Cerebral Cortex 2
Functions of association cortex
Brain imaging
Yasushi Nakagawa
Department of Neuroscience
University of Minnesota
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Areas of the neocortex
Brodmann’s areas (you do not need to remember
the area numbers)
-proposed in early 20th century
-based on thickness and cell density of each layer, as well as size, shape
and arrangement of neuronal cell bodies (“cytoarchitecture”)
-some areas have highly developed pyramidal layers (layer 3 and 5) and
poorly developed layer 4
primary motor area (Area 4)
-some areas have highly developed layer 4 and poorly developed
pyramidal layers
primary visual area (Area 17)
primary somatosensory area (Area 3)
primary auditory area (Area 41)
-recent functional studies (fMRI, etc.) have found that Brodmann areas
are correlated with functional areas
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The story of Phineas Gage
1823-1860
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The story of Phineas Gage
-a railroad worker in the nineteenth century Vermont
-suffered extensive prefrontal damage at the age of 25 when blasting powder exploded
and drove a 3-foot long (1.25 inch in diameter) tamping iron through his head
Weighing 13 1⁄4 pounds (6 kg), this "abrupt and intrusive visitor" was
said to have landed some 80 feet (25 m) away, "smeared with blood
and brain."
Amazingly, Gage spoke within a few minutes, walked with little or no
assistance, and sat upright in a cart for the 3⁄4-mile (1.2 km) ride to his
lodgings in town. The first physician to arrive was Dr. Edward H.
Williams:
I first noticed the wound upon the head before I alighted from my carriage,
the pulsations of the brain being very distinct. Mr. Gage, during the time I
was examining this wound, was relating the manner in which he was injured
to the bystanders. I did not believe Mr. Gage's statement at that time, but
thought he was deceived. Mr. Gage persisted in saying that the bar went
through his head .... Mr. G. got up and vomited; the effort of vomiting pressed
out about half a teacupful of the brain, which fell upon the floor.
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The story of Phineas Gage
http://www.nejm.org/doi/full/10.1056/NEJMicm031024
-recent examination shows that the brain injury must have been limited to the left
frontal lobe and spared the superior sagittal sinus.
-formerly “a shrewd, smart business man, very energetic and persistent in pursuing all
his plans”
-he became “pertinaciously obstinate, yet capricious and vacillating, devising many plans
of future operations, which are no sooner arranged than they are abandoned in turn for
others appearing more feasible”
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What does this story tell us?
The Phineas Gage case showed that frontal lobe damage may lead to:
-dramatic alterations of strategic thinking, personality, emotional integration and
conduct
but not
-language, memory, sensory-motor functions
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Association areas
The term association areas is traditionally used for parts of the cortex that neither receive
direct sensory information through the major sensory pathways (somatosensory, auditory, visual)
or motor thalamic nuclei (ventro-lateral, ventro-anterior).
How are different modalities of sensory information integrated in the cerebral cortex
and elicit various types of behavioral output?
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Association areas
Association areas have much more extensive input and output connections than
primary sensory and motor areas.
Association areas occupy much larger fraction of the total cerebral cortex in the
human brain than in monkey (and all other mammalian) brains
blue shows association areas
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Progression of connections in the cerebral
cortex (somatosensory information)
718
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a
5
5
6
4
The Cerebral Cortex and Complex Cerebral Functions
7
7
S
46
FROM:
5: somatosensory association
area (anterior parietal cortex)
FROM:
S: primary somatosensory area
b
8A
17
TO:
5: somatosensory association
area (anterior parietal cortex)
20
4: primary motor
area
TO:
STS: TG
multimodal sensory
association area (temporal
lobe)
45, 46: prefrontal cortex
STS
c
8B
9
STS
FROM:
7: higher order somatosensory
association
area (posterior
46
parietal cortex)
TO:
7: higher order
somatosensory
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association area (posterior
parietal cortex)20
6: premotor area
8B
45
10
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Functions of association areas
Parietal association areas: sensory guidance of motor behavior and spatial awareness
(connected to visual, somatosensory and motor areas)
Temporal association areas: recognition of sensory stimuli and storage of factual
knowledge (vision, touch, hearing)
Frontal association areas: organizing behavior and working memory
Limbic association areas: emotion and episodic memory
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Functions of parietal association areas
Parietal association areas are connected to visual, somatosensory and motor areas of the
cerebral cortex
Damage of parietal association areas will lead to:
1) impairment of body awareness, motor control and visual guidance of motor behavior (dorsal
part of posterior parietal cortex)
-patients deny the existence of the arm or leg contralateral to the lesion (asomatognosia)
-unable to execute certain movements (e.g., waving goodbye) on command or by imitation
-difficulty in reaching for an object in the peripheral visual field
“Balint syndrome” http://www.youtube.com/watch?v=4odhSq46vtU
1) impairment of using visual information to guide movement (“optic ataxia”)
2) inability to perceive the visual field as a whole
3) inability to voluntarily guide eye movements
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Functions of parietal association areas
2) spatial perception and cognition (ventral part of posterior parietal cortex)
-patients ignore objects in half of the space
opposite to the cortical lesion (“hemispatial
neglect”)
-patients may be unable to appreciate the
structure and arrangement of things by
looking at them (“constructional apraxia”)
http://www.youtube.com/watch?v=ymKvS0XsM4w
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Functions of association areas
Parietal association areas: sensory guidance of motor behavior and spatial awareness
(connected to visual, somatosensory and motor areas)
Temporal association areas: recognition of sensory stimuli and
storage of factual knowledge (vision, touch, hearing)
Frontal association areas: organizing behavior and working memory
Limbic association areas: emotion and episodic memory
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Functions of the temporal association
cortex
temporal association cortex receives information about the shape, color and texture of visual
images through the ventral visual pathway
Temporal association cortex mediate the recognition of objects in the environment, and through
projecting to the prefrontal cortex, trigger appropriate emotional responses to them.
