The cerebrum is responsible for complex behavior and consciousness

The cerebrum is the dominant structure in the mammalian brain. In humans it is so large that it covers all other parts of the brain except the cerebellum (Figure 46.4A). Layers of cells at the surface of the cerebrum form the cerebral cortex, which is about 4 millimeters thick and is folded into ridges (gyri; singular gyrus) and valleys (sulci; singular sulcus). If the cortex were flattened out, it would be about 1 square meter, but the foldings, or convolutions, enable that extensive surface area of cortex to fit within the skull.

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Figure 46.4 The Human Cerebrum (A) Each cerebral hemisphere is divided into frontal, temporal, parietal, occipital, and insular lobes. (B) Different functions are localized in particular areas of the four cerebral lobes.

Activity 46.1 The Human Cerebrum

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As we explore the infoldings of the cerebral cortex and other parts of the brain, we will occasionally mention an individual whose brain was damaged by an accident or other unfortunate event. Until recently the study of such individuals has been the main source of functional information about the human brain, but new imaging technologies such as positron emission tomography (PET) and magnetic resonance imaging (MRI) are providing a wealth of new information and opportunities to study the human brain.

A curious feature of the human nervous system is that the left side of the body is served (in both sensory and motor aspects) mostly by the right side of the brain, and the right side of the body is served mostly by the left side of the brain. Thus sensory input from the right hand goes to the left cerebral hemisphere, and sensory input from the left hand goes to the right cerebral hemisphere. The exception is the head, where the left side is controlled by the left cerebral hemisphere and the right side by the right cerebral hemisphere. The two hemispheres are not symmetrical with respect to all functions. Language abilities, for example, reside predominantly in the left hemisphere in most people.

Different regions of the cerebral cortex have specific functions (Figure 46.4B). Some of those functions are easily defined, such as receiving and processing sensory information or generating motor commands, but in most humans most of the cortex is involved in higher-order information processing that is less easy to define. These latter areas are given the general name of association cortex, so named because they integrate, or associate, information from different sensory modalities and from memory.

To understand the cerebral cortex, it helps to have an anatomical road map. Viewed from the left side, the left cerebral hemisphere looks like a boxing glove for the right hand with the fingers pointing forward, the thumb pointing out, and the wrist at the rear. The “thumb” area is the temporal lobe, the fingers the frontal lobe, the back of the hand the parietal lobe, and the wrist the occipital lobe (see Figure 46.4A). What you can’t see from the external view is the insular cortex, which is folded in between the thumb and finger areas of the boxing glove (between the frontal/parietal and the temporal lobes). The right cerebral hemisphere shows a mirror image of this arrangement. We will look at each lobe separately.

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investigating life

Place Cells Reveal Processes of Memory Consolidation during Sleep

experiment

Original Paper: Lee, A. K. and M. A. Wilson. 2002. Memory of sequential experience in the hippocampus during slow wave sleep. Neuron 36: 1183–1194.

Multiple hippocampal cells were recorded while a rat ran a maze. Individual cells fired only when the rat was at particular locations in the maze; thus the rat’s progress through the maze can be represented by a specific sequence of firing of these “place cells.” Since memory is consolidated during sleep, are the sequences of place cells expressed during the maze-running experience replayed and processed into long-term memory during sleep?

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work with the data

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To conclude that a particular sequence of cell firing observed during sleep is a significant event—meaning that it could be a replay of the waking experience—we have to compare it with the probability that it would occur by random chance. And we need to consider whether an observed sequence that includes some but not all of the components of the baseline “word” is nevertheless a significant event.

