FUNCTIONAL DIVISIONS

The cerebral cortex is responsible for the higher-order functions of the brain, such as language and the processing of information.  It is possible to separate the cerebral cortex by function, creating functional divisions, also known as cortical areas.

The image below shows the functional divisions of the left cerebral hemisphere. In most human individuals the left cerebral hemispheres is where the language centres, Broca's Area (shown in the image as the Speech Centre) and Wernicke's Area, can be found.

Functional Divisions of the Left Cortical Hemisphere.  Image editted for the purposes of this website using Photoshop CS4.  Original image taken from Wikimedia Commons and is in public domain.

Prefrontal Cortex

Executive function is the typical psychological term used for the functions carried out by the prefrontal cortex.  It relates to the human ability to differentiate among conflicting thoughts, determine morality, weigh out future consequences of actions, maintain social control, work towards a difined goal and a variety of other abilities.

There are three possible ways to define the prefrontal cortex:

► As the granular frontal cortex (by the presence of cortical granular layer IV, though this is only present primates).

► As the projection zone for the mediodorsal nucleus of the thalamus.

► As part of the frontal cortex which does not evoke movement when electrically stimulated.

Motor Association Cortex

The motor association area of the frontal cortex is thought to play a role in the planning of complex movements. It is said to be agranular because it lacks an internal granular cortical layer (layer IV).

Primary Motor Cortex

The primary motor cortex, also known as M1, is also agranular.  It works with the motor association cortex to plan and execute movement.

M1 contains large neurons in its internal pyramidal layer (layer V) called Betz cells which have long axons that are sent down the spinal cord to synapse with muscle alpha motor neurons.  These Betz cells are referred to as upper motor neurons (UMNs).

Motor Homunculus

A broad somatotopic representation of different body parts in relation to their area of innervation along the primary motor cortex can be created.  This representation, or map, is known as the motor homunculus.

In humans, the lateral area of the primary motor cortex is arranged from top to bottom in areas that correspond to the buttocks, torso, shoulder, elbow, wrist, fingers, thumb, eyelids, lips and jaw. Interior sections of the motor area folding into the medial logitudinal fissure correspond with the legs.

These areas are in no way proportional to their size in the body, nor are they fixed in one position as amputation or paralyisis can cause motor areas to shift to adopt new parts of the body.

The areas for the hand and arm are the largest as these two body parts require very intricate and precise movement.

The Motor Homunculus. Image courtesy of Wikimedia Commons. This image is in the public domain.

Primary Somatosensory Cortex

The primary somatosensory cortex receives tactile information from the body.  This is where the primary processing of the tactile information occurs.

Sensory Homunculus

Like the primary motor cortex it is possible to map the somatosensory cortex with certain areas of the body depending on the amount or importance of somatosensory input from that area.

The sensory homunculus holds many similarities to the motor homunculus. For example, there is a large area of cortex devoted to sensation in the hands, while the back has a much smaller area.

The area for the hand in the sensory homunculus, however, is smaller than the area for the hand in the motor homunculus.  The area for the lips, on the other hand, is larger.

The Sensory Homunculus. Image courtesy of Wikimedia Commons. This image is in the public domain.

Sensory Association Cortex

This area is involved in the processing of multisensory information and controls fine sensation (judgement of texture, weight, shape, size).

It works along with the primary somatosensory cortex in order to process and make sense of tactile information.

Visual Association Area

This is where the complex processing of visual information occurs.  It works in association with the visual cortex and is often considered as part of the visual cortex.

Visual Cortex

The visual cortex is responsible for detecting simple visual stimuli and is made up of the primary visual cortex (a.k.a. striate cortex or V1) and extrastriate visual cortical areas.

The name striate cortex is derived from the stria of Gennari, a stripe that is visible to the naked eye.  The stripe represents myelinated axons from the lateral geniculate body terminating in cortical layer IV of the grey matter of the visual cortex.

The primary visual cortex is the simplest and earliest cortical visual area and is highly specialised for processing information about both moving and static objects.  It is particularly adept at pattern recognition.

Wernicke's Area

Wernicke's area is involved in the understanding of written and spoken language.  Language development or usage can be seriously impaired by damage to the Wernicke's Area.

Speech Centre (Broca's Area)

The speech centre, often known as Broca's area, is the region of the cerebral cortex traditionally thought to be involved in the production of speech rather than the understanding of language.  However, recent studies have demonstrated that Broca's area can also play a significant part in language comprehension.

Patients suffering from Broca's aphasia are typically able to understand speech but are unable to produce it fluently.  This type of aphasia is also known as non-fluent aphasia.

Auditory Association Area

The auditory association area has very similar functions to Wernicke's area and is a significant region concerning the processing of acoustic signals in order for the brain to distinguish between speech, music, or other noises.

Auditory Cortex

The primary auditory cortex is the first region of the cerebral cortex to receive auditory input.  Its is involved in the detection of sound quality, such as loudness and tone.

Neurons in the auditory cortex are organised according the frequency of the sound to which they respond to best.  Such that neurons at one end of the auditory cortex respond best to high frequencies and vicé versa.