HigherOrder Cerebral Function

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Chapter 19 Higher-Order Cerebral
Function
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Anatomical and Clinical Review Most of the human
brain cortex is association cortex. Functions of
the association cortex have been more difficult
to discover than those of any other brain
areas. Association cortex is unimodal (modality
specific) or heteromodal (higher order).
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Cerebral Cortex

• Brains most complex area with billions of
  neurons and trillions of synapses the tissue
  responsible for mental activities
• Consciousness
• Perceives sensations
• Commands skilled movements
• Emotional awareness
• Memory, thinking, language ability
• Motivation
• All higher mental functions

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Types of Cerebral Cortex

• Neocortex
• Newest in evolution
• About 90 of total cortex in humans
• 6 layers, most complex

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Cerebral Cortex

• Highly developed in humans, cetaceans, and
  primates
• Makes up about 50 of total brain weight in
  humans
• Surface area of 2.5 square feet in humans
• 4.5 mm thick in precentral gyrus 1.5 mm thick
  in visual cortex
• Only 1/3 of surface area visible, 2/3 in banks of
  sulci
• Surface of gyri and sulci
• Estimated to contain about 15 billion neurons in
  humans

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Histology of the Cerebral Cortex 1

• 2 main cell types are pyramidal and granule cells
• Pyramidal cells have large apical dendrite and
  basal dendrites
• Axon projects downward into subcortical white
  matter may have collaterals
• Pyramidal cell is the primary output neuron

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Histology of Cerebral Cortex 2

• Pyramidal neurons are large and complex
• Similar orientation
• Process input from many sources

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Histology of Cerebral Cortex 3Dendritic Spines

• Surfaces of dendrites covered with spines
• Increase dendrite surface area for synapses
• Faulty development of dendrites and spines seen
  in Downs Syndrome
• Dendrite complexity and spine numbers first
  increase postnatally and then decrease with old
  age

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Dendritic Spines

• Spines become more complex postnatally
• Spine abnormalities occur in some conditions
  where mental performance is diminished

Trisomy 21
Trisomy 13
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Histology of Cerebral Cortex 3

• Granule (stellate) cells are interneurons
• Short dendrites extending in all directions
• Short axon projecting to adjacent pyramidal cells
• Granule cells are expecially numerous in sensory
  and association cortex
• Horizontal cells (layer I) and multiform cells
  (layer VI)

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Functional Histology of the Cerebral Cortex

• Neocortex has 6 layers designated I, II, III, IV,
  V, VI
• Pyramidal cells predominate in layers III and V
• Granule cells in layers II and IV

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Types of Cortex

• Cytoarchitecture varies in different areas
• Number and size of cells
• Thickness of layers

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Cortical Columns

• Functional units are cortical columns
• Columns are vertically oriented groups of
  thousands of neurons in synaptic contact
• Main input layer is layer IV which receives
  thalamic input
• Thalamus is the main source of input to the cortex

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Functional Histology

• Layers V and VI output
• V to Basal ganglia, brainstem and spinal cord
• VI to thalamus
• Layers I, II, III associative projecting to
  other cortical areas
• Layer IV layer receiving inputs from thalamus
  and other cortical areas

CC
V VI
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Cortical Connections 1

• Intracortical fibers
• Association fibers
• Commissural fibers
• Subcortical fibers

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Cortical Connections 2

• Intracortical fibers
• short, project to nearby cortical areas
• most from horizontal neurons in layer I
• some from axon collaterals of pyramidal cells

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Cortical Connections 3

• Association fibers
• gyrus to gyrus and lobe to lobe in the same
  hemisphere
• arcuate fibers connect adjacent gyri
• long association fibers connect distant gyri
• originate from pyramidal neurons in layers II and
  III

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Cortical Connections 4

• Commissural fibers
• connect homologous areas of the two hemispheres
• Corpus callosum rostrum, genu, trunk, splenium
• rostrum genu connect frontal lobes
• trunk connects posterior frontal lobes, parietal
  lobes, and superior temporal lobe
• splenium connects the occipital lobes
• Originate with pyramidal neurons in layers II and
  III

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Cortical Connections 5

• Anterior commissure connects the inferior and
  middle temporal gyri in opposite hemispheres
  also olfactory connections
• Posterior commissure carries fibers from the
  pretectal nuclei and other nearby neurons