What is the minimum number of elements that would have to match the baseline sequence for it to be considered a significant event with P < 0.01? To find out, we need to calculate the probabilities of the recorded neurons firing randomly and creating the sequence matches. First, let’s consider the probability of a recurrence by chance of the exact nine-“letter” baseline sequence being recorded in a single burst of that group of nine cells. The total number of possible combinations of those nine letters can be calculated as the factorial 9!:

9! = 9 × 8 × 7 × 6 × 5 × 4 × 3 × 2 × 1 = 362,880

In other words, the probability of repeating the entire nine-letter sequence would be 1 out of 362,880. That repeat would have an extremely low probability of occurring and would therefore be a highly significant event. But what if the recorded sequence has only six of the nine cells firing in the proper order? How do you determine the probability of that? First, calculate the probability of a combination of six cells out of the nine. So for the first cell you have nine choices, for the second you have eight choices, and so forth. Then you have to calculate the probability of that combination of six cells being in the proper sequence, and that will be 1 out of the total number of possible arrangements of those six cells, or 6!. Then what do you do for probabilities of successive events? You multiply the probabilities. For example, if you flip a coin, the probability of heads is 0.5. If you flip it twice, the probability of getting heads both times is 0.5 × 0.5, or 0.25.

QUESTIONS

Question 1

What is the probability of the sequence 1, 4, 6, 7, 8, 9 out of a random firing of nine cells?

The probability of this sequence out of a random firing of nine cells is first a function of the probability of that one combination from all of the combinations of cells that could constitute six elements of the nine possible. That would be 1/(9 × 8 × 7 × 6 × 5 × 4) = 1/60,480 = 1.6 × 10–5. Then, for the sequence, what is the probability that it would be in the correct order? There is one possible correct order of those six elements in 6 × 5 × 4 × 3 × 2 × 1 possible combinations or 1 in 720, thus the probability would be 1.4 × 10–3. So the probability of recording those specific six elements out of nine in the correct order would be 2.24 × 10–8.

Question 2

What is the probability of a sequence of four letters occurring out of a random firing of nine cells?

The probability of recording four elements in the sequence in the proper order would be: 1/(9 × 8 × 7 × 6) = 1/4,698 = 2.1 × 10–4, and the probability of getting those four elements in the proper order would be 1/24. Thus the probability of recording those specific four elements out of nine in the correct order would be 2.1 × 10–4 × 0.042 = 0.88 × 10–3.

Question 3

Are both of the recordings from Questions 1 and 2 low-probability events?

Both of these recordings have a probability of < 0.01, so they would be considered significant low-probability events.

Question 4

What do you note about the time frame of the sequence during sleep versus during wakefulness?

The time frame of the sequence recording during sleep is about five times faster than the occurrence of that sequence during wakefulness when the rat is running the maze.

A similar work with the data exercise may be assigned in LaunchPad.

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THE TEMPORAL LOBE The upper region of the temporal lobe receives and processes auditory information. The association areas of this lobe are involved in recognizing, identifying, and naming objects. Damage to the temporal lobe results in disorders called agnosias, in which the individual is aware of an object but cannot identify it.

Damage to a specific region of the temporal lobe results in the inability to recognize faces. Even old acquaintances cannot be identified by facial features, although they may be identified by other attributes such as voice, body features, and posture. Damage to other association areas of the temporal lobe can cause deficits in understanding spoken language, although speaking, reading, and writing abilities may be intact.

THE FRONTAL LOBE The frontal and parietal lobes are separated by a deep valley called the central sulcus. A strip of the frontal lobe cortex just in front of the central sulcus is called the primary motor cortex (see Figure 46.4B). The neurons in this region control muscles in specific parts of the body; the parts of the body map onto the primary motor cortex, with the head represented in the lower outside region and the legs and feet in the top region near the midline. The graphic drawing of body parts overlying the diagram of the cortex is called a homunculus, and its distortion represents disproportionate innervation. Parts of the body with fine motor control, such as the face and hands, have disproportionate representation (Figure 46.5A). Electrical stimulation of neurons in different regions of the primary motor cortex causes specific muscles of the body to twitch. Just anterior to the primary motor cortex is the pre-motor cortex, which is involved in planning more complex and coordinated movements.

