Introduction
The brain is one of the largest and most complex organs in the human body; it is composed of billions of neurons that communicate through numerous connections and synapses. Brain weight differs between men and women; the brain weighs approximately 1336 g in men and 1198 g in women, but this difference has no demonstrated effect on function or intelligence.[1] The brain has 3 main divisions: the cerebrum, cerebellum, and brainstem. The cerebrum consists of 2 cerebral hemispheres, each containing an outer layer called the cerebral cortex, which is composed of gray matter, and an inner layer composed of white matter. The cerebral cortex has 4 lobes: the frontal, parietal, temporal, and occipital lobes. This article reviews the functions of the cerebral cortex.
Function
The Frontal Lobe
The frontal lobe is the largest cerebral lobe and is located in the anterior portion of the cerebral hemispheres. Its major functions include prospective memory, speech and language, personality, decision-making, and movement control.
Prospective memory: Prospective memory involves remembering previously established plans, ranging from simple daily tasks to long-term goals.[2]
Speech and language: The frontal lobe contains Broca area, which is located in the posterior inferior frontal gyrus and is involved in speech production. Results from a recent study showed that Broca area mediates sensory representations originating in the temporal cortex and traveling to the motor cortex.[3]
Personality: Over the past several centuries, researchers have described personality changes occurring after frontal lobe injuries. One of the best-known examples involved Phineas Gage, a gentle, polite, and sociable young man before a large iron rod passed through his eye and damaged his prefrontal cortex. The injury resulted in emotional insensitivity, socially inappropriate behavior, and an inability to make rational judgments. Results from a recent study suggested that damage to the prefrontal cortex can produce 5 subtypes of personality change:
Decision-making: The ability to make decisions involves reasoning, learning, and creativity. Results from a study conducted in 2012 proposed a new model for understanding decision-making processes in the frontal lobe, specifically how the brain creates a new strategy for a recurrent but novel situation or an open-ended environment. The investigators called this framework the PROBE model.
Individuals typically adapt to a situation in 3 possible ways:
Selecting a previously learned strategy that precisely applies to the current situation
Adjusting an already learned approach
Developing a creative behavioral method
The PROBE model proposes that the brain can compare a maximum of 3 to 4 behavioral methods before selecting the best strategy for the situation.[5]
Movement control: The frontal lobe contains the motor cortex, which is divided into 2 regions: the primary motor cortex, located posterior to the precentral sulcus, and the nonprimary motor cortices, including the premotor cortex, supplementary motor area, and cingulate motor areas. The exact function of each structure and its role in movement remain areas of active research.[6]
The Parietal Lobe
The parietal lobe is located posterior to the frontal lobe and superior to the temporal lobe and is divided into 2 functional regions. The anterior parietal lobe contains the primary somatosensory cortex, located in the postcentral gyrus (Brodmann areas 3, 1, and 2). The primary somatosensory cortex receives most sensory input from the thalamus and interprets basic somatosensory signals, including touch, position, vibration, pressure, pain, and temperature.[7]
The posterior parietal lobe contains 2 regions: the superior parietal lobule and the inferior parietal lobule.
The superior parietal lobule contains the somatosensory association cortex (Brodmann areas 5 and 7), which is involved in higher-order functions such as motor planning. The inferior parietal lobule, which includes the supramarginal gyrus (Brodmann area 40) and angular gyrus (Brodmann area 39), contains the secondary somatosensory cortex. The secondary somatosensory cortex receives somatosensory input from the thalamus and contralateral secondary somatosensory cortex and integrates this information with other major sensory modalities, including visual and auditory input, to support the following higher-order functions:
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 temporal lobe is the second-largest cerebral lobe, occupies the middle cranial fossa, and lies posterior to the frontal lobe and inferior to the parietal lobe. The temporal lobe has 2 surfaces: lateral and medial.[9] The lateral surface is divided by the superior temporal sulcus and the inferior temporal sulcus into 3 gyri: the superior temporal gyrus, middle temporal gyrus, and inferior temporal gyrus. The superior temporal gyrus (STG) is further subdivided into two surfaces, the dorsal surface (superior temporal plane STP) and the lateral surface of the STG.
