Localisation of Function of the Cerebral Cortex

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The cerebral cortex | Main Anatomy Index | Blood supply to the brain

Last updated 30 March 2006

Localisation of Function of the Cerebral Cortex

There is a controversy about the localisation of function.
At one extreme, some maintain that particular patches of cortex are the unique sites of particular functions.
At the other extreme is that large areas of cortex form uniform fields in which functions are not localised and that complex activities depend on the amount of such cortex that is intact, not on the particular areas.

However, it has been shown clinically that reasonably predictable deficits are found after damage to various cerebral sites.
Thus it is convenient to consider functions as though they were localised to specific cortical areas.

Cortical Mapping

The mapping system devised by Brodmann has very widespread use.
He divided the cortex of each hemisphere into 52 areas.

Nolte (1993) The Human Brain. Table 10. p. 373.

Lobe	Number	Location	Other names
Frontal	4	Precentral gyrus, paracentral lobule	Primary motor area
	6	Superior and middle frontal gyri, precentral gyrus	Premotor area, supplementary motor area
	8	Superior and middle frontal gyri	Inferior portion = frontal eye field
	44, 45	Opercular and triangular parts of inferior frontal gyrus	Broca's area
Parietal	3, 1, 2	Postcentral gyrus, paracentral lobule	Primary somatosensory area; S1
	5, 7	Superior parietal lobule	Somatosensory association area
	39	Inferior parietal lobule	Angular gyrus
	40	Inferior parietal lobule	Supramarginal gyrus
Occipital	17	Banks of calcarine sulcus	Primary visual area; V1
Occipital	18, 19	Surrounding 17	Visual association area; V2, V3, V4, V5
Temporal	41	Superior temporal gyrus	Primary auditory area; A1
	42	Superior temporal gyrus	Auditory association area; A2
	22	Superior temporal gyrus	Wernicke's area

Cortical Areas

The neocortex of each hemisphere is traditionally divided into:

The primary sensory area (receives inputs from thalamic relay nuclei);
The primary motor area (giving rise to the pyramidal tract);
The association areas;
And limbic areas.

The association cortex is commonly divided into 2 broad types.

The area adjacent to a primary area is typically a unimodal association area, devoted to an elaboration of the business of that primary area.
Thus, areas 18 and 19, which surround the primary visual cortex are part of the visual association cortex.
Similarly, the superior parietal lobule, much of the superior temporal gyrus and the premotor cortex are involved in somatosensory, auditory and motor functions.

These multimodal or heteromodal association areas include the inferior parietal lobule, and large portions of the frontal and temporal lobes.
Neurons in these areas typically respond to multiple sensory modalities and may change their response properties under different circumstances.
These areas are therefore thought to be concerned somehow with high-level intellectual functions.

Sensory Areas

Somatosensory Cortex

Somatosensory information travels:

Medial lemniscus & spinothalamic & trigeminothalamic tracts --> VPL/VPM of thalamus --> posterior limb of internal capsule --> areas 3, 1, 2

These 3 areas are long, parallel strips of cortex that together comprise of almost the complete postcentral gyrus.
Most of area 3 is in the posterior wall of the central sulcus.

Area	Sensory Input
3	Slowly adapting cutaneous receptors
1	Rapidly adapting cutaneous receptors
2	Deep receptors (e.g., those of joints)

These three areas together are often referred to as the first somatosensory area or S1.
There is also a second somatosensory area or S2 that occupies the parietal operculum and much of it is buried in the lateral sulcus.

Visual Cortex

The primary visual cortex (striate cortex) corresponds to area 17.

Peripheral parts of the visual fields are represented anteriorly.
The fovea has a disproportionately large representation located posteriorly.

The two-part visual association cortex occupies the rest of the occipital lobe.

Area 18 surrounds area 17 and is itself surrounded by area 19.
Additional visual association areas occupy much of the temporal lobe.

Auditory Cortex

The superior surface of the temporal lobe forms on of the walls of the lateral sulcus.
Two transverse temporal gyri cross the posterior part of this surface and form areas 41 and 42.

Area 41 receives most of the auditory radiation from the medial geniculate nucleus via the sublenticular part of the internal capsule.
This it is the primary auditory cortex or A1.

