Unit 4. Cerebral Lobes, Cerebral Cortex, and Brodmann's Areas
Each cerebral hemisphere is divided into four lobes; the frontal, parietal, temporal, and the occipital.
The Frontal Lobe is the most anterior lobe of the brain. Its posterior boundary is the fissure of Rolando, or central sulcus, which separates it from the parietal lobe. Inferiorly, it is divided from the temporal lobe by the fissure of Sylvius which is also called the lateral fissure.
This lobe is deals with with higher level cognitive functions like reasoning and judgment. Sometimes called executive function, it is associated with the pre-frontal cortex. Most importantly for speech/language pathologists, the frontal lobe contains several cortical areas involved in the control of voluntary muscle movement, including those necessary for the production of speech and swallowing.
Broca's Area is found on the inferior third frontal gyrus in the hemisphere that is dominant for language. This area is involved in the coordination or programming of motor movements for the production of speech sounds. While it is essential for the execution of the motor movements involved in speech it does not directly cause movement to occur. The firing of neurons here does not generate impulses for motor movement; that is the function of neurons in the motor strip. The neurons in Broca's area generate motor programming patterns when they fire.
This area is also involved in syntax which involves the ordering of words in speech.
Injuries to Broca's area may cause apraxia or Broca's aphasia.
The precentral gyrus, which may also be called the primary motor area or, most commonly, the motor strip is immediately anterior to the central sulcus. It controls the voluntary movements of skeletal muscles; cell bodies of the pyramidal tract are found on this gyrus.
The amount of tissue on the precentral gyrus that is dedicated to the innervation of a particular part of the body is proportional to the amount of motor control needed by that area, not just its size. For example, much more of the motor strip is dedicated to the control of the articulators than to the legs.
The premotor area or supplemental motor area is immediately anterior to the motor strip. It is responsible for the programming for motor movements. It does not, however program the motor commands for speech as these are generated in Broca's area which is also located in the frontal lobe.
The most anterior part of the frontal lobe is involved in complex cognitive processes like reasoning and judgment. Collectively, these processes may be called biological intelligence. A component of biological intelligence is executive function. According to Denckla, 1996, executive function regulates and directs cognitive processes. Decision making, problem solving, learning, reasoning and strategic thinking are all part of executive functioning. Some characteristics of right hemisphere syndrome are considered problems of the executive function. They include left side neglect where there is a lack of awareness of the left side of the body.
The Parietal Lobe is immediately posterior to the central sulcus. It is anterior to the occipital lobe, from which it is not separated by any natural boundary. Its inferior boundary is the posterior portion of the lateral fissure which divides it from the temporal lobe.
The parietal lobe is associated with sensation, including the sense of touch, kinesthesia, perception of warmth and cold, and of vibration. It is also involved in writing and in some aspects of reading.
The postcentral gyrus which is also called the primary sensory area or the sensory strip is immediately posterior to the central sulcus. This area receives sensory feedback from joints and tendons in the body and is organized in the same manner as the motor strip.
Like the motor strip, the sensory strip continues down into the longitudinal cerebral fissure and so has both a lateral and a medial aspect.
The presensory, secondary sensory, or sensory association areas are located behind the postcentral gyrus. These areas are capable of more detailed discrimination and analysis than is the primary sensory area. They might, for example, be involved in sensing how hot or cold something is rather than simply identifying it as hot or cold. Information is first processed in the primary sensory area and is then sent to the secondary sensory areas.
The angular gyrus lies near the superior edge of the temporal lobe, immediately posterior to the supramarginal gyrus. It is involved in the recognition of visual symbols. Geschwind described this area as "the most important cortical areas of speech and language" and the "association cortex for association cortices". He also claims that the angular gyrus is not found in non-human species.
Fibers of many different types travel through the angular gyrus, including axons associated with hearing, vision, and meaning. The arcuate fasciculus, the groups of fibers connecting Broca's area in the frontal lobe to Wernicke's area in the temporal lobe also connects to the angular gyrus.
The following disorders may result from damage to the angular gyrus in the hemisphere that is dominant for speech and language: anomia, alexia with agraphia, left-right disorientation, finger agnosia, and acalcula.
Anomia according to Webb, Adler, and Love, 2008 is loss of power to name objects and people. It is difficulty with word-finding or naming. Someone suffering from anomia can list the functions of an object and explain it meaning, but cannot recall its name.
Alexia with Agraphia refers to difficulties with reading and writing.
Left-right disorientation is an inability to distinguish right from left.
Finger agnosia is the lack of sensory perceptual ability to identify which finger is which.
Acalcula refers to difficulties with arithmetic.
The Temporal Lobe is inferior to the lateral fissure and anterior to the occipital lobe. It is separated from the occipital lobe by an imaginary line rather than by any natural boundary.
The temporal lobe is associated with auditory processing and olfaction. It is also involved in semantics, or word meaning, as Wernicke's area is located there.
Wernicke's Area is located on the posterior portion of the superior temporal gyrus. In the hemisphere that is dominant for language, this area plays a critical role in the ability to understand and produce meaningful speech. A lesion here will case Wernicke's aphasia.
Heschl's Gyrus, which is also known as the anterior transverse temporal gyrus, is the primary auditory area.
There are two secondary auditory or auditory association areas which make important contributions to the comprehension of speech. They are not completely responsible for this ability, however, as many areas, including Wernicke's area, are involved in this process.
The Occipital Lobe, which is the most posterior lobe, has no natural boundaries. It is involved in vision.
The primary visual area receives input from the optic tract via the thalamus.