The superior temporal gyrus is further divided into 2 surfaces: the dorsal surface, also called the superior temporal plane, and the lateral surface. The superior temporal plane is located deep within the Sylvian fissure. The most significant anatomical landmark of the superior temporal plane is Heschl gyrus, which contains the primary auditory cortex. The primary auditory cortex translates and processes sounds and tones and is minimally affected by task requirements. A task requirement may involve an examiner pronouncing words and asking the participant to categorize them acoustically, phonemically, or semantically.[10] The superior temporal plane also contains Wernicke area adjacent to Heschl gyrus. Historically, Wernicke area was believed to play a significant role in speech perception and comprehension, but results from recent studies showed that this region does not independently mediate these processes. Researchers found that speech perception and comprehension represent complex processes distributed throughout the brain rather than a single, localized task. The primary function of the Wernicke area involves phonological representation, through which a spoken word is interpreted according to its tones and sounds and linked with a previously learned sound.[11]
The lateral surface of the superior temporal gyrus is thought to contain the secondary auditory cortex, which also functions in interpreting sounds, particularly during activities involving specific task requirements.[10] The middle temporal gyrus has 4 subregions: anterior, middle, posterior, and sulcal.[12]
The anterior middle temporal gyrus is primarily involved in:
Default mode network: The default mode network demonstrates characteristic activity when the brain is at rest. Activities requiring sustained attention or pursuit of a specific goal, such as studying or playing a game, deactivate this network.
Sound recognition: Sound recognition supports the functions of other auditory processing regions.
Semantic retrieval: Semantic retrieval assigns meaning to words or sounds by retrieving previously learned concepts when available.
The middle portion of the middle temporal gyrus has 2 functions:
Semantic memory: Semantic memory involves remembering concepts or information considered common knowledge, such as the location of a bathroom.
Semantic control network: The semantic control network consists of connections among several brain regions, including the middle temporal gyrus, that assign meaning to words and sounds through stored knowledge and semantic retrieval mechanisms.
The posterior middle temporal gyrus is thought to be part of the classic sensory language area. The sulcal portion of the middle temporal gyrus is involved in decoding gaze direction and processing speech. The inferior temporal gyrus is involved in visual and facial perception and contains the ventral visual pathway. This pathway carries information from the primary visual cortex to the temporal lobe and helps determine the content of visual perception.[13]
The medial surface of the temporal lobe, also called the mesial temporal lobe, includes the hippocampus and the entorhinal, perirhinal, and parahippocampal cortices. These anatomically related structures are essential for declarative memory. Declarative memory is a type of long-term memory that involves remembering concepts, ideas, and events experienced or learned throughout life. Declarative memory is further divided into 3 types:
Semantic memory: Semantic memory was discussed previously in the section on the middle temporal gyrus.
Recognition memory: Recognition memory involves recognizing an object and recalling related details. Recognition memory has 2 forms: recollection and familiarity.
Recollection: Recollection allows an individual to remember an object and nearly every related detail, including the time and place of the encounter.
Familiarity: Familiarity occurs when an individual remembers previously encountering an object but cannot recall specific details about the encounter. For example, a person may recognize that a face is familiar without remembering where or when the encounter occurred.
Episodic memory: Episodic memory involves recalling an event and its associated details. Unlike recognition memory, episodic memory allows an individual to consciously recall a specific life event without exposure to a similar situation.
The medial temporal lobe memory system remains an area of active research, particularly regarding the exact function of each component structure.[14]
The Occipital Lobe
The occipital lobe is the smallest lobe of the cerebral cortex. Located in the most posterior region of the brain, the occipital lobe lies posterior to the parietal and temporal lobes. Its primary functions are visual processing and interpretation. Based on function and structure, the visual cortex is typically divided into 5 areas (V1 through V5). The primary visual cortex (V1; Brodmann area 17) is the first area to receive visual information from the thalamus and is located around the calcarine sulcus. The visual cortex receives, processes, and interprets visual information before transmitting it to other brain regions, such as the inferior temporal lobe, for further analysis. Visual information enables individuals to identify, recognize, and compare objects.[15]
Clinical Significance
Cerebral cortex dysfunction can result from various lesions, including tumors, trauma, infections, autoimmune diseases, and cerebrovascular accidents. The clinical features depend on the affected cerebral lobe. The following sections summarize selected clinical features associated with lesions in each lobe.
Frontal Lobe Lesion Clinical Presentation
Flaccid hemiplegia
Weakness
Apraxia
Personality changes
Parietal Lobe Lesion Clinical Presentation
Astereognosis
Aphasia
Apraxia
Temporal Lobe Lesion Clinical Presentation
Deafness
Phenomic paraphasia
Auditory or memory, visual hallucinations
[17]
Occipital Lobe Lesions
Visual field deficits like complete blindness or color blindness [15]