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Figure 46.5 The Body Is Represented in Primary Motor and Primary Somatosensory Cortexes Neurons in the primary motor cortex (A) control muscles in specific parts of the body, while neurons in the primary somatosensory cortex (B) receive information from specific parts of the body. The locations of these neurons within each cortex correspond to “maps” on which regions of the body are represented in proportion to the amount of innervation they receive.

The association functions of the frontal lobe are diverse and best described as having to do with feeling and planning. They are said to have executive function and they contribute significantly to personality. People with frontal lobe damage have drastic alterations of personality and difficulty planning future events. A dramatic case of frontal lobe damage is that of Phineas Gage, who in 1848 was an industrious and responsible young railroad construction foreman. Then a blasting accident shot a meter-long, 3-centimeter-wide iron tamping rod through his brain. The rod entered Gage’s head below his left eye, passed through his frontal lobe, and exited the top of his head (Figure 46.6).

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Figure 46.6 A Mind-Altering Experience Phineas Gage miraculously survived a nineteenth-century railroad construction accident that blew an iron rod through his brain. His personality, however, was permanently altered from that of a responsible foreman to a sometimes quarrelsome drifter.

Remarkably, Gage survived, but he had a different personality. In the years following his recovery, he was quarrelsome, impatient, obstinate, and used profane language, which he did not do before. He lost his railroad job and earned money by telling his story and exhibiting his scars (and the tamping iron). Several years later, however, he took a job as a stage coach driver in Chile—a challenging job indicating considerable behavioral recovery. He died of a seizure in 1860, at the age of 38. If you are in Boston, you can pay him a visit—his skull, death mask, and the tamping iron are on display in the Warren Anatomical Museum of Harvard Medical School.

THE PARIETAL LOBE The strip of parietal lobe cortex just behind the central sulcus is the primary somatosensory cortex (see Figure 46.4B). This area receives touch and pressure information relayed from the body through the thalamus.

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As with the primary motor cortex, the entire body surface can be mapped onto the primary somatosensory cortex (Figure 46.5B). Areas of the body that have a high density of tactile mechanoreceptors and are capable of making fine discriminations in touch (such as the lips and fingers) have disproportionately large representation. If a very small area of the primary somatosensory cortex is stimulated electrically, the subject reports feeling specific sensations, such as touch, in a localized part of the body.

A major association function of the parietal lobe is attending to complex stimuli. Damage to the right parietal lobe causes a condition called contralateral neglect syndrome, in which the individual tends to ignore stimuli from the left side of the body or the left visual field. Such individuals have difficulty performing complex tasks, such as dressing the left side of the body; an afflicted man may not be able to shave the left side of his face. When asked to copy simple drawings, a person who exhibits this syndrome can do well with the right side of the drawing but not the left.

The parietal cortex is not symmetrical with respect to its role in attention. Damage to the left parietal cortex does not cause the same degree of neglect of the right side of the body. You will see similar asymmetries in cortical function later in the chapter when we discuss language.

THE OCCIPITAL LOBE The occipital lobe receives and processes visual information. The association areas of the occipital cortex are essential for making sense of the visual world and translating visual experience into language. Some deficits resulting from damage to these areas are specific. In one case, a woman with limited damage was unable to see motion. Her vision was intact, but she could see a waterfall only as a still image, and an approaching car only as a series of a stationary object at different distances.

THE INSULAR CORTEX The insular cortex, buried deep in the forebrain, receives a great variety of afferent information. The posterior regions receive somatosensory information (e.g., touch, pain, temperature); the middle region combines that with autonomic regulatory information and drives (e.g., hunger, thirst, sex); and the anterior region, which is well developed in higher primates, elephants, and marine mammals, receives information about social interactions. Thus in mammals the insular cortex appears to integrate physiological information from all over the body to create a sensation of how the body “feels,” and in the higher mammals it may extend that function into a sense of self.