Lesions in the visual association areas cause visual agnosia (they cannot recognize things but
can draw them). “The Man Who Mistook His Wife for a Hat” by Oliver Sacks
Visual object agnosia may be general or specific to fine distinction within a category of objects
such as faces (=prosopagnosia).
http://www.youtube.com/watch?v=vwCrxomPbtY&feature=related
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Functions of association areas
Parietal association areas: sensory guidance of motor behavior and spatial awareness
(connected to visual, somatosensory and motor areas)
Temporal association areas: recognition of sensory stimuli and storage of factual
knowledge (vision, touch, hearing)
Frontal association areas: organizing behavior and working memory
Limbic association areas: emotion and episodic memory
15
Functions of the prefrontal cortex
The prefrontal cortex is involved in many forms of executive control (acting on intentions)
-simple ones: particular action, simple mental math
-complex ones: general plans, career path
Patients with damaged prefrontal cortex (like Phineas Gage) are typically normal in perceptual
ability or motor behavior and may perform normally on tests of intelligence. But...
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Functions of the prefrontal cortex
patients with prefrontal cortex lesions are ineffective in carrying out plans (dorsal and lateral
prefrontal cortex)
-tasks involving planning and adjustment of strategy:
Wisconsin Card Sort Test
working memory (ability to hold information in mind and manipulate it mentally, such as
dialing a telephone numbers or doing mental arithmetic) is also affected
lesions in the ventral and medial prefrontal cortex causes emotional impairment
-flat, shallow, apathetic, indifferent
-loss of religious feeling, loss of appreciation for literature or music
-insensitive to feeling of others
-emotional changes occur in nonhuman primates
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Brain imaging
How can we understand biological basis of cognitive functions?
-use lab animals for invasive experiments (e.g., electrophysiology on monkey)
-clinical studies of patients with cognitive disorders
-structural imaging
obtain detailed anatomical images of the brain
-functional imaging
non-invasive methods used to observe the areas of the human brain during cognitive
processes
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Structural imaging
Two classical methods:
CT (X-ray computerized tomography)
MRI (magnetic resonance imaging)
-developed in the 70s and 80s
-still widely used as diagnostic tools
-detailed 3D (but static) anatomical images of the brain in a living patient
CT
-use an X-ray tube that emits a narrow beam of X-rays and rotates around the head of
the subject.
-detects different brain structures that vary in density
subdural hematoma
glioblastoma multiforme
stroke due to MCA
occlusion
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Structural imaging
MRI
-provides more detailed anatomical images than CT
-measures current caused by the changing magnetic field around tissue protons
-computes two different time constants that characterize the recovery from altered
magnetic field
-can localize the signal in 3D volume of the brain
(voxel)
-current minimum voxel size (resolution) is 1-3mm
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Structural imaging
Diffusion tensor imaging (DTI)
-measures how far water diffuses within the brain
-A voxel in white matter typically shows greater diffusion of water in the direction of
the fiber tract and less diffusion in other directions.
-Measurement is repeated for each of several directions and images are combined.
-The most advanced application of DTI is fiber tracking, the only non-invasive method
currently available to characterize anatomical connectivity in the living human brain.
-The minimum cross-section diameter that can be detected is ~5mm
coronal DTI (what is shown in blue?)
long association fiber bundles
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Functional imaging
Positron Emission Tomography (PET)
-developed in the 80s
-detects specific molecules that reflect increased blood flow (water) or increased
energy consumption (deoxyglucose)
-requires introduction of radio-labeled substances (11C, 18F, 15O, 13N)
H215O for blood flow, 18F-deoxyglucose for energy consumption
-the extra proton in the radionucleotide breaks down into positron and neutron
-the positron travels (2-3mm) and hit an electron, which leads to emission of two
gamma-rays in opposite directions
-PET scanner detects gamma ray
-spatial resolution ~6-8mm (not high)
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Functional imaging
fMRI (functional magnetic resonance imaging)
-developed in the 90s
-detects increased blood flow associated with neural activity
-measures blood oxygen levels (BOLD: blood oxygen level dependent)
-with greater neuronal activity (more homogeneous magnetic field), T2 decay time is
longer and image intensity is brighter
-unlike PET, no need for injecting foreign substances into bloodstream
-higher spatial (~1mm) and temporal (~100ms) resolution than PET
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An example of fMRI data
30-year-old man listening
-to white noise
-to spoken words
another subject watching a red and black checkerboard
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Reconstruction of vision using fMRI
https://www.youtube.com/watch?
v=6FsH7RK1S2E&feature=youtu.be
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