Cerebral Cortex – Anatomy, Types, Structure, Functions

The cerebral cortex is composed of a complex association of tightly packed neurons covering the outermost portion of the brain. It is the gray matter of the brain. Lying right under the meninges, the cerebral cortex divides into four lobes: frontal, temporal, parietal, and occipital lobes, each with a multitude of functions. It is characteristically known for its bulges of brain tissue known as gyri, alternating with deep fissures known as sulci. The enfolding of the brain is an adaptation to the dramatic growth in brain size during evolution. The various folding of brain tissue allowed large brains to fit in relatively small cranial vaults that had to remain small to accommodate the birth process. Notable sulci include the Sylvian fissure which divides the temporal lobe from the frontal and parietal lobe, the central sulcus which separates the frontal and parietal lobes, the parieto-occipital sulcus which divides the parietal and occipital lobes, and the calcarine sulcus which divides the cuneus from the lingual gyrus.

Sensory Areas

Sensory areas of the brain receive and process sensory information, including sight, touch, taste, smell, and hearing.

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:

Somatosensory System

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.

Taste

The primary gustatory area is near the face representation within the postcentral gyrus.

Olfaction

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.

Vision

The visual area is located on the calcarine sulcus deep within the inside folds of the occipital lobe.

Hearing

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.

Motor Areas

The motor areas, arranged like a pair of headphones across both cortex hemispheres, are involved in the control of voluntary movements.

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

Motor Cortex: Topography of the human motor cortex, including the premotor cortex, SMA, primary motor cortex, primary somatosensory cortex, and posterior parietal cortex.

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.

Association Areas

Associative areas of the cortex integrate current states with past states to predict proper responses based on sets of stimuli.

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 the 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.

Hemispheric Lateralization

The human brain is composed of a right and a left hemisphere, and each participates in different aspects of brain function.

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.

The hemispheres of the cerebral cortex: The human brain is divided into two hemispheres–left and right. Scientists continue to explore how some cognitive functions tend to be dominated by one side or the other; that is, how they are lateralized.

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 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.

Split-Brain Phenomenon

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 task efficiency.

Blood Supply and Lymphatics

The brain weighs 2% of total body weight. It receives about 15% of the cardiac output.

Anterior Circulation

The anterior portion of the brain is supplied mainly by branches of the paired internal carotid artery. It accounts for 80% of the blood supply of the brain.

Internal Carotid Arteries (ICA) – The internal carotid artery runs upward through the neck and enters the skull through the carotid canal, located in the petrous portion of the temporal bone just superior to the jugular fossa. The internal carotid branches into the anterior cerebral artery and continues to form the middle cerebral artery. The ICA provides the anterior supply to the circle of Willis.

Anterior Cerebral Arteries (ACA) – The anterior cerebral arteries are branches of the ICA and supply the frontal and superior medial parietal lobes; this includes part of the motor cortex that controls the movement of the contralateral lower limb, the sensory cortex that controls sensation in the contralateral lower limb, Broca’s area, and the prefrontal cortex. Both ACAs connect to each other via the anterior communicating artery. Although ACA infarcts are rare due to the collateral circulation provided by the anterior communicating arteries, one would experience contralateral motor and sensory deficits in the lower limbs.

Middle Cerebral Arteries (MCA) – The middle cerebral artery is the most common site for a stroke, accounting for up to 80% of ischemic strokes that occur in the brain.[rx] It arises from the ICA and courses laterally through the sphenoid ridge to the Sylvian fissure. It is responsible for supplying the majority of the lateral hemispheres except for the superior portion of the parietal lobe (ACA) and the inferior portions of the temporal and occipital lobes (PCA). Lenticulostriate branches of the MCA supply the basal ganglia and internal capsule. Damage to the middle cerebral artery on the Left can cause deficits due to damage to Broca’s area, Wernicke’s area, and contralateral sensorimotor deficits in the upper extremities and head. Damage to the MCA on the right side would spare Wernicke’s and Broca’s area given the patients’ dominant hemisphere is on the left. It is important to note the resultant contralateral sensorimotor deficits in the upper extremities with an MCA stroke versus contralateral sensorimotor deficits in the lower extremities with an ACA stroke.

Posterior Circulation

The posterior cerebral circulation supplies the occipital lobes, cerebellum, and brainstem via branches of the vertebral arteries. It accounts for 20% of cerebral blood flow.

