Frontal-Lobe

The , which contains the primary motor cortex and the prefrontal cortex, extends from the central sulcus to the anterior limit of the . The posterior portion of the frontal lobe just anterior to the central sulcus, the pre central gyrus, is specialized for the control of fine movements, such as moving one finger at a time. Separate areas are responsible for different parts of the body, mostly on the contralateral (opposite) side but also with slight control of the ipsilateral (same) side shows the traditional map of the precentral gyrus, also known as the primary motor cortex. The most anterior portion of the frontal lobe is the prefrontal cortex. In general, the larger a species’ , the higher the percentage of it is devoted to the prefrontal cortex. For example, it forms a larger portion of the cortex in humans and all the great apes than in other species (Semendeferi, Lu, Schenker, & Damasio, 2002). It is not the primary target for any sensory system, but it receives information from all of them in different parts of the prefrontal cortex. The dendrites in the prefrontal cortex have up to 16 times as many dendritic spines as neurons in other cortical areas (Elston, 2000). As a result, the prefrontal cortex integrates an enormous amount of information.

In the human brain, the frontal lobes comprise all the tissue in front of the central sulcus. This vast area, constituting 20% of the neocortex, is made up of several functionally distinct regions that we shall group into three general categories—motor, premotor, and prefrontal. The motor cortex is area 4. The premotor cortex includes areas 6 and 8, which can be divided into four regions: lateral area 6: premotor cortex medial area 6: supplementary motor cortex area 8: frontal eye field area 8A: supplementary eye field

Asymmetry of Frontal-Lobe Function In keeping with the general complementary organization of the left and right hemispheres, as a rule the left frontal lobe has a preferential role in language-related movements, including speech, whereas the right frontal lobe plays a greater role in nonverbal movements such as facial expression. Like the asymmetry of the parietal and temporal lobes, the asymmetry of frontal-lobe function is relative rather than absolute; the results of studies of patients with frontal lesions indicate that both frontal lobes play a role in nearly all behavior.

Symptoms of Frontal-Lobe Lesions

Fine Movements, Speed, and Strength - Damage to the primary motor cortex is typically associated with a chronic loss of the ability to make fine, independent finger movements, presumably owing to a loss of direct corticospinal projections onto motor neurons. In addition, there is a loss of speed and strength in both hand and limb movements in the contralateral limbs. The loss of strength is not merely a symptom of damage to area 4, because lesions restricted to the prefrontal cortex also lead to a reduction in hand strength. The observation that frontal injury severely disrupts the copying of facial but not arm movements implies that the frontal lobe may play a special role in the control of the face, perhaps even including the tongue.

Voluntary Gaze A number of studies using quite different procedures have been reported in which frontal-lobe lesions produce alterations in voluntary eye gaze. Corollary Discharge If you push on your eyeball, the world appears to move. If you move your eyes, the world remains stable. Why? Teuber proposed that, for a movement to take place, a neural signal must produce the movement as well as a signal that the movement is going to take place. If the eyes are moved mechanically, there is Visual search task used by Teuber. The subject must locate a duplicate of the shape inside the central box by pointing to it. No such signal and the world moves. However, when you move your eyes, there is a neural signal that movement will happen and the world stays still. This signal has been termed corollary discharge or reafference.

Speech Speech is an example of movement selection. Passingham suggested that words are responses generated in the context of both internal and external stimuli. If the frontal lobe has a mechanism for selecting responses, then it must select words, too. The frontal lobe contains two speech zones: Broca’s area, which can be regarded as an extension of the lateral premotor area, and the supplementary speech area, which may be an extension of the supplementary motor area

Loss of Divergent Thinking One of the clearest differences between the effects of parietal- and lesions and the effects of frontal-lobe lesions is in performance on standard intelligence tests. Posterior lesions produce reliable, and often large, decreases in IQ scores, but frontal lesions do not. The puzzle is why patients with frontal lobe damage appear to do such “stupid” things. Guilford noted that traditional intelligence tests appear to measure what can be called convergent thinking, in the that there is just one correct answer to each question. Thus, definitions of words, questions of fact, arithmetic problems, puzzles, and block designs all require correct answers that are easily scored. Another type of intelligence test, in which the number and variety of responses to a single question rather than a single correct answer are emphasized, can measure divergent thinking. An example is a question asking for a list of the possible uses of a coat hanger. Frontal- lobe injury interferes with the intelligence required by divergent thinking, rather than the convergent type measured by standard IQ tests. Several lines of evidence support Guilford’s idea.

