Transcript Cortex

To study changes in the brain. ..
• Multiple memory
system
• Top down approach
• Lesion of part(s) of
CNS
• Behaviours (system
level)
• Cortical involvement
in memory
• Bottom up approach
• Manipulation of the
sensory experience
• Neurophysiology &
neural anatomy
(cellular level)
• Examining changes
– Same region/cell
– (Hopefully) same
region/cell
Psych3FA3, Part III.
The Cerebral Cortex and Memory
STUDY QUESTIONS
• What is the functional organization of the cerebral
cortex?
• Is the organization of the cortex fixed during the
course of development?
• Is the organization of the cortex fixed in
adulthood?
Part III.
The Cerebral Cortex and Memory
STUDY QUESTIONS
• What is the functional organization of the cerebral
cortex?
• Is the organization of the cortex fixed during the
course of development?
• Is the organization of the cortex fixed in
adulthood?
Functional Specializations of Cortex
– General Principles
1. Different functional cortical regions
• Processing hierarchies
Functional Specializations of Cortex
– General Principles
1. Different functional cortical regions
• Processing hierarchies
2. Topographic organization
Functional Specializations of Cortex
– General Principles
1. Different functional cortical regions
• Processing hierarchies
2. Topographic organization
• Receptive field
Receptive field
a small circumscribed spatial region of the
sensory field within which cells respond to
stimulation.
Cells often respond preferentially to specific
features of the stimulus.
video
Functional Specializations of Cortex
– General Principles
1. Different functional cortical regions
• Processing hierarchies
2. Topographic organization
• Receptive field
3.Columnar organization
Primary visual cortex
i. cells with these response properties are
organized topographically along two
dimensions.
(1) Ocular dominance
preferences for activation by the
ipsilateral (same side of the head) or
contralateral (opposite side) eye.
(2) Orientation selectivity
preferences for an optimal
orientation of the contrasting edge
Representation of
these dimensions
1. Between layers
1. through the depths
2. similar properties
2. Within layers,
1. Ipsilateral and
contralateral ocular
dominance columns
alternate
2. orientation columns
are arranged in a
systematic
sequence.
Representation of these dimensions (con’t)
3. The combination of a full set of ocular dominance
and orientation columns that represent the same
small receptive field area is known as a
"hypercolumn."
4. Sets of such modules are organized
systematically to provide a full representation of
the contralateral visual field for each hemisphere.
Part III.
The Cerebral Cortex and Memory
STUDY QUESTIONS
• What is the functional organization of the cerebral
cortex?
• Is the organization of the cortex fixed during the
course of development?
• Is the organization of the cortex fixed in
adulthood?
The tuning and modification of
cortical processing networks by
experience (the classic studies by
David Hubel and Torsten Wiesel)
i.first observed in the primary visual cortex
associated with development in young animals.
ii.showed that response properties of primary
visual cortex neurons are plastic, that is,
modifiable by experience, during a "critical
period" of the first 4 weeks of life.
David Hubel and Torsten Wiesel
1.Plasticity in ocular dominance
1.closure of one eye  a shift in ocular
dominance of all cells toward a
preference for the active eye.
David Hubel and Torsten Wiesel
1.Plasticity in ocular dominance
1.closure of one eye  a shift in ocular
dominance of all cells toward a
preference for the active eye.
2.Plasticity in orientation selectivity
1.restricting exposure to stimuli with only
certain orientations of visual contrast  a
shift of all cells toward selectivity for the
trained orientation.
David Hubel and Torsten Wiesel (Con’t)
1. Such manipulations are not nearly so effective
after this early critical period ends,
1. a finding that led most investigators to
conclude that cortical organization becomes
fixed in adulthood.
2. Mechanisms
1. the reorganization of sensory maps, as well
as the normal initial organizing of sensory
maps, arises from a competition of activity
among different inputs to each cell.
Psych3FA3, Part III.
The Cerebral Cortex and Memory
STUDY QUESTIONS
• What is the functional organization of the cerebral
cortex?
