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

Hubels-research[www.savevid.com].flv

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 cortexi. cells with these response properties are

organized topographically along two dimensions.

(1) Ocular dominancepreferences for activation by the ipsilateral (same side of the head) or contralateral (opposite side) eye.

(2) Orientation selectivitypreferences for an optimal

orientation of the contrasting edge

Representation of these dimensions

1.Between layers1. 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 Wiesel1.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 Wiesel1.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 selectivity1.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.Mechanisms1. 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 manipulation1.a laser light very small lesion

of the retina

Results1. 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 area1.become responsive to stimulation of intact

visual field areas. 2.expand in size

1 2 3 4 5 1 2 3 4 5

1 2 3 4 5 1 2 3 4 5

Retina (screen) cortex

Baseline

WithoutReorganization

WithReorganization

1 1 3 5 51 2 3 4 5

Boundary of the lesion

Reorganization of the visual cortex in adulthood, part 2

Cortical changes under normal sensory and behavioural experience (Gilbert)

a.Experimental manipulationa."artificial scotoma"

Resultsa.After a 10-minute period of conditioning the cell,

its receptive field expanded severalfold in lengthc. 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.

B A B A

Retina or screen cortex

Before lesion

B A B B

After lesion

A

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 ITb. 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.

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 backgroundform, location in the visual fieldmotion.

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 neuronsChange 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 tobe 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 20seconds 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.