Introduction
The brain is one of the largest and most complex organs in our body; it is composed of billions of neurons that communicate together by forming numerous connections and synapses. The brain weight is different between men and women; The male brain weighs about 1336 grams, and the female comes in at about 1198 grams, but this difference in weight has shown no effect on function or intelligence.[1] There are three main divisions cerebrum, cerebellum, brain stem. The cerebrum consists of two cerebral hemispheres the outer layer called the cortex (gray matter) and the inner layer (white matter). There are four lobes in the cortex, the frontal lobe, parietal lobe, temporal lobe, occipital lobe.
This review article will focus on the functions of the cerebral cortex.
Development
The cerebral cortex develops from the most anterior part, the forebrain region, of the neural tube.
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.[2]
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.[3]
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[4]
- 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 situation
Adjusting 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.[5]
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.[8]
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.[9]
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.[10]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.[11]
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.[10]
- The middle temporal gyrus (MTG) has four sub-regions, the anterior, middle, posterior, and sulcus MTG.[12]
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).
- 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.[13]
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.[14]
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.[15]
Clinical Significance
Cerebral cortex dysfunction can occur due to various causes (lesions) like tumors, trauma, infections, autoimmune diseases, cerebrovascular accident. The clinical features for each cause will depend on which lobe is affected. I will review some of the clinical features and their relation to each lobe.
Frontal Lobe Lesions Presentation[16]
- Flaccid hemiplegia
- Weakness
- Apraxia
- Personality disorders
- Aphasia
Parietal Lobe Lesions Presentation[8]
- Astereognosis
- Aphasia
- Apraxia
- Loss of sensation
Temporal Lobe Lesions[17]
- Deafness
- Phenomic paraphasia
- Auditory or memory, visual hallucinations
Occipital Lobe Lesions
Visual field deficits like complete blindness or color blindness.[15]
Physiology, Cerebral Cortex Functions