Association Cortices: The Posterior Parietal And Prefrontal Cortex
The association cortices are parts of the cerebral cortex that do not have a primary motor or sensory role, but instead are involved in the higher order processing of sensory information necessary for perception and movement initiation. These association areas include:
• The posterior parietal cortex (PPC; defined in monkeys as corresponding to Brodmann’s areas 5 and 7, and in humans including areas 39 and 40);
• The prefrontal cortex (corresponding to Brodmann’s areas 9–12 and 44–47);
• The temporal cortex (corresponding to Brodmann’s areas 21, 22, 37 and 41–43). The temporal cortex is involved in audition and language, complex visual processing (such as face recognition) and memory (discussed in Chapters 26–28, 45–47).
Posterior parietal cortex
This area has developed greatly during evolution and relates to specific forms of human behaviour, such as the extensive use of tools, collaborative strategic planning and the development of language. It has two main subdivisions:
• One Involved Mainly With Somatosensory Information (Centred On Area 5);
• The Other With Visual Stimuli (Centred On Area 7).
Neurophysiologically, area 5 contains many units with a complex sensory input often with a convergence of different sensory modalities, such as proprioceptive and cutaneous stimuli. These units with such a dual input are probably involved in the sensory control of posture and movements. Other units with mul- tiple cutaneous inputs are probably more involved in object recognition. However, in addition to having these complex sensory inputs, units in this area are often only maximally activated when the sensory stimulus is of interest or behavioural significance. Clinical features of lesions in area 5 of the posterior parietal cortex include:
• A Contralateral Sensory Loss That Is Often Subtle, E.G. A Failure To Recognize Objects On Tactile Manipulation (Astereognosis).
• An Inattention To Stimuli Received On The Contralateral Side Of The Body. This Can Be So Severe That The Patient Denies The Existence Of That Part Of His Or Her Body, Which Can Then Interfere With The Actions Of The Normal Non-Neglected Side (Intermanual Conflict Or Alien Limb). More Commonly, The Patient Fails To Perceive Sensory Stimuli Contralaterally When Stimuli Are Simultaneously Applied To Both Sides Of The Body (Extinction).
In contrast, area 7 is more involved in complex visual processing, with many of the units in this area responding to stimuli of interest or behavioural significance (e.g. food). Many different units are found in this cortical area some of which maximally respond to the visual fixation and tracking, while others are more involved in the process of switching attention from one visual object of interest to another (light sensitive or visual space neurones). There are individual neurones in area 7 that respond to both sensory and visual stimuli. Some of these neurones are maximally activated when a stimulus is moved towards the neurone’s cutaneous receptive field from extrapersonal (distant) space, while others are maximally activated during visual fixation of a desired object in which there is concomitant movement of the arm towards that object.
Clinical features of lesions in area 7 of the posterior parietal cortex include:
• A neglect of visual stimuli in the contralateral hemifield;
• Defects in eye movement and the visual control of movement. In some patients, more striking deficits occur in the realm of complex visual processing such as route finding, the construction of complex shapes and the copying of motor actions/gestures (dyspraxia).
Finally, in humans, and to a lesser extent in other primates and animals, some units in the posterior parietal cortex are maximally activated by vestibular and auditory inputs (see Chapters 28 and 29). Therefore damage to this area in humans can lead to complex difficulties in vision and visually guided movements, balance and language processing, including arithmetic skills. This includes an inability to write (agraphia), to read (alexia) and calculate simple sums (acalculia).
This cortical area has increased in size with phylogenetic development and has its greatest representation in humans. It is involved in the purposeful behaviour of an organism and thus is intimately involved in the planning of responses to stimuli that include a motor component (see Chapter 35). Within this structure are specialized cortical areas such as the frontal eye fields (FEF; see Chapter 56) and Broca’s area (see Chapter 28). Although the pre- frontal cortex is treated as a functional whole, this is a gross simplification.
Many different types of units are encountered neurophysiologically in this area of cortex, but they generally respond to complex sensory stimuli of behavioural relevance, which can then be translated into a cue for movement.
Damage to this site in animals leads to increased distractibility with corresponding deficits in working memory (the ability to retain information for more than a few seconds) and a change in locomotor activity and emotional responsiveness. A patient with frontal lobe damage anterior to the motor areas has a characteristic syndrome without insight (as occurs in frontal variant frontotemporal dementia (FTD)).
• Is often disinhibited, which results in him or her behaving in an atypical, often childish fashion;
• Has very poor attention and is easily distractible, cannot retain information and is sometimes unable to form new memories, with a tendency to perseverate (the repetition of words or phrases and actions) and pursue old patterns of behaviour even in the face of environmental change;
• Is unable to formulate and pursue goals and plans, to generalize and deduce, and may have difficulties in judging risk;
• Displays a marked reduction in verbal output, which is also reflected in motor behaviour as evidenced by a lack of spontaneous movement;
• Has a change in food preference, typically favouring sweet over savoury foods;
• Can become apathetic with severe blunting of his or her emotional responses, although in some cases the converse is true with the patient becoming aggressive;
• Show overall changes in their personality and it is typically others who bring the patient to medical attention, as the patient usually denies there is any problem (no insight).
The reliance on the clinical symptomatology to describe the function of the prefrontal cortex relates to the fact that this part of the cortex is most developed in humans. However, extensive damage of the frontal lobes can also affect the cortical motor areas (see Chapter 38), eye movements (see Chapter 56), the ability to talk (an expressive dysphasia; see Chapter 28) and the control of micturition.