Basilar Artery – As the vertebral arteries course superiorly into the skull through the foramen magnum, they fuse to form the basilar artery. Often referred to as the vertebrobasilar system, the combination of the vertebral arteries with the basilar arteries provides the posterior supply to the circle of Willis. The basilar artery runs cranially in the central groove of the pons within the pontine cistern. It travels adjacent to CNVI to the upper pontine border and the appearance of CNIII where it terminates. The basilar artery gives off various branches including the anterior inferior cerebellar artery, labyrinth arteries, pontine arteries, superior cerebellar artery, and then finally bifurcates and terminates as the posterior cerebral arteries. Basilar artery occlusion represents up to 4% of all ischemic strokes. Clinical features localizing to the cerebellum or brainstem such as hearing loss, truncal ataxia, extraocular movement abnormalities, and nystagmus, may help to differentiate ischemia in the posterior circulation from other clinical diagnoses.[rx] One of the most disabling locations for a basilar artery occlusion is a mid-basilar occlusion with bilateral pontine ischemia. Patients with this condition appear comatose but can be fully conscious and paralyzed with only limited vertical eye movements. This phenomenon termed “locked-in syndrome” has a high mortality rate of approximately 75% in the acute phase.[rx] Another basilar artery occlusion can occur at the distal “top of the basilar syndrome” where the superior cerebellar artery and posterior cerebral artery terminate. It may result in cortical blindness. Physical examination findings may include vertical gaze and convergence disorders, slowed smooth pursuit movements, skew deviation, and convergence-retraction nystagmus, and light-near dissociation. The top of the basilar syndrome can have further clinical findings if there is an involvement of the superior cerebellar or posterior cerebral arteries.

Posterior Cerebral Arteries (PCA) – The posterior cerebral arteries are the terminal branch of the basilar artery and supply the overwhelming majority of the occipital lobe. It is joined with the MCA in the circle of Willis via the posterior communicating artery. As the posterior cerebral arteries branch off from the basilar artery, they travel around the midbrain, through the quadrigeminal cistern, and with the calcarine artery in the calcarine sulcus. The posterior cerebral arteries have various branches including the posterior communicating artery, the thalamoperforating branches, and the posterior choroidal arteries. The most significant manifestation of a PCA stroke is contralateral hemianopia with macular sparing. The macula is spared due to the dual collateral circulation provided by the MCA. If the PCA stroke involves the dominant hemisphere (usually left) patients may exhibit alexia without agraphia (patients can write but cannot read). Larger infarcts involving the internal capsule and thalamus may cause contralateral hemiparesis and hemisensory loss.

Function

The Frontal Lobe

It is the largest lobe, located in front of the cerebral hemispheres, and has significant functions for our body, and these are:

• Prospective memory a type of memory that involves remembering the plans made, from a simple daily plan to future lifelong plans.[rx]
• Speech and language

The frontal lobe has an area called Broca’s area located in the posterior inferior frontal gyrus involved in speech production. A recent study shows that the exact function of Broca’s area is to mediate sensory representations that originate in the temporal cortex and going to the motor cortex.[rx]

• Personality – During the past centuries, several researchers have described that there are personality changes that occurred after frontal lobe injuries. One of the most important cases was about Phineas Gage, who was a gentle, polite sociable young, man until a large iron rod went through his eye-damaging his prefrontal cortex. This injury made him emotionally insensitive, perform socially inappropriate behaviors, and was unable to make a rational judgment. A recent study suggests that when there is damage to the prefrontal cortex, there are five sub-types of personality changes that occur, and these include:

• Executive disturbances
• Disturbed social behavior
• Emotional Dysregulation
• Hypo-emotionality/de-energization
• Distress[rx]
• Decision making

The ability to decide on something involves reasoning, learning, and creativity. A study conducted in 2012 proposed a new model to understand how the decision-making process occurs in the frontal lobe, specifically how the brain creates a new strategy to a new-recurrent situation or an open-ended environment; they called it the PROBE model.

There are typically three possible ways to adapt to a situation:

Selecting a previously learned strategy that applies precisely to the current situationAdjusting an already learned approach Developing a creative behavioral method

The PROBE model illustrates that the brain can compare three to four behavioral methods at most, then choose the best strategy for the situation.[rx]

• Movement control – The frontal lobe has the motor cortex divided into two regions: the primary motor area located posterior to the precentral sulcus and non-primary motor areas, including the premotor cortex, supplementary motor area, and cingulate motor areas. The exact function of each structure and its role in the movement is still an active research area.[6]

The Parietal Lobe

It is located posterior to the frontal lobe and superior to the temporal lobe and classified into two functional regions.

The anterior parietal lobe contains the primary sensory cortex (SI), located in the postcentral gyrus (Broadman area BA 3, 1, 2). SI receives the majority of the sensory inputs coming from the thalamus, and it’s responsible for interpreting the simple somatosensory signals like (touch, position, vibration, pressure, pain, temperature).[7]

The posterior parietal lobe has two regions: the superior parietal lobule and the inferior parietal lobule.

