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).
Prefrontal cortex
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)).
The patient:
• 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.
