Association areas of the brain

Unimodal vs heteromodal (multimodal) cortical areas

The cerebral cortex can be divided into unimodal and heteromodal (multimodal) areas based on the type of information they primarily process. Primary sensorimotor areas are strictly unimodal, they process information from a single modality and form specific, modality-restricted connections.

On the other hand, the association cortical areas support higher-order processing and include both unimodal and heteromodal (multimodal) regions. Unimodal association areas integrate information coming from different parts of one sensory or motor system. For example, occipital association areas integrate inputs from the primary visual cortex to further analyse visual information.

Heteromodal (multimodal) association areas integrate information across different areas with different functionality, such as visual, auditory, somatosensory, motor, and memory-related inputs. Association areas in the parietal and frontal lobes combine such diverse sources to support abstract and high-level cognitive functions.

Topologically, cortical regions located closer to primary cortices tend to be more specialized and unimodal, whereas more distal regions are heteromodal (multimodal) exhibiting greater flexibility, longer processing timescales, and broader integrative capacities leading to an hierarchy gradient.

There are four main association areas in the cerebral cortex:

• Occipital association cortex
• Temporal association cortex
• Prefrontal association cortex
• Parietal association cortex

The cerebral cortex has four main association areas, each serving a distinct region and functional role. The table below compares them.

Visual association cortex

.The occipital or visual association cortex is located in Brodmann areas 18 and 19, around the primary visual cortex of the occipital lobe, it extends into parts of the temporal cortex and processes visual information beyond the basic feature detection carried out by V1. The primary visual cortex, also called V1 area or striate cortex, is located in Brodmann area 17 on both sides of the calcarine sulcus, where the earliest stage of visual processing is carried out, including basic feature detection and the initial steps of visual analysis.

The visual association cortex is subdivided in areas V2, V3, V4, and V5, all of which are strongly and reciprocally connected to V1. The secondary visual cortex, also known as V2 area or parastriate cortex, receives strong feedforward visual information from V1 area, as well as from the pulvinar nucleus of the thalamus. In turn, it projects extensively to V3, V4, and V5 areas. It plays an important role in integrating visual information, refining or suppressing certain tuning patterns from V1 to enable more complex interpretations of natural images.

The V3 and V4 areas provide additional filtering and transformation of visual information coming from V1 and V2 areas before routing it to higher-order visual areas. V3 contains neurons selective for shapes and contours, as well as various aspects of motion processing related to orientation, curvature and direction. V4 contains neurons responsive to complex features involved in object recognition, scene understanding and color processing.

The V5 area or middle temporal visual area is an extrastriate region with a high concentration of direction-selective neurons. In primates, it plays a major role in motion perception, integrating local motion signals into global motion patterns and contributing to eye-movement guidance. In the human and primate brain another association visual area has been described. It is called V6 area, located in the parieto-occipital sulcus, and it is responsible for making representations of the entire contralateral visual hemifield and participates in wide-field motion and visuospatial processing.

The association visual cortex also receives input from other lobes, including the temporal lobe, allowing it to integrate visual signals with memory, reward information, and other sensory modalities. Because of these interconnections, it contributes to higher cognitive functions such as working memory, associative memory, visual comparison, and learning. Studies suggest that increased activation of visual association regions during memory encoding may support improved associative memory across development.

A clinical example is visual agnosia. Lesions in the occipital association cortex, particularly when they extend into the parieto-occipital and inferior temporal regions, can leave basic vision intact while impairing the ability to recognize objects or faces. The deficit arises because the connection between visual perception and stored knowledge is interrupted. Face recognition specifically (prosopagnosia) is most commonly associated with bilateral inferior temporal lesions.

Auditory association cortex

The temporal association cortex (auditory association area) is found in the superior temporal gyrus, adjacent to the primary auditory cortex. The temporal association cortex is connected through long fiber tracts linking visual and auditory regions with the prefrontal cortex to support perception, language, and higher-order semantic processing.