• The superior parietal lobule contains the somatosensory association (BA 5, 7) cortex which is involved in higher-order functions like motor planning action.
• The inferior parietal lobule (supramarginal gyrus BA 40, angular gyrus BA 39) has the  Secondary somatosensory cortex (SII), which receives the somatosensory inputs from the thalamus and the contralateral SII, and they integrate those inputs with other major modalities (examples: visual inputs, auditory inputs) to form higher-order complex functions like:

  • Sensorimotor planning
  • Learning
  • Language
  • Spatial recognition
  • Stereognosis: the ability to differentiate between objects regarding their size, shape, weight, and any other differences.[rx]

The Temporal Lobe

The second most substantial portion occupies the middle cranial fossa and lies posterior to the frontal lobe and inferior to the parietal lobe. There are two surfaces, the lateral surface and the medial surface.[rx]

The lateral surface is classified by the superior temporal sulcus and the lateral temporal sulcus into three gyri; the superior temporal gyrus and the middle temporal gyrus, and the inferior temporal gyrus.

• The superior temporal gyrus (STG) is further sub-divided into two surfaces, the dorsal surface (superior temporal plane STP) and the lateral surface of the STG.

The STP is located deep in the Sylvain fissure. The most significant anatomical landmark in STP is the Heschl gyrus (HG) which contains the primary auditory cortex. It is responsible for translating and processing all sounds and tones, and it is minimally affected by task requirements. Task requirement: a test where the examiner pronounces some words and asks the participant to categorize them acoustically, or phonemically, or semantically.[rx]The STP has another important area next to the HG called Wernicke’s area. In the past, this area was thought to have a significant role in speech perception and comprehension, but recent evidence shows that this area is not involved in this process. Researchers found that this process is not a simple task, but moreover, it is a complex task that is distributed all over the brain. The primary function of this area is the phonological representation, a process where the pronounced word is interpreted based on their tones and sound and trying to link it to a previously learned sound.[rx]

The lateral surface of the STG is thought to be the secondary auditory cortex that also functions in interpreting sounds, but mostly in the activities that involve task requirements.[rx]

• The middle temporal gyrus (MTG) has four sub-regions, the anterior, middle, posterior, and sulcus MTG.[rx]

The Anterior MTG is primarily involved in:

The default mode network has a specific activity that exists naturally in the brain at rest. So if one is studying or engaging in a game or doing any other activity that demands staying focused or setting a particular goal this mode will be deactivated.

• Sound recognition helps the other areas that we talked about before.
• Semantic retrieval a process that assigns meaning to the words or sounds by trying to retrieve the previously learned concepts if they existed.

The Middle MTG has two functions:

• Semantic memory a type of memory involved in remembering the thoughts or objectives that are common knowledge (for example, where the bathroom is located).
• A semantic control network a system of connections between different areas of the brain, including the middle MTG, to assign meaning to words, sounds that require both stored knowledge and mechanisms of semantic retrieval.

The Posterior MTG is thought to be part of the classical sensory language area.

The Sulcus MTG is involved in decoding gaze directions and in speech.

• The inferior temporal gyrus (IT) is involved in visual perception and facial perception by containing the ventral visual pathway, the pathway that carries the information from the primary visual cortex to the temporal lobe, to determine the content of the vision.[rx]

The medial surface of the temporal lobe (mesial temporal lobe) includes important structures (Hippocampus, Entorhinal, Perirhinal, Parahippocampal cortex) that are anatomically related and are mandatory for declarative memory. Declarative memory is a type of long-term memory that involves remembering the concepts or ideas and the events that happened or learned throughout life. It is further divided into three types of memory:

• Semantic memory was discussed previously (see middle MTG).
• Recognition memory the memory involved in recognizing an object and all the other details that relate to this object. There are two forms: recollection and familiarity.
• Recollection means one can remember the object and almost every detail related to that object, such as time and place.
• Familiarity means one remembers encountering the object previously but doesn’t recall any specific detail about it. For example, when someone says to a person, “Your face is familiar, but I can’t remember where and when we met.”
• Episodic memory is the type of memory that specializes in recalling an event and its associated details; this is different from recognition memory, in which someone can consciously memorialize a specific event that happened throughout their life without being exposed to a similar situation.

The medial temporal lobe (memory system) is still an active research area; more precisely, the exact function of each structure in this lobe is currently being studied.[rx]

The Occipital Lobe

The occipital lobe is the smallest lobe in the cerebrum cortex. It is located in the most posterior region of the brain, posterior to the parietal lobe and temporal lobe. The role of this lobe is visual processing and interpretation. Typically based on the function and structure, the visual cortex is divided into five areas (v1-v5). The primary visual cortex (v1, BA 17) is the first area that receives the visual information from the thalamus, and its located around the calcarine sulcus. The visual cortex receive, process, interpret the visual information, then this processed information is sent to the other regions of the brain to be further analyzed (example: inferior temporal lobe). This visual information helps us to determine, recognize, and compare the objects to each other.[rx]

References