drinking, temperature regulation, sexual behavior, and the fight-or-flight response.
The hippocampus (from a Latin word meaning “sea horse,” so called because of its shape) is a large structure in the limbic system that is important in the creation of new memories. Memories are not stored in the hippocampus; rather, it is like a factory that creates new memories and then sends them to other areas of the brain for storage. A famous case study of a man known as HM illustrates the function of the hippocampus. HM had severe seizures that originated in the hippocampus and could not be controlled by medication. His hippocampus was surgically removed in a drastic attempt to eliminate his debilitating seizures. As a result, HM developed anterograde amnesia, which is the inability to form new memories. All the memories that had been stored in his brain before the surgery were unharmed. HM was essentially stuck in a perpetual present. He could read the same newspaper repeatedly and have no memory of ever reading it before. He could be introduced to a stranger and have a normal conversation, but when the person left the room and later returned, he would have no recollection of meeting the person or having the conversation.
The thalamus is also part of the forebrain. It looks like two small footballs in the center of the brain. The thalamus is a relay center for sensory information. Sensory receptors, such as those located in the retina or the eardrum, absorb physical stimuli from the environment, such as light or sound waves. The sensory systems send the information to the thalamus, which relays it on to the cerebral cortex for further processing. One exception is the olfactory system (the sense of smell), which has a direct connection to the cortex.
Cerebral Cortex The cerebral cortex (“cortex” being a Latin word meaning “bark” or “covering”) is the outer part of the brain, which is most developed in humans. The cortex consists of two hemispheres, one on the left and one on the right. The corpus callosum connects the two hemispheres, allowing communication between them. Each hemisphere of the cortex is composed of four lobes: the occipital, parietal, temporal, and frontal lobes.
The occipital lobes, which are located in the back of the brain, process visual information. The receptor cells in the retina of the eye absorb light and then send the information along the optic nerve to the thalamus, which then relays it to the occipital lobes. Visual perception, which is the process of recognizing and interpreting sensory information, occurs in the occipital lobes. If certain parts of the occipital lobes are damaged, it may result in visual agnosia, which is the inability to interpret visual information. In his book The Man Who Mistook His Wife for a Hat (1985),neurologist
Oliver Sacks describes a music teacher, Dr. P, who had a visual agnosia. Dr. P first noticed that something was amiss when he became unable to recognize his students. However, he could recognize them when they spoke, since his auditory perception was intact. Sacks describes an interview with Dr. P in which he showed him a glove and asked him to identify it. Dr. P examined the glove carefully and described it as some sort of container with five pouches that could be used to carry coins of different sizes. He was able to describe the different features of the object, indicating that he was not blind, but was unable to recognize the object as a glove.
The parietal lobes lie between the occipital lobes and the central sulcus, which is one of the deepest grooves in the surface of the cortex. The parietal lobes are important for processing tactile information (the sense of touch), as well as body position and location. The lobe of each hemisphere records and interprets tactile stimulation from the opposite side of the body. A quarter placed in a person’s right hand will be recognized by the agency of the left parietal lobe, for instance. The parietal lobes also play an important role in proprioception, a sensory system that gives the brain information about the position and movement of the body without relying on vision.
People who experience damage to the right parietal lobe sometimes show a fascinating condition called hemi-inattention, or hemispatial neglect. When this occurs, the person is unable to attend to the left side of the body and the world. A person with hemi-inattention may shave or apply makeup to only the right side of the face. While dressing, he or she may put a shirt on the right arm but leave the left side of the shirt hanging behind the body. The person may eat from only the right side of the plate, not noticing the food on the left side. This condition is not due to visual problems or the loss of sensation on the left side of the body; rather, it is a deficit in the ability to direct attention to the left side of the body and the world.
The temporal lobes of the cortex are located on each side of the head, near the temples. They play an important role in recognizing and interpreting auditory information. Temporal lobes enable a person to identify familiar sounds, such as a police siren or a crying baby. Wernicke’s area (named after Carl Wernicke, the neurologist who discovered its function), located in the left temporal lobe, mediates the ability to understand spoken language. When this area of the temporal lobe is damaged, the person may exhibit Wernicke’s aphasia, a condition marked by poor language comprehension and difficulty remembering the names of objects.
The frontal lobes, located just behind the forehead, are specialized to control motor movements, spoken language, and higher-level thinking skills. The decision to scratch one’s head or turn the page of a book is transmitted by the frontal lobes through the spinal cord to the muscles needed to perform the task. Like tactile sensation in the parietal lobes, each frontal lobe controls the opposite side of the body.
