The Cerebral Cortex and Memory

1
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?
2
Cortical localization general principles a.
anterior -- motor processing. b. posterior fields
perceptual processing c. both the anterior and
posterior cortex involve processing
hierarchies. i. In the anterior cortex, the
primary motor the muscles of the body are mapped
out in a topographic organization, with adjacent
areas of cortex representing muscle groups in
adjacent areas of the body. ii. In the
posterior cortex there are distinct primary areas
for each sensory modality.
3

• the posterior cortex
• distinct primary areas for each sensory modality.
• Receptive field
• a small circumscribed spatial region of the
  sensory field within which cells respond to
  stimulation. Cells often respond preferentially
  to specific trigger features of the stimulus.
• (2) The receptive fields and other trigger
  features are organized in a topographic map of
  the sensory field and of other relevant sensory
• dimensions.

4

• Primary visual cortex
• i. cells with these response properties are
  organized topographically along two dimensions.
• 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

5

• Representation of these two dimensions
• Between layers, arranged in columns,
• through the depths of the cortical layers,
  neurons have very similar properties
• Within layers,
• Ipsilateral and contralateral ocular dominance
  columns alternate, and
• orientation columns are arranged in a systematic
  sequence.
• 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."
• Sets of such modules are organized systematically
  to provide a full representation of the
  contralateral visual field for each hemisphere.

6
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.
7

• David Hubel and Torsten Wiesel
• Plasticity in ocular dominance
• closure of one eye ? a shift in ocular dominance
  of all cells toward a preference for the active
  eye.
• Plasticity in orientation selectivity
• restricting exposure to stimuli with only certain
  orientations of visual contrast ? a shift of all
  cells toward selectivity for the trained
  orientation.
• Such manipulations are not nearly so effective
  after this early critical period ends,
• a finding that led most investigators to conclude
  that cortical organization becomes fixed in
  adulthood.
• Mechanisms
• 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.

8

• 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)
• Experimental manipulation
• a laser light ? very small lesion of the retina
• Results
• initially produces a correspondingly small area
  of silent primary visual cortex.
• Short period of time (minutes or hours) after
  lesion
• cells at the border of the deafferented area
  become responsive to stimulation of intact visual
  field areas.
• Cortical cells with receptive fields located near
  the boundary of the lesion expand in size
• after a 2-month recovery period,
• the topography of the cortex is reorganized such
  that the formerly disconnected zone becomes
  responsive to neighboring parts of the visual
  field

9

• Reorganization of the visual cortex in adulthood,
  part 2
• Cortical changes under normal sensory and
  behavioural experience (Gilbert)
• Experimental manipulation
• "artificial scotoma"
• Results
• 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, adapting in different ways to
  different scenes.
• 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.

10
cortex
Retina or screen
Before lesion
B
A
B
A
After lesion
B
A
B
B
11

• Rearrangements of the somatotopic representation
  (Merzenich group ) on monkeys through a variety
  of traumatic and more natural interventions in
  somatosensory input.
• digit removal ?
• the cortical representation of neighbouring
  digits invades the cortical zone whose afferents
  have been removed
• Surgical joining of the digits ?
• the establishment of a continuous somatic
  representation of formerly discontinuous zones
  for each digit.
• c. Tactile discrimination training (for several
  weeks) ?
• 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.

12
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
qualities-and this area is thought to be the site
of long-term storage of memory about visual
objects.
13
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

14

• c. Normal sensory response properties of IT cells
• i. 1st type
• 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.

15

• The selectivity of IT cells is sometimes highly
  specific.
• the first explorations of IT described a cell
  that responded best to the silhouette of a
  monkey's hand,
• 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

16
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).
17
Joaquin Fuster and his colleagues performed the
first studies (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. (c) Other studies have
revealed that many inferotemporal cells show
suppressed responses to repeated stimuli,
and suggested that this phenomenon may provide
signals for short-term memory.
18
(No Transcript)
19
(No Transcript)
20
(No Transcript)
21
(No Transcript)
22
(No Transcript)
23
(No Transcript)
24
(No Transcript)
25
(No Transcript)