• Is the organization of the cortex fixed during the
course of development?
• Is the organization of the cortex fixed in
adulthood?
Reorganization of the visual
cortex in adulthood, part 1
Plasticity of the adult cortex revealed
by creating small and selective
damage to a part of the normal
inputs to the cortex (Gilbert & Wiesel,1992.)
1.Experimental manipulation
1.a laser light  very small lesion
of the retina
Results
1. initially
1. produces a correspondingly small area of
silent primary visual cortex.
2. Short period of time (minutes or hours) after
lesion & after a 2-month recovery period
3. cells at the border of the deafferented area
1. become responsive to stimulation of intact
visual field areas.
2. expand in size
Retina (screen)
Baseline
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cortex
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Boundary of the lesion
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Reorganization
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Reorganization of the visual
cortex in adulthood, part 2
Cortical changes under normal
sensory and behavioural
experience (Gilbert)
a.Experimental manipulation
a."artificial scotoma"
Results
a. After a 10-minute period of conditioning the cell,
its receptive field expanded severalfold in length
c. The results suggested
• an ongoing process of modulation of receptive field
size,
• normal sensory experience, not just peripheral
lesions,
• the changes take place on a brief time scale.
e. Implications
the short-term plasticity must involve in some way a
change in the synaptic weight of existing
connections, altering the patterns of activation of
intrinsic circuits.
cortex
Retina or screen
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lesion
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lesion
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Rearrangements of the somatotopic
representation (Merzenich group ) on
monkeys through a variety of traumatic
and more natural interventions in
somatosensory input.
a.digit removal 
a.the cortical representation of neighbouring
digits invades the cortical zone whose
afferents have been removed
b.Surgical joining of the digits 
a.the establishment of a continuous somatic
representation of formerly discontinuous
zones for each digit.
Rearrangements of the
somatotopic representation
(Merzenich group) (Cont’)
Tactile discrimination training 
larger cortical representations of the
stimulated digits, and larger receptive fields
in the expanded areas.
cortical physiological activity acquired
during training is well correlated with the
behavioural performance in a discrimination
task.
How do higher areas of cortex
respond to learning - inferotemporal
cortex (IT)?
a. Two Visual pathways and IT
b. IT is the highest-order cortical visual
processing area, whose function is the
identification of objects by their visual
qualitiesand this area is thought to be the site of
long-term storage of memory about
visual objects.
evidence
• a. effects of damage to this area
– i. in humans, results in visual agnosia, a
selective deficit in visual object recognition
• b. observed to be activated in various PET
or fMRI studies of neurologically intact
individuals performing tasks requiring
visual object recognition
c. Normal sensory response properties of
IT cells
i. 1st type
(1) responsive to whole objects positioned almost
anywhere' within the visual fields.
(2) respond similarly to a particular stimulus
regardless of its
size,
contrast from the background
form,
location in the visual field
motion.
The selectivity of IT cells is sometimes highly
specific.
(1)the first explorations of IT described a cell that
responded best to the silhouette of a monkey's
hand,
(2)Other cells responded to the shape of a banana or
a toilet brush (used to clean monkey cages)
(3) most widely studied are IT cells that respond best
to faces
(a) The responses of these cells are relatively
invariant to size, color, contrast, and position.
(b) some neurons respond to
(i) particular features of faces
(ii) particular face orientation
(iii) face identity
2nd type of IT neurons
Change their firing patterns in accordance with
their recent past history (short-term or working
memory).
(1) delayed match to sample task: an animal is
presented with a sample cue, followed by a
memory delay during which that sample has to
be remembered.
Then one or more choice stimuli are presented
and the animal is required to respond
depending on whether the choice cue is the
same as the sample (a match) or not (a
nonmatch).
Joaquin Fuster and colleagues
(a) In one version of their task the monkey
was presented with a color cue and was
required to retain it for up to 20
seconds prior to the choice.
They identified cells that fired differentially to
specific colors of the sample and choice.
(b) Some of these cells maintained high levels
of activity during the memory delay, and this
activity was specific to the sample cue.