Sensory areas of the brain receive and process sensory information, including sight, touch, taste, smell, and hearing.
Describe the sensory areas of the cerebral cortex
- The cortex can be divided into three functionally distinct areas: sensory, motor, and associative.
- The main sensory areas of the brain include the primary auditory cortex, primary somatosensory cortex, and primary visual cortex.
- In general, the two hemispheres receive information from the opposite side of the body. For example, the right primary somatosensory cortex receives information from the left limbs, and the right visual cortex receives information from the left eye.
- Sensory areas are often represented in a manner that makes topographical sense.
- calcarine sulcus: An anatomical landmark located at the caudal end of the medial surface of the brain.
- primary somatosensory cortex: The main sensory receptive area for the sense of touch.
- primary auditory cortex: A region of the brain that processes sound and thereby contributes to our ability to hear.
- primary visual cortex: Located in the posterior pole of the occipital cortex, the simplest, earliest cortical visual area. It is highly specialized for processing information about static and moving objects and is excellent in pattern recognition.
Sensory areas are the areas of the brain that receive and process sensory information. The cerebral cortex is connected to various subcortical structures such as the thalamus and the basal ganglia. Most sensory information is routed to the cerebral cortex via the thalamus. Olfactory information, however, passes through the olfactory bulb to the olfactory cortex, bypassing the thalamus. The cortex is commonly described as composed of three parts: sensory, motor, and association areas. Parts of the cortex that receive sensory inputs from the thalamus are called primary sensory areas. Each of the five senses relates to specific groups of brain cells that categorize and integrate sensory information.
The Five Sensory Modalities
The five commonly recognized sensory modalities, including sight, hearing, taste, touch, and smell, are processed as follows:
The primary somatosensory cortex, located across the central sulcus and behind the primary motor cortex, is configured to generally correspond with the arrangement of nearby motor cells related to specific body parts.
The primary gustatory area is near the face representation within the postcentral gyrus.
The olfactory cortex is located in the uncus, found along the ventral surface of the temporal lobe. Olfaction is the only sensory system that is not routed through the thalamus.
The visual area is located on the calcarine sulcus deep within the inside folds of the occipital lobe.
The primary auditory cortex is located on the transverse gyri that lie on the back of the superior temporal convolution of the temporal lobes.
Organization of Sensory Maps
In general, each brain hemisphere receives information from the opposite side of the body. For example, the right primary somatosensory cortex receives information from the left limbs, and the right visual cortex receives information from the left eye. The organization of sensory maps in the cortex reflects that of the corresponding sensing organ, in what is known as a topographic map. Neighboring points in the primary visual cortex, for example, correspond to neighboring points in the retina. This topographic map is called a retinotopic map.
Similarly, there is a tonotopic map in the primary auditory cortex and a somatotopic map in the primary sensory cortex. This somatotopic map has commonly been illustrated as a deformed human representation, the somatosensory homunculus, in which the size of different body parts reflects the relative density of their innervation.
A cortical homunculus is a physical representation of the human body located within the brain. This neurological map of the anatomical divisions of the body depicts the portion of the human brain directly associated with the activity of a particular body part. Simply put, it is the view of the body from the brain’s perspective. Areas with lots of sensory innervation, such as the fingertips and the lips, require more cortical area to process finer sensation.
The motor areas, arranged like a pair of headphones across both cortex hemispheres, are involved in the control of voluntary movements.
Describe the motor areas of the cerebral cortex
- The primary motor cortex is involved in the planning of movements.
- The posterior parietal cortex guides movements in space.
- The dorsolateral prefrontal cortex acts as a decision maker for which planned movements will actually be made.
- The basal nuclei receive input from the substantia nigra of the midbrain and motor areas of the cerebral cortex and send signals back to both of these locations.
- primary motor cortex: A brain region located in the posterior portion of the frontal lobe of humans. It plans and executes movements in association with other motor areas including the premotor cortex, supplementary motor area, posterior parietal cortex, and several subcortical brain regions.
- cognitive flexibility: Ability to switch between thinking about two different concepts and to think about multiple concepts simultaneously.
- dorsolateral prefrontal cortex: The highest cortical area responsible for motor planning, organization, and regulation. It plays an important role in the integration of sensory and mnemonic information and the regulation of intellectual function and action.
- posterior parietal cortex: Plays an important role in producing planned movements by receiving input from the three sensory systems that help localize the body and external objects in space.
The motor areas of the brain are located in both hemispheres of the cortex. They are positioned like a pair of headphones stretching from ear to ear. The motor areas are very closely related to the control of voluntary movements, especially fine movements performed by the hand. The right half of the motor area controls the left side of the body, and the left half of the motor area controls the right side of the body.