Temporal lobe The temporal lobe is the lateral portion of each hemisphere, near the temples. It is the primary cortical target for auditory information. The human temporal lobe—in most cases, the left temporal lobe—is essential for understanding spoken language. The temporal lobe also contributes to complex aspects of vision, including of movement and recognition of faces. A tumor in the temporal lobe may give rise to elaborate auditory or visual hallucinations, whereas a tumor in the ordinarily evokes only simple sensations, such as flashes of light. In fact, when psychiatric patients report hallucinations, brain scans detect extensive activity in the temporal lobes (Dierks et al., 1999). The temporal lobes also play a part in emotional and motivational behaviors. Temporal lobe damage can lead to a set of behaviors known as the Klüver-Bucy syndrome (named for the investigators who first described it). Previously wild and aggressive monkeys fail to display normal fears and anxieties after temporal lobe damage (Klüver & Bucy, 1939). They put almost anything they find into their mouths and attempt to pick up snakes and lighted matches (which intact monkeys consistently avoid). Interpreting this behavior is difficult. For example, a monkey might handle a snake because it is no longer afraid (an emotional change) or because it no longer recognizes what a snake is (a cognitive change).

Subdivisions of the Temporal Cortex Brodmann identified 10 temporal areas, but many more areas in the monkey were identified in more recent studies. Likely there are more areas in the human as well. We can divide the temporal regions on the lateral surface into those that are auditory (Brodmann’s areas 41, 42, and 22 in Figure 15.1B) and those that form the ventral visual stream on the lateral temporal lobe (areas 20, 21, 37, and 38 in. The visual regions are often referred to as inferotemporal cortex or by von Economo’s designation, TE. Symptoms of Temporal-Lobe Lesions Nine principal symptoms are associated with disease of the temporal lobes: (1) Disturbance of auditory sensation and perception, (2) Disorders of music perception, (3) Disorders of , (4) Disturbance in the selection of visual and auditory input, (5) Impaired organization and categorization of sensory input, (6) Inability to use contextual information, (7) Impaired long-term memory, (8) Altered personality and affective behavior, and (9) Altered sexual behavior.

Disorders of Auditory Perception Damage to the primary visual or somatic cortex leads to a loss of conscious sensation; so it is reasonable to predict that bilateral damage to the will produce cortical , an absence of neural activity in the auditory regions. The results of neither clinical nor animal laboratory studies support this prediction, however. Auditory hallucinations, which result from spontaneous activity in the auditory regions, are essentially the opposite of cortical deafness. Auditory hallucination is the perception of sounds ( voices) that are not actually present. The auditory cortex does play an actual role in discriminating two forms of auditory processing— namely, rapidly presented stimuli and complex patterns of stimuli. Language is fast and must be analyzed quickly, whereas music generally contains relatively slower changes in frequency, but the ear must be sensitive to the small differences in frequency important in music.

Speech Perception Impaired auditory processing can be seen in the difficulty that temporal-lobe patients have in discriminating speech sounds. Although related to the common complaint among patients with left-temporal-lobe damage that people are talking too quickly, the problem is not so much the quickness of the speech but rather the patient’s inability to discriminate sounds presented quickly. This difficulty is commonly encountered by normal people trying to learn a new language. The problem is not just in discriminating the speech sounds, however, but also in judging the temporal order in sounds heard. If a normal subject is presented with two sounds, a temporal separation of only 50 to 60 ms is sufficient to identify which sound was presented first. Subjects with temporal-lobe lesions may require as much as 500 ms between two sounds (a 10- fold increase) to perform at the same level.

Disorders of Music Perception The fact that the brain appears to have neural networks dedicated to the processing of language and music leads to the conclusion that both language and music have biological roots. Although this conclusion seems obvious for language, it is less obvious for music, which has often been perceived as an artifact of culture. But considerable evidence suggests that humans are born with a predisposition for processing music.

