The association cortices (ACs)

In primates they constitute the largest area of the cortex (75% in humans). They respond to a combination of two or more inputs from the higher order sensory areas and pass output to both the motor and sensory cortices. The association cortices also receive information from the other hemisphere via the corpus callosum and anterior commissure and also from the brainstem. So inputs to the ACs are already highly processed, unlike inputs to the rest of the neocortex. The cognitive functions of the ACs are listed below.

ACs of the Parietal lobe (Specifically, right inferior parietal cortex)

• Lesions lead to attention and perception deficits ``contralateral neglect syndrome'' - deficit occurs in the left perceptual area.
  • Getting lost in familiar places, although topographical memory and ability to describe the route remain intact

  • Amnesia for body parts on the left side (neglect, or denying awareness)

  • Inability to perceive the left half of objects Neuroscience p. 471 fig. 24.4

• The right parietal cortex is thus found to mediate attention to both the left and right halves of the body and extrapersonal space, while the left hemisphere attends to only the right - confirmed by PET scan (damage to left side can therefore be compensated by right but not vice versa).

• ``Attention neurons'' have been found in this region of rhesus monkeys.

ACs of the Temporal lobe

• Lesions lead to deficits of recognising, identifying and naming objects ``agnosias'': lexical or mnemonic, or faces: ``prosopagnosia''. In the latter case, ability to identify other objects and subtle shape differences might be unaffected. also persons might still be recognised by voice, body shape and gait.

• ``Recognition neurons'' have been found in the inferior temporal gyrus of rhesus monkeys, which respond selectively to monkey faces in front view - other cells respond maximally to profiles, some others to hand shapes ... but none are so specific as to be ``grandmother cells.''

ACs of the Frontal lobe - the most complexly connected region

• Lesions lead to ``planning deficits'': cannot use previous information to guide subsequent behaviour.
  • Poor performance in the Wisconsin Card Sorting Task (guessing the category rule)

  • Difficulty in carrying out complex behaviours that are either temporally, spatially or even socially appropriate - thus a change in personality.

• ``Planning neurons'' have been found in rhesus monkeys around the principal sulcus of the prefrontal cortex: are active in the ``remembering'' phase of a delayed-response task; return to low activation during the motor phase of the behaviour.