Motor Cortex Divisions
The motor cortex is divided into three areas:
- Primary motor cortex: Main contributor to the generation of neural impulses that control the execution of movement.
- Premotor cortex: Located anterior to the primary motor cortex and responsible for some aspects of motor control.
- Supplementary motor area (SMA): Functions include internally generated planning of movement, planning of sequences of movement, and the coordination of the two sides of the body. It is located on the midline surface of the hemisphere anterior to the primary motor cortex.
Motor Cortex Functions
Motor functions are also controlled by these additional structures:
- Posterior parietal cortex: Guides planned movements, spatial reasoning, and attention.
- Dorsolateral prefrontal cortex: Important for executive functions, including working memory, cognitive flexibility, and abstract reasoning.
Buried deep in the white matter of the cerebral cortex are interconnected subcortical masses of cerebral gray matter called basal nuclei (or basal ganglia) that are involved in motor control. The basal nuclei receive input from the substantia nigra of the midbrain and motor areas of the cerebral cortex and send signals back to both of these locations.
Motor Cortex Map
The majority of neurons in the motor cortex project to the spinal cord synapse on interneuron circuitry in the spinal cord. The view that each point in the motor cortex controls a muscle or a limited set of related muscles has been debated. Various experiments examining the motor cortex map showed that each point in motor cortex influences a range of muscles and joints, indicating significant overlapping in the map.
Associative areas of the cortex integrate current states with past states to predict proper responses based on sets of stimuli.
Describe the association areas of the cerebral cortex
- Many areas of the brain are required to form a cohesive view of the world and permit perception.
- The prefrontal association cortex is involved in planning actions and abstract thought.
- The association areas integrate information from different receptors or sensory areas and relate the information to past experiences. Then the brain makes a decision and sends nerve impulses to the motor areas to generate responses.
- Wernicke’s area: The posterior section of the superior temporal gyrus in the dominant cerebral hemisphere, one of two parts of the cerebral cortex linked with speech (the other being Broca’s area).
- prefrontal association complex: A region of the brain located in the frontal lobe that is involved in planning actions and movement, as well as abstract thought.
- agraphia: An acquired neurological disorder causing a loss in the ability to communicate through writing.
- Broca’s area: A region in the frontal lobe of the dominant hemisphere (usually the left) of the hominid brain with functions linked to speech production.
Association areas produce a meaningful perceptual experience of the world, enable us to interact effectively, and support abstract thinking and language. The parietal, temporal, and occipital lobes, all located in the posterior part of the cortex, organize sensory information into a coherent perceptual model of our environment centered on our body image. The frontal lobe or prefrontal association complex is involved in planning actions and movement, as well as abstract thought.
Language abilities are localized in the left hemisphere in Broca’s area for language expression and Wernicke’s area for language reception. The association areas are organized as distributed networks, and each network connects areas distributed across widely spaced regions of the cortex. Distinct networks are positioned adjacent to one another, yielding a complex series of interwoven networks. In humans, association networks are particularly important to language function.
The processes of language expression and reception occur in areas other than just the perisylvian structures such as the prefrontal lobe, basal ganglia, cerebellum, pons, caudate nucleus, and others. The association areas integrate information from different receptors or sensory areas and relate the information to past experiences. Then the brain makes a decision and sends nerve impulses to the motor areas to elicit responses.
Methods of Brain Function Analysis
Behavioral and neuroscientific methods are used to get a better understanding of how our brain influences the way we think, feel, and act. Many different methods help us analyze the brain and give an overview of the relationship between brain and behavior. This promotes understanding of the ways in which associations are made by multiple brain regions, allowing the appropriate responses to occur in a given situation. Well-known techniques are EEG (electroencephalography), which records the brain’s electrical activity, and fMRI (functional magnetic resonance imaging), which tells us more about brain functions. Other methods, such as the lesion method, are not as well-known, but still very influential in modern neuroscientific research.
In the lesion method, patients with brain damage are examined to determine which brain structures were damaged and to what extent this influences the patient’s behavior. The concept of the lesion method is based on the idea of finding a correlation between a specific brain area and an occurring behavior. From experiences and research observations, it can be concluded that damage to part of the brain causes behavioral changes or interferes in performing a specific task.
For example, a patient with a lesion in the parietal-temporal-occipital association area has an agraphia, which means he is unable to write although he has no deficits in motor skills. Consequently, researchers deduce that if structure X is damaged and changes in behavior Y occur, X has a relation to Y.
The human brain is composed of a right and a left hemisphere, and each participates in different aspects of brain function.