Disorders of Visual Perception Although persons with temporal lobectomies do not normally have large defects in their visual fields, they do have deficits in visual perception. Such deficits were first demonstrated by Milner, who found that her patients with right temporal lobectomies were impaired in the interpretation of cartoon drawings in the McGill Picture-Anomalies Test. For example, one item illustrating a monkey in a cage features an oil painting on the wall of the cage—an obvious oddity or anomaly. But, although patients with right temporal lesions can describe the contents of the cartoon accurately; they are impaired at recognizing the anomalous aspects of this picture and others. Similarly, on a test such as the Mooney Closure Test or tests requiring the discrimination of complex patterns, patients with temporal-lobe damage perform very poorly.

THE OCCIPITAL LOBES

The occipital lobes form the posterior pole of the cerebral hemispheres, It lies under the occipital bone at the back of the skull. On the medial surface of the hemisphere, the occipital lobe is distinguished from the by the parieto-occipital sulcus the fusiform gyrus.

Subdivisions of the Occipital Cortex

Brodmann divided the monkey brain into three regions (areas 17, 18, and 19), but studies using imaging, physiological, and newer anatomical techniques have produced much finer subdivisions. to detect in a black-and-white scene. Connections of the Visual Cortex

By the late 1960s, the consensus was that the visual cortex is hierarchically organized, with visual information proceeding from area 17 to area 18 to area 19. Each visual area was thought to provide some sort of elaboration on the processing of the preceding area. 1. V1 (the striate cortex) is the primary vision area: it receives the largest input from the lateral geniculate nucleus of the and it projects to all other occipital regions. V1 is the first processing level in the hierarchy. 2. V2 also projects to all other occipital regions. V2 is the second level. 3. After V2, three distinct parallel pathways emerge en route to the parietal cortex, superior temporal sulcus, and inferior temporal cortex, for further processing.

Functions of Occipital Lobe 1. Vision for Action This category is required to direct specific movements. For example, when reaching for a particular object such as a cup, the fingers form a specific pattern that allows grasping of the cup. This movement is obviously guided by vision, because people do not need to shape their hands consciously as they reach. In addition to that for grasping, there must be visual areas that guide all kinds of specific movements, including those of the eyes, head, and whole body. 2. Action for Vision In a more “top down” process, the viewer actively searches for only part of the target object and attends selectively. When we look at a visual stimulus, we do not simply stare at it; rather, we scan the stimulus with numerous eye movements. These movements are not random but tend to focus on important or distinct features of the stimulus. When we scan a face, we make a lot of eye movements directed toward the eyes and mouth. When people are asked to rotate objects mentally in order to answer simple questions about the objects’ appearance, they usually make many eye movements, especially to the left. When people are doing things in the dark, such as winding photographic film onto spools for processing, they also make many eye movements. Curiously, if the eyes are closed, these movements stop.

Visual Recognition

We enjoy the ability both to recognize objects and to respond to visual information. For example, we can both recognize specific faces and discriminate and interpret different expressions in those faces. Similarly, we can recognize letters or symbols and assign meaning to them. We can recognize different foods, tools, or body parts, but it is not reasonable to expect that we have different visual regions for each category or object.

Visual Space

Visual information comes from specific locations in space. This information allows us to direct our movements to objects in space and to assign meaning to objects. But spatial location is not a unitary characteristic. Objects have location both relative to an individual (egocentric space) and relative to one another (allocentric space). Egocentric visual space is central to the control of your actions toward objects. It therefore seems likely that visual space is coded in neural systems related to vision for action. In contrast, allocentric properties of objects are necessary for you to construct a memory of spatial location.

Visual Attention We cannot possibly process all the visual information available. This page has shape, color, texture, location, and so on, but the only really important characteristic is that it has words. Thus, when we read the page, we select a specific aspect of visual input and attend to it selectively. In fact, neurons in the cortex show various attentional mechanisms. For example, neurons may respond selectively to stimuli in particular places or at particular times or if a particular movement is to be executed. Independent mechanisms of attention are probably required both for the guidance of movements (in the parietal lobe) and for object recognition (in the temporal lobe).

