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The Neocortex

The human neocortex is huge; if it were stretched out, it would be much too large to fit in the skull. This is why the human brain is all folded on the surface. The neocortex has many convolutions or folds that enable it to cram a large number of neurons into a head small enough to be supported by the human neck.

The neocortex is highly developed in humans and is responsible for many of our most complex behaviors, including language and thought. Although some of the func-tions of neocortical regions are not well understood, neuroscientists agree that within the neocortex, there is localization of function. This means that certain parts of the neocortex are important for specific behaviors or abilities.

At the most macroscopic level, the neocortex can be subdivided into four different parts or lobes, as shown in Figure 4-9: occipital, temporal, parietal, and frontal. Within each of these regions, there are two major classifications:

1. Primary sensory and/or motor areas. These areas are responsible for processing basic information about the senses as well as for producing signals that lead to voluntary movement. As we will see, many of the primary sensory and motor parts of the neocortex process information related to the opposite, or contralateral, side of the body.

Parallel processing Air traffic controllers must react to an array of sensory stimuli and make quick deci-sions. Communication among the association cortex within and between the lobes of the brain allows us to perform such complex functions simultaneously.

FIGURE 4-9The lobes of the neocortex The neocortex can be subdivided into four lobes—frontal, parietal, temporal, and occipital.

2. Association cortex. The association cortex in each region is responsible for many complex functions, including higher-order sensory processing, integrating information from different senses (how you know that an object that looks like a violin is producing the music), thinking, planning, and other complex functions.

The Occipital Cortex The occipital cortex, the cortical area at the back of the skull, contains primary sensory regions important for processing very basic informa-tion about visual stimuli, such as orientainforma-tion and lines. As shown in Figure 4-10, visual information arrives in the occipital cortex through partially crossed connections. The visual information from each eye that is closest to the midline between the two eyes is actually projected to the opposite side of the brain. As a result, the representation of your left visual field is on your right primary visual cortex, and vice versa.

association cortex areas of the neocortex responsible for complex functions, including higher-order sensory processing, thinking, and planning.

occipital cortex lobe of the neocortex at the back of the skull, important for processing very visual information.

Left visual field

Right visual field

Visual area of left hemisphere

Visual area of right hemisphere FIGURE 4-10 The visual system is a partially crossed pathway

Visual cues from the temporal (toward the side) part of the visual field are sent to the opposite side of the brain, while those of the medial (toward the nose) part of the visual field are transmitted to the same side of the brain.

Structures of the Brain 131

Broca's area

Wernicke's area

Association areas in the occipital cortex integrate information about color, com-plex patterns, and motion. Since vision is such an important sense for primates, the occipital cortex is very well developed in humans. Although the occipital cortex is often referred to as the visual cortex, it’s important to realize that visual

informa-tion is also processed in other parts of the neocortex. Some estimates sug-gest that 25 percent of the human neocortex is devoted to some sort of visual task! Connections to other parts of the neocortex enable us to hook up visual information with information from other sensory modalities as well as with our memory stores (for example, connecting the sight of a potato chip with its smell, taste, sound when crunched, feel, and memories of having that type of food in your past). This serves as an important reminder that no brain region operates entirely on its own. Each receives input from other areas and communi-cates with many other regions to produce integrated responses.

The Temporal Cortex The temporal cortex is located on the sides of the head with-in the temporal lobe. It wraps around the hippocampus and amygdala. The temporal cortex includes areas important for processing information about auditory stimuli, or sounds. Abnormal electrical activity in the temporal cortex, such as what occurs with seizures or epilepsy, has been shown to result in auditory hallucinations. People who have epileptic seizures centered in this region sometimes “hear” in their minds very loud music during seizures. Neurosurgery to remove a region causing seizures is particularly dangerous in this part of the brain because so many critical functions may be disrupted.

For instance, the temporal cortex also contains regions important for language comprehension (Damasio et al., 2004). Shown in Figure 4-11, this area, called Wernicke’s area, is located on the left side of the brain in the vast majority of humans (over 90 percent). (This is a good example of lateralization of function; this phenom-enon means that the particular ability is localized to one side of the brain. We will return to this general issue later in the chapter.) Wernicke’s area communicates with other areas, including a region located in another cortical area important for recogniz-ing appropriate syntax (language rules) and producrecogniz-ing speech.

In addition to the temporal cortex involvement in hearing and language compre-hension, this lobe plays important roles in learning and memory as well as in recogniz-ing objects through visual cues. Regions of the temporal cortex respond to complex visual stimuli, such as faces (Gross, 2005). Neuroimaging studies have shown that parts of this brain region are activated when people view photos of faces, particularly those of familiar faces. In additional studies, recording electrodes placed into these same brain regions show changes in neuronal activity, or firing rate, when the same complex visual stimuli are presented (Seeck et al., 1993). At first consideration, the presence of neurons that respond to faces in the temporal cortex might suggest that direct projections from the eye activate a set of cells in the temporal lobe that are pro-grammed to respond to complex visual stimuli. This is not the case though. The “face cells” in the temporal cortex respond to faces because they receive inputs from visual areas in the occipital cortex as well as memory centers in the brain, allowing for the recognition of faces previously seen.

The Parietal Cortex The parietal cortex is localized on the top middle of the brain.

The primary sensory parts of this cortical region are critical for processing information about touch or somatosensory stimuli: our senses of touch, pressure, vibration, and pain. The parietal cortex contains a region known as the somatosensory strip; this band of cortex processes tactile information about different body parts. As Figure 4-12 shows, this area of the brain forms a systematic body map, but one in which some parts of the body are represented more than others. For instance, somatosensory informa-tion about the lips (which are particularly sensitive) occupies a greater amount of cor-tex than does somatosensory information about the elbow.

temporal cortex part of the neocortex important in processing sounds, in speech comprehension, and in recognizing complex visual stimuli, such as faces.