While the primary auditory cortex (A1) processes basic sound features (such as pitch, volume, and timbre), the auditory association area integrates these features into more complex perceptions, including the recognition of sounds, speech, and music. It is involved in interpreting sounds based on past experiences, attention, and memory, allowing us to understand and respond meaningfully to sounds.

An important mention is about the role of the auditory association area in speech comprehension. Wernicke's area is located in the posterior third of the superior temporal gyrus of the dominant hemisphere, with very strong connections to the supramarginal and angular gyri of the parietal lobe. Although its exact boundaries vary between individuals, it is consistently associated with the comprehension of spoken and written language. Some authors extend Wernicke's area to the posterior two-thirds of the STG and include adjacent BA 37, 39, and 40 (supramarginal and angular gyri), because lesions extending into those areas also produce Wernicke's aphasia. Language comprehension involves several connected processes: the reception of auditory or visual input, extraction of linguistic features, and assignment of meaning. Auditory language processing begins when sound waves are processed in the auditory cortex of the temporal lobe, and this information is then relayed to Wernicke’s area. Recent research has also shown that the homologue of Wernicke’s area in the non-dominant hemisphere contributes to aspects of speech comprehension, particularly for non-literal meaning and prosody.

A lesion to the temporal association cortex can result in diverse clinical manifestations, including:

Phonagnosia: The inability to recognize familiar voices, though spoken words may be still recognized.

Verbal auditory agnosia (pure word deafness): The inability to comprehend spoken words, while reading, writing, and speaking remain relatively normal.

Non-verbal auditory agnosia: The inability to comprehend nonverbal sounds and noises, but speech comprehension is preserved.

Amusia: The inability to recognize or comprehend music.

Wernicke's aphasia: A condition where speech output is fluent with normal rate and intonation, but comprehension of spoken and written language is severely impaired. The patient's own speech is typically full of paraphasic errors (inappropriate word substitutions and phonemic distortions), neologisms, and circumlocutions, making it meaningless to the listener. Repetition is also impaired. Patients are often unaware of their deficit.

Parietal association cortex

The parietal association cortex comprises the superior and inferior parietal lobules, and the precuneus, while it extends to cortical areas of the intraparietal, parieto-occipital, and lunate sulci. By processing somatic, visual, auditory, and vestibular sensory information, the parietal association cortex plays a pivotal role in spatial cognition, sensory integration, and control of movements. It includes areas devoted to integrating neural signals from diverse sensory modalities and strongly interconnected to areas involved in higher intellectual functions, such as language comprehension (mainly in the dominant hemisphere) and internal representation of spatial surroundings (mainly in the non-dominant hemisphere).

Damage to the parietal association cortex can result in deficits in the complex sensory perception and memory of spatial relationships, inaccurate action planning, difficulty controlling eye movements, and inattention. The two most notable clinical syndromes are described with the terms apraxia and hemispatial neglect.

Prefrontal association cortex

The prefrontal association cortex is located in the anteriormost portion of the frontal lobe and occupies portions of all three surfaces of the frontal lobe - orbital, medial, and lateral. It is connected to the cortical association regions of the occipital, the parietal and temporal lobes, the limbic system, the brainstem, and the thalamus. These extensive connections allow the prefrontal association cortex to play a crucial role in the top-down processing and regulation of both sensory and motor information.

The prefrontal association cortex is responsible for higher cognitive functions such as decision-making, planning, working memory, emotional regulation, self-control, social cognition, and cognitive flexibility.

A key example contributing to our understanding of the prefrontal association cortex comes from the case of Phineas Gage. Phineas Gage was a railroad worker who survived a severe brain injury in 1848 when a metal rod was driven through his skull, damaging his prefrontal cortex. Remarkably, he survived, but his personality changed drastically, becoming impulsive, rude, and irresponsible. This case became a landmark in neuroscience, providing early evidence of the prefrontal cortex's role in personality, decision-making, and social behavior. Gage's injury contributed significantly to the understanding of how brain damage can alter cognitive and emotional functions.

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