DISORDERS OF VISUAL PATHWAYS

A lesion of the medial region of the optic chiasm severs the crossing fibers, producing bitemporal hemianopia—loss of vision of both temporal fields. This symptom can arise when a tumor develops in the pituitary gland, which sits medially, next to the chiasm.

A lesion of the lateral chiasm results in a loss of vision of one nasal field, or nasal hemianopia.

Complete cuts of the , lateral geniculate body, or area V1 result in homonymous hemianopia—blindness of one entire visual field.

Small lesions of the occipital lobe often produce scotomas, small blind spots in the visual field . A curious aspect of scotomas is that people are often totally unaware of them because of (constant, tiny, involuntary eye movements) and “spontaneous filling in” by the .

The eyes are usually in constant motion; so the scotoma moves about the visual field, allowing the brain to perceive all the information in the field.

Disorders of Cortical Function

1. Visual Visual can be divided into object agnosias and other agnosias. 1.1 Object Agnosias The traditional way to classify visual-object agnosia is to distinguish two broad forms: apperceptive agnosia and associative agnosia

Apperceptive Agnosia

Any failure of object recognition in which relatively basic visual functions (acuity, color, motion) are preserved is apperceptive. This agnosia category has been applied to an extremely heterogeneous set of patients, but the fundamental deficit is an inability to develop a percept of the structure of an object or objects. Many patients have another unusual symptom, too—often referred to as simultagnosia. In this case, patients can perceive the basic shape of an object, but they are unable to perceive more than one object at a time.

Associative Agnosia

The inability to recognize an object despite an apparent perception of the object is associative agnosia. Thus, the associative agnosic can copy a drawing rather accurately, indicating a coherent percept, but cannot identify it. Associative agnosia is therefore conceived as being at a “higher cognitive” level of processing that is associated with stored information about objects— that is,with memory.

2. Other Agnosias a. : Patients with prosopagnosia or facial agnosia cannot recognize any previously known faces, including their own as seen in a mirror or photograph. They can recognize people by face information, however, such as a birthmark, moustache, or characteristic hairdo. Prosopagnosics may not accept the fact that they cannot recognize their own faces, probably because they know who must be in the mirror and thus see themselves. b. Alexia: It is an inability to read has often been seen as the complementary symptom to facial- recognition deficits. Alexia is most likely to result from damage to the left fusiform and lingual areas. c. Visuospatial Agnosia: People have difficulty in topographical disorientation which is the inability to find one’s way around familiar environments such as one’s neighborhood. People with this deficit seem unable to recognize landmarks that would indicate the appropriate direction in which to travel.

THE PARIETAL LOBES

The parietal lobe is the region of cerebral cortex between the frontal and occipital lobes, underlying the parietal bone at the roof of the skull. This area isroughly demarcated anteriorly by the central fissure, ventrally by the Sylvian fissure, dorsally by the cingulate gyrus, and posteriorly by the parieto-occipital sulcus. The principal regions of the parietal lobe include: a. The postcentral gyrus (Brodmann’s areas 1, 2, and 3) b. The superior parietal lobule (areas 5 and 7) c. the parietal operculum (area 43) d. The supramarginal gyrus (area 40), and the e. Angular gyrus (area 39) . Together, the supramarginal gyrus and angular gyrus are often referred to as the inferior parietal lobe. The parietal lobe can be divided into two functional zones: an anterior zone including areas 1, 2, 3, and 43; and a posterior zone, which includes the remaining areas. The anterior zone is the somatosensory cortex; the posterior zone is referred to as the posterior parietal cortex.

Parietal Lobe Functions

The anterior zone processes somatic sensations and ; the posterior zone is specialized primarily for integrating sensory input from the somatic and visual regions and, to a lesser extent, from other sensory regions, mostly for the control of movement. The posterior parietal lobe controls the visuomotor guidance of movements in egocentric (that is, viewer-centered) space. This control is most obvious in regard to reaching and to eye movements needed to grasp or manipulate objects. The eye movements are important, because they allow the visual system to attend to particular sensory cues in the environment.