Wernicke’s area an area of the temporal cortex important in helping us understand language.

parietal cortex lobe of the neocortex involved in processing information related to touch and complex visual information, particularly about locations.

somatosensory strip an area of the parietal cortex that processes tactile information coming from our body parts.

FIGURE 4-11Major brain regions important for speech production and language comprehen-sion Broca’s area, located in the frontal lobe, is criti-cal for speaking and Wernicke’s area, located in the temporal lobe, is critical for understanding language.

KneeHip

corpus callosum brain region that allows communication from one side of the neocortex to the other.

hemispheres halves of the brain.

The parietal cortex also plays an important role in the higher-order processing of visual stimuli. As we will see in Chapter 5 on sensation and

per-ception, processing visual stimuli in-volves localizing visual cues in space.

The parietal cortex contains a system known as the “where pathway” that enables us to see

and respond to visual informa-tion in a spatially appropriate

way. People with damage to the “where pathway” can

find it impossible to pour water from a pitcher into a glass. This deficiency

is not due to a motor disturbance, but rather to an inability to prop-erly determine where the glass is located rela-tive to the pitcher.

The Frontal Cortex Located at the front of the brain (behind the forehead) is the frontal cortex. The frontal cortex is a relatively large cortical region and is proportionately larger in humans compared to less complex animals. Like the other cortical regions, however, the frontal cortex is not just one area, but a large collection of regions that serve numerous functions. The frontal cor-tex is important for planning and movement. Voluntary movements begin in the frontal cortex, in a part referred to as the primary motor strip, also shown in Figure 4-12. It has long been known that stimula-tion of different parts of the primary motor strip invokes movement in specific groups of muscles. However, recent research suggests that parts of the motor cortex are not just involved in contracting specific muscles. They are also im-portant in coordinating the use of these muscles in complex movements (Graziano, 2006).

Brain Implants. With their ever more sophisticated knowledge of brain struc-ture and function, neuroscientists are developing new methods to treat mal-functions in the brain. For example, scientists at the University of Pittsburgh are currently testing brain implants that they hope will enable paralyzed patients to move prosthetic arms with their thoughts. In some of these experiments, electrodes implanted in the motor cortex of monkeys will interpret brain signals and send them to a computer, which will in turn control the movement of a prosthetic arm and hand (Velliste et al., 2008). People who have suffered limb amputations might someday benefit from this research.

In addition to its role in controlling movement, the frontal cortex contains a region called Broca’s area, which is critical for speech production. Individuals with damage to this region, or to the connections between Wernicke’s and Broca’s area, find it impos-sible to generate speech, despite normal language comprehension.

The part of the frontal cortex closest to the front of the head, the prefrontal cortex, is important for a large number of functions. Among them is short-term memory or working memory (Soto et al., 2008). When you call information for a phone number and hold that number in your mind while you dial, you are using frontal cortex lobe of the neocortex involved

in many functions, including movement and speech production.

Broca’s area brain region located in the frontal lobe that’s important for speech production.

prefrontal cortex portion of the frontal cortex involved in higher-order thinking, such as memory, moral reasoning, and planning.

FIGURE 4-12Motor and sensory cortices are organized according to body parts Areas of motor cortex that control the movement of specific body parts and those of somatosensory cortex that receive tactile information are grouped according to body parts. Some regions are overrepresented, including the mouth and hands.

Structures of the Brain 133 your prefrontal cortex. In addition, when you execute complex

plans, such as planning a party and inviting friends on Facebook or creating a Facebook page for the first time, you are using your prefrontal cortex.

Moral reasoning (discussed in Chapter 3) has also been localized, at least in part, to a component of the prefrontal cortex. Children with damage to the prefrontal cortex can have difficulty understand-ing ethical principles despite normal IQ (Anderson et al., 1999). The prefrontal cortex has also been implicated in some aspects of mood regulation. Studies have shown that individuals with a positive out-look on life tend to have more activity on one side of their prefrontal cortex (Urry et al., 2004).

One of the earliest examples of localization of function involved an individual who sustained damage to the prefrontal cortex. In the mid-1800s, a railroad worker named Phineas Gage experienced se-vere brain damage when a metal railroad spike penetrated his frontal lobes during an explosion (Figure 4-13

).

recov-ered physically, but those who knew him previously reported that his personality was never the same again. Once a mild-mannered individual, Gage became hot-tempered and prone to outbursts of anger. Stories such as this, as well as some experimental data, led to the suggestion that the prefrontal cortex is important for personality. Such claims, which do have some basis but were perhaps overstated, led to the development of a procedure called the prefrontal lobotomy. This surgical procedure was used to treat individuals with problems ranging from severe mental illness to nonconformity and rebellion (Heller et al., 2006). Due to its lack of scientific basis and its side effects, this surgery has (appropriately) fallen out of fashion. However, more limited destruction of the prefrontal cortex is still carried out for a small number of patients suffering from severe depression or other forms of mental illness who do not respond to drug therapy (Abosch & Cosgrove, 2008).

The four general regions of the neocortex can be further subdivided into many areas that serve different functions and have different neural connections.

However, all parts of the neocortex share some neuroanatomical features. The neocortex consists of six layers, whether occipital, temporal, parietal, or frontal.

Although the composition of the layers does vary somewhat across regions, in general, the output neurons (those that project to subcortical structures) are lo-cated in the deepest layers.