Area 42 receives auditory information from both area 41 and the medial geniculate nucleus and is designated A2.
This is also the area of the planum temporale.
This is referred to as part of the primary auditory cortex by some authors and as part of the auditory association cortex by others.
In the dominant hemisphere, the planum temporale is thin and long while in the non-dominant hemisphere is it fatter and shorter.
The overall volume of this area is the same, however.

Area 22 forms much of the superior temporal gyrus and is called the auditory association cortex.

Motor Areas

Cortex	Brodmann areas	Area	Electrical stimulation
Primary motor cortex	4	Precentral gyrus	Discrete contralateral movements of one muscle or small groups of muscles. Movements of the palate, pharynx, masseter and often the tongue (but not face) are bilateral.
Premotor area	6	Anterior to precentral gyrus	Movements are slower and involve larger groups of muscles.
Supplementary motor area	6	On the medial surface of the hemisphere, anterior to the representation of the foot in the primary motor cortex	Cause movements that are assumption of postures and may involve muscles on both sides of the body.

Higher Functions

Cerebral Dominance

Language tends to be lateralised in the human brain.
The hemisphere that is more important for the comprehension and production of language is called the dominant hemisphere.

Nearly all right-handed people (> 95%) have a dominant left hemisphere.
Although left-handed people are more likely than right-handed people to have a dominant right hemisphere, the majority still have a dominant left hemisphere.

Broca's Area

This traditionally occupies the opercular (44) and triangular parts (45) of the inferior frontal gyrus.
Recent research has, however, suggested that the orbital part (47) of the inferior frontal gyrus is also part of this area.

Broca's area is considered to contain the motor programs for the generation of language.

A lesion to Broca's area leads to Broca's aphasia:

Broca's aphasics produce few words, either written or spoken and have great difficulty producing them.
They tend to leave out all but the most meaningful words in a sentence and to speak or write in a telegraphic manner.
These patients have relatively little difficulty in comprehending language.

Broca's aphasia is all called non-fluent, motor or expressive aphasia.

Wernicke's Area

This area occupies the posterior part of the superior temporal gyrus (22) and much of the inferior parietal lobule (40, 39).
Recent research would suggest, however, that this area is also, larger than traditionally thought including areas 20, 21, 41, 42, and 37.

Wernicke's area is considered to contain the mechanisms for the formation of language.

A lesion in this area leads to Wernicke's aphasia:

Wernicke's aphasics are able to produce written and spoken words, but the words or the sequence in which they are used are defective in their linguistic content.
There may be substitution of one word for another (paraphasia);
The insertion of new and meaningless words (neologisms);
Or the stringing of words and phrases in an order that conveys little or no meaning (jargon aphasia).

These patients have difficulty in comprehending whether their own speech makes sense and indeed, they are deficient in language comprehension generally.
This type of aphasia is also called fluent, sensory or receptive aphasia.

Conduction Aphasia

There are neural projects from Wernicke's area to Broca's area.
This is through the superior longitudinal fasciculus.

A selective lesion of these fibres would lead to:

A fluent aphasia similar to that seen in a lesion of Wernicke's area.
However, since Wernicke's area itself is undamaged, the patient still has intact comprehension.

This syndrome is called conduction aphasia.

Parietal Lobe Syndromes

The consequences of large lesions of the right parietal lobe include:

Difficulty with spatial orientation to everything on the left side.
The patient may completely ignore the halves of objects to the left side as well as the left side of their own body.
In some cases, there is a general difficulty in spatial orientation that shows up as a difficulty if following maps or in finding locations even in familiar surroundings.

The consequences of large lesions of the left parietal lobe include:

There is sometimes contralateral neglect, but much less frequently.
The lesion may encroach on Wernicke's area and cause more prominent aphasic disturbances.

Other deficits that accompany damage to the parietal-occipital-temporal association cortex include various agnosias and apraxias.

Some agnosias tend to be associated with bilateral damage.

Apraxias often accompany certain forms of aphasia and it can result from a lesion to either side.

Prefrontal Cortex

These are parts of the frontal lobe anterior to areas 4, 6 and 8.
When they are stimulated, they do not cause movements.

A lesion to the prefrontal cortex can result in:

The individual becomes carefree and apparently euphoric;
There is a loss in their capacity to do things for a delayed reward;
There is less inclination to observe social norms;
Lowered powers of concentration;
Lowered attention span;
Lowered initiative;
Lowered spontaneity;
And lowered abstract reasoning.