The Association Cortex consists of areas of cortex which combine many external and internal conscious processes to achieve a particular higher level function of the brain. These different high levels functions are localised in different parts of the brain and are interconneted by bundles of subcortical association fibres.
All areas of the cortex are interconnected using subcortical axons. Higher level functions of the brain, i.e. these involving attention and thought, depend on sensory inputs of all types, memory, emotion, etc, in order to generate an appropriate response, be it movement, speech, or other output.
Areas of the cortex concerned with specific higher level functions, such as conversational speech, depend on many factors, internal and external. For example, auditory inputs that need to be interpreted, and combined with an appreciation of three-dimensional space (where the sounds originate), facial recognition (of friends or foes?), auditory memory of dialects, accents, etc, memory of contexts, previous experience, etc, etc. Different parts of the brain are concerned with all of these separate functions and subcortical association fibres connect them, so that a mental image involving many senses, memory traces and emotions are combined almost simultaneously in order to produce a thoughtful reaction.
Key Words: localisation of higher level functions of the brain, understanding words and speech, attention, emotion, thought, spatial awareness, behaviour, motivation, subcortical association fibres.
Subcortical white matter: bundles of association fibres
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The diagram shows the patterns of short and long association fibres connecting different areas of the cortex. These are the means by which various functions are integrated to generate the higher level activity characteristic of the cortex
The white matter beneath the cortex contains axons that connect all areas of cortex. These are association fibres and some connect adjacent areas of cortex, while others travel long distances within the hemisphere or cross to the opposite hemisphere.
These axons allow association areas of the cortex to access information processed by other parts of the brain. So, for example, movement of a finger towards an object in the visual field requires visual, proprioceptive and cutaneous feedback.
The association areas are adjacent to the primary receiving areas, e.g. the parietal lobe uses information from the somatosensory and visual cortices to guide the limbs, fingers and eye movements and generate complex movements requiring inputs from skin and eyes.
Lesions of these areas leads to a failure to integrate sensory information. Areas behind the postcentral gyrus integrate visual and spatial inputs and are involved in perceiving an awareness of trajectories of moving objects. Unsurprisingly proprioception (awareness of the position of body parts in space) is also represented in this area.
The somatosensory association cortex (Brodmann's areas 5 and 7) receives inputs from SI and SII and lies directly behind the sensory cortex in the superior parietal lobes. Damage to this area causes tactile agnosia, an inability to recognize objects even though the objects can be felt.
Lesions of the parietal lobe also cause a neglect syndrome in which the patient is unaware of the opposite side of the body, and does not wash or clothe it.
Areas of Association Cortex
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This diagram shows the functions of different areas of association cortex. The primary receiving areas for vision, hearing and touch are seen in the occipital, superior temporal and post-central regions; between these are areas that utilise all these inputs in various ways using sub-cortical connections.
Areas of the Association Cortex
The cortex is organised into areas with different functions, connected with each other by subcortical association fibres, and by connections with the thalamus and basal ganglia.
There are separate areas for understanding speech and processing written words, and spoken language is organised by a different area, Broca's Area.
Speech depends on Broca's area and this is present only in the left hemisphere of the majority of people.
Emotional behaviour and motivation depend on the limbic system and its connections with the temporal lobe.
Memories also involve combinations of sensory inputs, and the hippocampal cortex in the temporal lobe is closely involved in memory
Attention and thought is carried out by the pre-frontal areas of the cortex.
A consequence of this arrangement is that normal functioning of the brain involves the simultaneous integrated activity of many areas of the cortex, and the association fibres and association cortex are essential for these complex activities.