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Cortical Connections 6

• Projection fibers connect cortex with subcortical
  neurons
• corticofugal/efferent, project from cortex
• corticopetal/afferent, project to cortex
• Corticofugal project to corpus striatum,
  brainstem, and spinal cord
• Corticopetal projections arise mainly from the
  thalamus - the thalamic radiations
• Internal capsule carries most of these
  connections

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Functional Areas of Cerebral Cortex 1

• Anatomically the cortex is divided into 6 lobes
  frontal, parietal, temporal, occipital, limbic
  and insular
• Each lobe has several gyri
• Functionally the cortex is divided into numbered
  areas first proposed by Brodmann in 1909
• Brodmanns areas were described based on
  cytoarchitecture later they were found to be
  functionally significant

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Cortex cytoarchitecture Area 3 postcentral
gyrus Area 5 superior parietal lobule Area
4 frontal lobe primary motor cortex
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Functional Areas of Cerebral Cortex 2

• Cytoarchitecture is based on the density and
  numbers of different cortical neurons and
  thickness of layers

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Functions include higher-order sensory
processing, visual-spatial orientation, motor
planning, language processing and
production, determining socially appropriate
behavior and abstract thought. Numerous
subcortical structures are involved in these
functions as well including thalamus, basal
ganglia, and subcortical white matter. Complex
functions of the brain are thought to involve
both distributed networks of neurons and
localized groups of neurons in specific cortical
areas.
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Unimodal sensory association cortex receives most
input from primary sensory cortex of a specific
modality and performs higher order sensory
processing for that modality. Unimodal motor
association cortex projects primarily to primary
motor cortex and functions in formulating motor
programs for complex actions involving multiple
joints. Heteromodal association cortex has
reciprocal connections with both motor and
sensory association cortex of all modalities.
Heteromodal assoc. cortex also has reciprocal
connections with limbic cortex.
This organization enables heteromodal assoc.
cortex to perform the highest-order mental
functions. This includes integration of
abstract sensory and motor information from
unimodal assoc. cortex, together with emotional
and motivational influences provided by limbic
cortex.
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Principles of Cerebral Localization and
Lateralization For over 100 years there have
been 2 different theories about how the
human brain is organized. One group supported
the widespread network theory and the other
group the localized functional regions
theory. We now know that the brain has both
types of functional organization. Localized
regions carry out specific functions, but they do
so through network interactions with many other
regions of the brain. Focal brain lesions can
cause specific deficits as will be seen in
several examples such as aphasia and unilateral
neglect, but most functions are also carried out
by interconnected networks located in
different brain regions, so false localization
can occur in different injuries. Frontal lobe
functions involve networks of neurons in
different brain regions including frontal,
parietal, and limbic cortices, thalamus, basal
ganglia, cerebellum and brainstem.
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Disconnection syndromes occur when fiber
connections between different parts of a
functional network are damaged. For example, if
an injury damages white matter connections
between visual cortex and the language
processing areas in the adjacent association
cortex, a patient may lose the ability to
read. Hemispheric specialization is the tendency
of some functions to be lateralized to the left
or right hemisphere. One example is that the
brain areas involved in language understanding
and production are typically in the left
hemisphere in humans. Handedness is another
lateralized function. About 90 of humans are
right- handed, so that performing tasks such as
writing or closing buttons with the nondominant
hand is very difficult. Evidence shows that
skilled complex motor tasks for both right and
left limbs are programmed mainly by the dominant
hemisphere, so injury to the dominant
hemisphere will lead to more severe motor
deficits.
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Language is lateralized in humans. The left
hemisphere is dominant for language in over 95
of right-handers and 60-70 of left-handers. Many
left-handers have significant bilateral language
function. After a left hemisphere injury left
handers tend to recover language more quickly
than right-handers. Nondominant hemisphere is
specialized for certain nonverbal
functions. Complex visual-spatial skills,
imparting emotional significant to events and
language, and music perception are typically
nondominant hemisphere functions.
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The Dominant Hemisphere Language Processing and
Related Functions Wernickes area (22)