The secondary visual areas integrate visual information, giving meaning to what is seen by relating the current stimulus to past experiences and knowledge. A lot of memory is stored here. These areas are superior to the primary visual cortex.
Damage to the primary visual area causes blind spots in the visual field, or total blindness, depending on the extent of the injury. Damage to the secondary visual areas could cause visual agnosia. People with this condition can see visual stimuli, but cannot associate them with any meaning or identify their function. This represents a problem with meaning, as compared to anomia, which involves a problem with naming, or word-recall.
The Island of Reil or Insula is a cortical area which lies below the fissure of Sylvius and is considered by some anatomists to be the fifth lobe of the cerebrum. It can only be seen by splitting the lateral fissure. Little is known about the connections of this area, but it may be linked to the viscera. Drunkers, 1996 feels that it may be involved in programming for speech for speech sounds.
It is important to remember that while some functions can be localized to very specific parts of the brain, others cannot be classified in this way because many areas are involved in their performance. Word-finding, for example, is associated with several different areas. Also, we cannot say that all higher level cognitive functioning is associated with the frontal lobe; the processing of word meaning carried out by Wernicke's certainly involves a sophisticated type of cognition. Also, right hemisphere lesions often result in cognitive/perceptual problems.
The cortex is about four millimeters thick and is composed of six layers. Listed from most superior to most inferior, these layers are; the molecular layer, the external granular layer, the internal pyramidal layer, the internal granular layer, the ganglionic layer, and the fusiform or multiform layer.
The molecular layer is the most superior layer of the cortex. It contains the cell bodies of neuroglial cells.
The external granular layer is very dense and contains small granular cells and small pyramidal cells.
The external or medial pyramidal layer contains pyramidal cells arranged in row formation. The cell bodies of some association fibers are found here.
The internal granular layer is thin, but its cell structure is the same as that of the external granular layer.
The ganglionic layer contains small granular cells, large pyramidal cells as well as the cell bodies of some association fibers. The association fibers that originate here form two large tracts: The Bands of Baillarger and Kaes Bechterew.
The fusiform layer is also known as the multiform layer; its axons enter white matter, it function is unknown.
All layers are present in all parts of the cortex. However, they do not have the same relative density in all areas. Depending upon the function of a particular area, some of these layers will be thicker than others in that location.
The cortex wraps around the brain, covering its inferior surface and lining the gap between the right and left cerebral hemispheres, which is called the longitudinal or interhemispheric fissure.
The part of the cortex covering the sides of the hemispheres is called the lateral cortex while the part covering the sides of the hemispheres that lie within the longitudinal cerebral fissure is called medial cortex.
Brodmann's Classification System
Studies done by Brodmann in the early part of the twentieth century generated a map of the cortex covering the lobes of each hemisphere. These studies involved electrical probing of the cortices of epileptic patients during surgery. Brodmann numbered the areas that he studies in each lobe and recorded the psychological and behavioral events that accompanied their stimulation.
The Frontal Lobe contains areas that Brodmann identified as involved in cognitive functioning and in speech and language.
Area 4 corresponds to the precentral gyrus or primary motor area.
Area 6 is the premotor or supplemental motor area.
Area 8 is anterior of the premotor cortex. It facilitates eye movements and is involved in visual reflexes as well as pupil dilation and constriction.
Areas 9, 10, and 11 are anterior to area 8. They are involved in cognitive processes like reasoning and judgment which may be collectively called biological intelligence.
Area 44 is Broca's area.
Areas in the Parietal Lobe play a role in somatosensory processes.
Areas 3, 2, and 1 are located on the primary sensory strip, with area 3 being superior to the other two. These are somastosthetic areas, meaning that they are the primary sensory areas for touch and kinesthesia.
Areas 5, 7, and 40 are found posterior to the primary sensory strip and correspond to the presensory to sensory association areas.
Area 39 is the angular gyrus.
Areas involved in the processing of auditory information and semantics as well as the appreciation of smell are found in the Temporal Lobe.
Area 41 is Heschl's gyrus, or the primary auditory area.
Area 42 immediately inferior to area 41 and is also involved in the detection and recognition of speech. The processing done in this area of the cortex provides a more detailed analysis than that done in area 41.
Areas 21 and 22 are the auditory association areas. Both areas are divided into two parts; one half of each area lies on either side of area 42.
Area 37 is found on the posterior-inferior part of the temporal lobe. Lesions here will cause anomia.
The Occipital Lobe contains areas that process visual stimuli.
Area 17 is the primary visual area.
Areas 18 and 19 are the secondary visual areas.
A pedagogical device called the homunculus, which literally means "little man," is often used to explain the organization of the motor strip and to demonstrate that specific areas of this gyrus are responsible for sending commands to specific parts of the body. The body is represented on the motor strip in an upside-down fashion. The lower parts of the body, like the feet and the legs, receive motor movement commands from the superior part of the precentral gyrus. Parts of the face, on the other hand are innervated by the inferior part of the motor strip. Although the homunculus is without a body it was easier for me to do it as shown.
The motor strip extends down some distance into the longitudinal cerebral fissure. The portion inside this fissure is its medial aspect. The part on the lateral surface of the hemisphere is called its lateral aspect. The medial cortex controls the movements of the body from the hips on down while the lateral aspect sends commands to the upper body including the larynx, face, hands, shoulders, and trunk.
The medial and lateral aspects of the motor strip have different blood supplies. Blood comes to the medial area from the anterior cerebral artery while the lateral cortex is supplied by the middle cerebral artery.
