Neuroanatomy: Association Cortex

#Note: Font in red may be the probable MCQs in PG entrance exams.
Association cortex is the cerebral cortex outside the primary areas. It is essential for mental functions that are more complex than detecting basic dimensions of sensory stimulation, for which primary sensory areas appear to be necessary.
In humans the association areas are by far the most developed part of the cerebral cortex, and the brain in general. These areas are necessary for perceptual activities, like recognizing objects (toasters, horses, trees, words, etc), rather than simple contours, edges or sensory qualities like color or pitch.

The increasing size of association cortex correlates with the complexity of behavior and inferred mental functions that different species show.

In humans there is immence growth of 3 major association areas
  1. Frontal - In front of pre-motor area
  2. Parietal-temporal-occipital
  3. Temporal

  • Each sensory system has its own association areas on the cerebral cortex. The sensory systems (vision, hearing, etc.) each have their own primary area on the cortex, which gets the most direct connections from its sense. Each primary sensory area sends information to its own cortical association areas, which are next to their primary areas, as shown in Figure 1. (The motor system is organized in the same way, but in the reverse direction: from motor association areas to the primary motor area to the motor systems in the brain stem and spinal cord.)

Figure 1. Primary sensory and motor areas and sensory and motor association areas. Arrows show the direction of information flow from primary areas to sensory association areas. The information flows in the opposite direction for the motor system.

  • The processing that occurs in the sensory association areas is the basis of complex mental processes associated with each sense. Each sensory association area appears necessary for perception of objects and events in its sensory modality. The information that each sensory association area gets from its primary area is about simple contours, boundaries, and sensory qualities like color or pitch. Sensory association areas combine this kind of information to represent complex objects. For example, the visual association area on the lower part of the temporal lobe plays a primary role in your ability to recognize faces, dogs, cars, trees, etc., whereas the primary visual cortex is required for detecting basic features of the visual world: edges, light and dark, location, etc.
For example, a group of neurons in a monkey's visual association area on the temporal lobe respond only when it looks at a specific person (Young & Yamane, 1992). This suggests that activity in those neurons tells the brain/mind that the monkey looks at that specific person. So the next time you see your best friend, remember that you can see him/her because a few thousand neurons in the visual association area of your temporal lobe have become active. Give them a pat on the back for the great job they do for you, without you even asking

  • Damage to the auditory association cortex (around the primary auditory cortex on the top of the temporal lobe) leaves sensitivity to sound unaffected, but disturbs recognition of what sounds mean. Neurons in the auditory association areas respond much better to complex sound patterns like bird calls and speech sounds than to simple pure tones. 
  • Damage to the somatosensory association cortex (on the parietal lobe behind the primary somatosensory cortex) leaves sensitivity to touch unaffected, but disrupts ability to recognize objects by touch.

Higher Order Association Areas

Higher order association cortex carries out complex mental processes not associated with any particular sense. Each sensory and motor association areas sends signals to higher order association areas, which combine this information to form the basis of the highest mental processes. These highest mental processes, like language, thinking, and planning, do not depend on specific sensory information. For example, language can use vision (reading, sign language) and touch (Braille for the blind), as well as hearing. Figure 2-2e shows the primary areas and their association areas in colors and the higher order association areas in grey. The arrows show the flow of information from primary areas to sensory association areas to higher order association areas.

Figure 2-2e. Primary sensory and motor areas, sensory and motor association areas, and higher order association areas. Arrows show the direction of information flow from primary areas to sensory association areas, to higher order association areas. The information flows in the opposite direction for the motor system.

Figure 3-2e. Location of the the two main language sites in the higher order association areas of the left hemisphere.

The higher order association areas combine information from several sensory association areas. Figure 3-2e shows the higher order association areas in grey and shows the location of two parts that play a crucial role in language: Broca's area at the lower back of the left frontal lobe, and Wernicke's area, at the junction of the left temporal and parietal lobes. Both are shown on the left hemisphere, because it is dominant for phonetics (speech sounds) and for grammar in 99% of right-handed people and about 2/3 of left-haded people. (These are not the only brain areas that are important in language.)

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