functions to enable particular sequences of
sounds to be identified and comprehended as
meaningful words. Parts of adjacent areas 37,
39, and 40 are often included because lesions of
these areas can produce Wernickes aphasia.
Speech production function is carried out by
Brocas area (44, 45). Adjacent parts of
cortical areas 9, 46, 47 or parts of even more
distant areas 6, 8, 10 are sometimes also
included. Ability to hear a word and then repeat
it aloud requires the connecting path from
Wernickes area to Brocas area be intact. This
path is called the arcuate fasciculus.
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Sounds are converted into words by Wernickes
area and then neural representations of words
are converted back into sounds by Brocas
area. Brocas and Wernickes areas do not carry
out their functions alone. They have reciprocal
connections with a large network of
cortical areas that are also involved in
language functions.
Brocas network connections prefrontal
cortex, premotor cortex, supplementary motor
area. Function in speech formulation, planning
and grammatical structure. Wernickes network
connections supramarginal gyrus, angular
gyrus, and temporal lobe area 37. Function in
language comprehension and lexicon for
converting sounds to words and meanings.
31
The angular gyrus is especially important in
reading. Visual inputs go to visual cortex and
then are processed in visual association cortex,
then to the angular gyrus and on to Wernickes
area. Connections through the corpus callosum to
the nondominant hemisphere function in
language processing. Damage to nondominant
hemisphere often produces difficulty judging the
emotional tone of voice or may have
problems producing speech with the correct
emotional content. These connections can also be
important if the dominant hemisphere is damaged
and the nondominant hemisphere is needed to carry
out language function (often requiring
therapy/training). Lesions of thalamus, basal
ganglia or adjacent white matter in the
dominant hemisphere can produce aphasia that can
be mistaken for a cortical lesion.
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Aphasia A defect in language processing caused
by dysfunction of the dominant cerebral
hemisphere. Aphasia can be confused with other
brain disorders.
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The most common cause of acute onset aphasia is
cerebral infarct.
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Brocas Aphasia Most common cause is stroke in
the superior division of the middle
cerebral artery. Most obvious symptom is
decreased fluency in which phrase length is
short (less than 5 words) more content words
(nouns) than function words (prepositions and
articles). Prosody, the normal melodious
intonation of speech that conveys meaning, is
lacking. Naming difficulties are common
comprehension is relatively intact.
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Wernickes Aphasia Most commonly caused by
stroke in the inferior division of the middle
cerebral artery. Markedly impaired comprehension
of language may not be able to follow
any instructions. Spontaneous speech has normal
fluency, prosody and grammatical structure
but is empty, meaningless, and full of
nonsensical paraphasic errors. Patients are often
unaware they have any language deficit. Anger or
paranoid behavior is common.
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• Functional Areas of Cerebral Cortex
• Frontal Lobe
• Primary motor cortex, precentral gyrus, area 4
• Premotor cortex, area 6, motor programs, apraxia
  (inability to perform
• movements in absence of paralysis)
• Frontal eye field, area 8
• Supplementary motor cortex, parts of areas 6 8,
  programming for
• complex movements
• Prefrontal cortex, areas 9, 10, 11, 12, 32, 46,
  and 47, orbitofrontal area
• functions in visceral and emotional activities
  dorsolateral area functions
• in intellectual activities such as planning,
  judgement, problem solving
• and conceptualizing
• 6. Brocas area, 44 45, production of speech

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• Parietal Lobe
• Primary somatosensory cortex, postcentral gyrus,
  areas 1, 2, 3
• Secondary somatosensory cortex, insula, posterior
  part of area 43
• Primary gustatory cortex, anterior part of area
  43
• Parietal association cortex, areas 5, 7, 39, 40

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Parietal Neglect SyndromeClinical Illustration

• Failure to recognize side of body contralateral
  to injury
• May not bathe contralateral side of body or shave
  contralateral side of face
• Deny own limbs
• Objects in contralateral visual field ignored

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• Temporal Lobe
• Primary auditory cortex, superior temporal gyrus,
  areas 41, 42
• Auditory association cortex, Wernickes area 22,
  Wernickes aphasia
• Limbic cortex, areas 20,21, 27,28,29,30,
  34,36,38 functions in
• emotions, complex behaviors and memory
  amnesia, Alzheimers
• disease, prosopagnosia (20, 21)

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• Occipital Lobe
• Primary visual cortex, upper and lower banks of
  calcarine fissure, area 17
• Visual association cortex, areas 18, 19 complex
  processing for color,
• movement, direction, visual interpretation
  lesion can cause
• visual agnosia