Broca’s area (named after Paul Broca, the neurologist who discovered its function) is located in the frontal lobe, close to the motor area that controls facial and tongue movements. Most people who experience damage to this area in the left frontal lobe exhibit Broca’s aphasia, which is an impairment of expressive language. People with Broca’s aphasia may speak only in nouns and verbs, omitting other parts of speech such as prepositions, conjunctions, adjectives, adverbs, helping verbs, and word endings that indicate number or tense. They have particular trouble applying grammar rules for word order, although their pronunciation may be adequate. People with Broca’s aphasia who communicate via sign language also show such difficulties, even though they are able to use their hands well in other ways.
The frontal lobes are also important in planning, initiating, and inhibiting behavior. They help people adapt to changes in the environment, including developing strategies to solve problems, monitoring the progress of such strategies, and being able to switch tactics when necessary.
In 1848, Phineas Gage, a railroad worker, suffered a severe accident in which an explosion caused an iron rod to pierce his cheek, slice through his frontal cortex, and emerge through the top of his head, where it lodged. Miraculously, Gage survived, but he experienced a dramatic personality change. He went from being a gentle, competent worker to being aggressive, emotionally volatile, and incapable of functioning normally. This case is considered the first known natural prefrontal lobotomy. The prefrontal lobotomy, in which the prefrontal cortex is surgically damaged or the connections between the prefrontal cortex and the rest of the cortex are cut, later became a drastic treatment for certain mental disorders.
In the United States, Walter Freeman and James Watts, professors of neurology and neurosurgery, published a report in 1942 titled “Psychosurgery in the Treatment of Mental Disorders and Intractable Pain.” In the late 1940s and early 1950s, about forty thousand prefrontal lobotomies were performed in the United States. The therapeutic goal of such psychosurgeries
was to make difficult, aggressive patients calmer without damaging their sensory or motor abilities. Freeman often employed crude methods in these surgical procedures, sometimes using electric drills or metal picks. He performed many of these operations in his office rather than in a hospital, often carrying his equipment, which he called his “lobotomobile,” around with him. Prefrontal lobotomies were performed on a wide range of people thought to be mentally disordered, with common results including apathy, loss of the ability to plan and take initiative, generally blunted emotions, and the loss of facial expression. Patients also became unable to inhibit socially unacceptable behaviors and tended to act impulsively, without the ability to predict the consequences of their behavior. In the mid-1950s, when effective drug therapies became available to treat many mental disorders, the use of lobotomies declined drastically. Freeman eventually lost his privilege to practice in most hospitals, and other neuroscientists subsequently denounced his practices.
Damage to the frontal lobes due to stroke or other trauma often impairs a person’s ability to initiate and organize behavior, as well as the ability to inhibit socially unacceptable behavior. After frontal lobe damage, a person who was once a model of social grace may become emotionally volatile, behaviorally explosive, and rude. Such people may use crude and profane language that they would never have uttered before the trauma.
Learning and Neuronal Connections Learning results in three different types of memory: semantic, procedural, and episodic. Semantic memories involve encoding and storing factual information, such as the name of one’s first-grade teacher. Learning to type at a keyboard is an example of procedural memory, which itself is a form of implicit memory. When asked to recall the location of the “L” key, most people who know how to touch-type find themselves moving the third finger on their right hand. Recalling letters in the sequence in which they appear on a keyboard without using the fingers to mimic the required movement is a task that most people find quite difficult. This is because the memory is encoded and stored in the brain as a sequence of motor movements rather than as factual information. Memories of life experiences, or episodic memories, are represented in the brain as visual scenes that can be relived through imagination.
How is it that no matter how old a person becomes, there is always room in the brain to store new information and experiences? How can something as simple as learning a new word, which may take seconds to occur, form a memory in the brain that can potentially last a lifetime? The answer to these questions lies in the examination of the brain at the cellular level.
Neurons
are specialized cells that have the ability to communicate with one another electrochemically. Most neurons do not physically touch each other but are separated by small gaps called synapses. When a neuron receives a message from another neuron, it triggers an electrical impulse, which travels from the receiving end of the neuron to the transmitting end. When the electrical impulse reaches the end of the neuron, it causes chemicals known as neurotransmitters
to be released into the synaptic gap. The neurotransmitters then flood across this gap, triggering another electrical impulse in the next neuron, and the sequence continues. Since there are billions of neurons in the human brain and any one neuron can form synaptic connections with hundreds of other neurons, the potential for forming unique patterns of neuronal connections is virtually infinite. When learning a new word, a memory forms as a unique pattern of neuronal connections in the brain. Hearing the word only once and never recalling or using it again will likely result in the fading away of the new pattern of neuronal connections. However, recalling a word and using it repeatedly will likely cause its unique pattern of neuronal connections to become more durable, with the potential to last a lifetime. Thus, the neural representations of everything a person learns and remembers are unique patterns of neuronal connections in the brain.
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