Somatosensory Symptoms of Parietal-Lobe Lesions

Afferent paresis - Lesions of the postcentral gyrus may also produce a symptom that Luria called afferent paresis. Here the movements of the fingers are clumsy because the person has lost the necessary feedback about their exact position.

Somatoperceptual Disorders 1. - It is derived from the Greek stereo, meaning “solid, which is the inability to recognize the nature of an object by touch. 2. Simultaneous extinction- When a person is confronted by an environment in which many sensory stimuli impinge simultaneously, the person is unable to distinguish and perceive each of these individual sensory impressions. 3. Blind Touch- Patients can identify the location of a visual stimulus even though they deny seeing it.

Somatosensory Agnosias There are two major types of somatosensory agnosias: astereognosis and asomatognosia—the loss of knowledge or sense of one’s own body and bodily condition. Asomatognosia is one of the most curious of all agnosias. It is an almost unbelievable syndrome—until you actually observe it. The varieties of asomatognosias include : Anosognosia-: the unawareness or denial of illness. Anosodiaphoria: indifference to illness Autopagnosia: an inability to localize and name body parts; and Asymbolia for : the absence of normal reactions to pain, such as reflexive withdrawal from a painful stimulus. Asomatognosias may affect one or both sides of the body, although most commonly the left side, as a result of lesions in the right hemisphere. An exception comprises the autopagnosias, which usually result from lesions of the left parietal cortex. The most common autopagnosia is finger agnosia, a condition in which a person is unable either to point to the various fingers of either hand or show them to an examiner.

Symptoms of Posterior Parietal Damage 1. Balint’s Syndrome- In 1909, R. Balint described a patient whose bilateral parietal lesion was associated with rather peculiar visual symptoms. The patient had full visual fields and could recognize, use, and name objects, pictures, and colors normally. Nevertheless, he had three unusual symptoms: Although he spontaneously looked straight ahead, when an array of stimuli was placed in front of him, he directed his gaze 35º to 40º to the right and perceived only what was lying in that direction. Thus, he could move his eyes but could not fixate on specific visual stimuli. When his attention was directed toward an object, he did not notice other stimuli. With urging, he could identify other stimuli placed before him, but he quickly relapsed into his former neglect. Balint concluded that the patient’s field of attention was limited to one object at a time, a disorder that made reading very difficult because each letter was perceived separately. (This disorder is often referred to as simultagnosia.) The patient had a severe deficit in reaching under visual guidance. Balint described this symptom as optic ataxia. He noted that the patient could still make accurate movements directed toward the body, presumably by using tactile or proprioceptive information, but could not make visually guided movements.

2. Contralateral Neglect: A perceptual disorder subsequent to right parietal lesions was described by John Hughlings-Jackson in 1874. The symptoms include unawareness of sensations on one side of the body.

3. : In 1924, Josef Gerstmann described a patient with an unusual disorder subsequent to a left parietal : finger agnosia, and asomatognosia. Gerstmann’s patient was unable to name or indicate recognition of the fingers on either hand. This symptom aroused considerable interest, and, in the ensuing years, three other symptoms were reported to accompany finger agnosia: right–left confusion, agraphia (inability to write), and acalculia. These four symptoms collectively became known as the Gerstmann syndrome.

Apraxia is a disorder of movement in which loss of skilled movement is not caused by weakness, an inability to move, abnormal muscle tone or posture, intellectual deterioration, poor comprehension, or other disorders of movement such as tremor. Among the many types of apraxia two important are: ideomotor apraxia and constructional apraxia. In ideomotor apraxia, patients are unable to copy movements or to make gestures (for example, to wave “hello”). Patients with left posterior parietal lesions often present ideomotor apraxia. In constructional apraxia, a visuomotor disorder, spatial organization is disordered. Patients with constructional apraxia cannot assemble a puzzle, build a tree house, draw a picture, or copy a series of facial movements. Constructional apraxia can develop after injury to either parietal lobe, although debate over whether the symptoms are the same after left- and right side lesions is considerable. Nonetheless, constructional apraxia often accompanies posterior parietal lesions.