Describe the impact of hemispheric lateralization on brain function
- The corpus collosum connects the hemispheres of the brain.
- Lateralization of function between the two hemispheres does occur but after injury, other regions of cortex can often compensate.
- There is no such thing as being left-brained or right-brained.
- Functional lateralization often varies between individuals.
- corpus collosum: A wide, flat bundle of neural fibers beneath the cortex that connects the left and right cerebral hemispheres and facilitates interhemispheric communication.
- lateralization: Localization of a function such as speech to the right or left side of the brain.
- hemisphere: Either of the two halves of the cerebrum..
- prosody: Properties of syllables and larger units of speech that contribute to linguistic functions such as intonation, tone, stress, and rhythm.
A longitudinal fissure separates the human brain into two distinct cerebral hemispheres connected by the corpus callosum. The two sides resemble each other and each hemisphere’s structure is generally mirrored by the other side. Yet despite the strong anatomical similarities, the functions of each cortical hemisphere are distinct.
Broad generalizations are often made in popular psychology about one hemisphere having a broad label, such as “logical” for the left side or “creative” for the right. But although measurable lateral dominance occurs, most functions are present in both hemispheres. The extent of specialization by hemisphere remains under investigation. If a specific region of the brain or even an entire hemisphere is either injured or destroyed, its functions can sometimes be taken over by a neighboring region even in the opposite hemisphere, depending upon the area damaged and the patient’s age. When injury interferes with pathways from one area to another, alternative (indirect) connections may develop to communicate information with detached areas, despite the inefficiencies.
While many functions are lateralized, this is only a tendency. The implementation of a specific brain function significantly varies by individual. The areas of exploration of this causal or effectual difference of a particular brain function include gross anatomy, dendritic structure, and neurotransmitter distribution. The structural and chemical variance of a particular brain function, between the two hemispheres of one brain or between the same hemisphere of two different brains, is still being studied. Short of having a hemispherectomy (removal of a cerebral hemisphere), no one is a “left-brain only” or “right-brain only” person.
Lateralization and Handedness
Brain function lateralization is evident in the phenomena of right- or left-handedness, but a person’s preferred hand is not a clear indication of the location of brain function. Although 95% of right-handed people have left-hemisphere dominance for language, 18.8% of left-handed people have right-hemisphere dominance for language function. Additionally, 19.8% of left-handed people have bilateral language functions. Even within various language functions (e.g., semantics, syntax, prosody), degree and even hemisphere of dominance may differ.
Language functions such as grammar, vocabulary and literal meaning are typically lateralized to the left hemisphere, especially in right-handed individuals. While language production is left-lateralized in up to 90% of right-handed subjects, it is more bilateral or even right-lateralized in approximately 50% of left-handers. In contrast, prosodic language functions, such as intonation and accentuation, often are lateralized to the right hemisphere of the brain.
Further Lateral Distinctions
The processing of visual and auditory stimuli, spatial manipulation, facial perception, and artistic ability are represented bilaterally, but may show right-hemisphere dominance. Numerical estimation, comparison, and online calculation depend on bilateral parietal regions. Exact calculation and fact retrieval are associated with left parietal regions, perhaps due to their ties to linguistic processing. Dyscalculia is a neurological syndrome associated with damage to the left temporoparietal junction. This syndrome is associated with poor numeric manipulation, poor mental arithmetic skill, and the inability to understand or apply mathematical concepts.
Lateralization and Evolution
Specialization of the two hemispheres is general in vertebrates including fish, frogs, reptiles, birds, and mammals, with the left hemisphere specialized to categorize information and control routine behavior. The right hemisphere is responsible for responses to novel events and behavior in emergencies, including the expression of intense emotions. Feeding is an example of a routine left-hemisphere behavior, while escape from predators is an example of a right-hemisphere behavior. This suggests that the evolutionary advantage of lateralization comes from the capacity to perform separate parallel tasks in each hemisphere of the brain.
Patients with split-brain are individuals who have undergone corpus callosotomy, a severing of a large part of the corpus callosum (usually as a treatment for severe epilepsy). The corpus callosum connects the two hemispheres of the brain and allows them to communicate. When these connections are cut, the two halves of the brain have a reduced capacity to communicate with each other.
The widespread lateralization of many vertebrate animals indicates an evolutionary advantage associated with the specialization of each hemisphere. The evolutionary advantage of lateralization comes from the capacity to perform separate parallel tasks in each hemisphere of the brain. In a 2011 study published in the journal of Brain Behavioral Research, lateralization of a few specific functions as opposed to overall brain lateralization was correlated with parallel tasks efficiency.