lecture 10 - cortical maps and plasticity Flashcards
(21 cards)
what is a map
- A map is a representation.
- ‘..the map is not the territory it represents, but, if correct, it
has a similar structure to the territory, which accounts for its
usefulness.’. Alfred Korzybski, ’31. - ANYTHING can be represented, theoretically, in a map
maps in the Brain?
- Rodents use their
WHISKERS to explore
the world – not their
forepaws - MUCH of the rodent
brain is occupied by
REPRESENTATIONS
of the whiskers in a
specialised area
called… - BARREL CORTEX –
another name for
rodent somatosensory
cortex
diagram in notes
…maps in the brain?* Look at the red and blue
whiskers
INFORMATION about the
world is transmitted to the
brain through the
stimulation – or whisking
by the rodent - of each
whisker
* AT EACH LEVEL –
brainstem, thalamus,
(barrel) cortex, the
SPATIAL RELATIONSHIP
between the two whiskers
is conserved
INFORMATION about the
world is transmitted to the
brain through the
stimulation – or whisking
by the rodent - of each
whisker
* So barrel cortex in
rodents is a MAP of the
spatial organisation of
the rodent’s whiskers in
the periphery
sensory maps are not limited to touch
- visual ‘space’ is mapped in a similar fashion
- diagrams in notes
- this ‘mapping’ links an location on the retina with groups of neurons within the lateral geniculate nucleus and primary visual cortex (V1)
- this is a ‘retinotopic map’
And Code For More Than
Stimulus Location…
Many neurons in V1 (primary
visual cortex) are most
sensitive to visual stimuli with
a particular orientation.
* Here the neuron on the left
fires most (most vertical lines
in a short time) when the
stimulus is vertically
oriented.
Carlson page 147
* When we combine many of
these neurons we get an
orientation selective map in
V1
Maps are often explored by
exploring their ‘feature space’ ..and are
ubiquitous
across
species
A - feature space
B - experiment
C - recordings
D - response
Neurons in cortical maps have ‘RECEPTIVE FIELDS’
* The receptive field of a neuron is the region of ‘feature space’ that it
responds strongest to (fires most action potentials to)
* This can be a LOCATION (a particular whisker in RAT BARREL CORTEX),
or here an ORIENTATION (the line’s angle for a cat V1 neuron
diagram in notes - Primary visual area expands up the tree
…and link structure
to function by their
relative
sizes/specialisations
- An illustration of the burrowing
blind mole rat (Spalax ehrenbergi,
top) and the organization of its
neocortex (bottom). - Although skin has grown over
the eyes and the visual system is
used primarily for circadian
functions, these animals still
have a V1 and a retino-
geniculo-cortical pathway. - However, visual cortex has
been co-opted by the auditory
system
blind-mole rat neocortex in notes
..if map
structure
between species
reflects inter-
species
differences,
what about
intra-species
differences?
Plasticity…
- “Organic matter, especially
nervous tissue, seems endowed
with a very extraordinary
degree of plasticity.“
William James (1890)
The Principles of Psychology
Can brain function be changed by
experience in development?
- All experience is not created equal…
- The capacity for large scale changes in brain
organisation and behaviour peaks early in post-natal
life - The ‘critical period’ – where exposure to the right (or
wrong) stimuli at a crucial moment can largely
determine adult behaviour. - cf Imprinting – Lorenz and his geese
Can brain function be changed by
experience in development?
- These are more complex cognitive abilities (and may
not have a spatial correlate)… - But first language acquisition in human infants follows a
complex but largely stereotypical time course. - Similarly with bilingualism – while there appears to be no
true ‘window’ for learning another language, fluencey
similar to a native speaker appears to require exposure
before the age of 12 in humans.
Environmental Exposure During Critical
Periods Can Drive Cortical Map
Development - Blakemore and Cooper 1970
Background and aim: The aim of this experiment was to investigate the physiological and behavioural effects of a limited visual experience and whether brain development/plasticity occurs due to experiences rather than nature.
Method: This study was a laboratory experiment, taking place in a controlled artificial environment. The participants in the study were new born kittens who were immediately placed into a dark room. At two weeks of age the kittens were then randomly placed into one of two conditions for five hours a day: this was either a horizontal or a vertical environment (IV).
The kittens had to stand on a clear glass platform which was inside a tall cylinder of which the inner surface was covered with either horizontal or vertical black-and-white stripes. Additionally, there were no corners or edges in their environment. The kittens’ visual field was restricted to 130 degrees, as they were required to wear a wide black collar. This prevented them from seeing their own body and ‘beyond their world of stripes’. Blakemore and Cooper defended the ethics of the study by stating that the kittens did not seem distressed, as they inspected the walls of the tube for long periods of time.
After five months, exposure to the experimental conditions ended and the kittens were then placed for several hours a week from their dark cage to a small, well-lit furnished room. The Dependent Variable (DV) was then measured: this was whether kittens raised in a horizontal environment could detect vertically aligned objects and vice-versa. After 7 and a half months, two of the kittens, one from each environment were anaesthetised and their neurophysiology was examined.
Results: All the kittens were extremely visually impaired; they demonstrated no visual placing when brought up to a table top and had no startle response when an object was thrust towards them. However, their papillary reflexes were normal and they guided themselves mainly by touch. All kittens showed behaviour blindness, meaning that they could not detect objects or contours that were aligned in the opposite way to their previous environment. They also demonstrated fear when they were standing on the edge of a surface.
There was recovery of some deficiencies from their early deprivation. After about 10 hours the kittens showed visual placing and some startled responses; they could also easily jump from a chair to the floor. But the kittens did suffer some permanent damage such as trying to touch things well beyond their reach and following objects with clumsy head movements. Moreover, the neurophysiological examination found no evidence of astigmatism (blurred vision), but there was evidence that horizontal plane recognition cells did not ‘fire-off’ in the kitten from the vertical environment and vice-versa – meaning that kittens were unable to perform orientation selectivity and therefore they suffered from ‘physical blindness’.
Conclusion: Brain development is clearly affected by early experiences and environmental factors rather than just genetics and there is clear evidence of brain plasticity – ‘the visual experience of these animals had modified their brain’ and therefore has serious perceptual consequences. The kittens’ visual cortex adjusts during development as a result of its visual experiences.
Sensory stimulation from Environment can have profound effect on how cells are tuned
No cells responding just to vertical
diagram in notes
Exposure BEYOND the critical
period?
- We accept that post ‘critical period’, we can LEARN
and ACQUIRE new skills - There simply CANNOT be enough genetic material to code
for the acquisition, say, of pretending to be a sled dog pulling
a shopping trolley pretending to be a sled down a street in
San Francisco…
(MOSTLY MAMMALIAN)
MECHANORECEPTORS
- Primary sense subserving sensation of the body is the
somatosensory system. - Comprises all information from the skin (…and some other info…)
– cutaneous sensibility - Skin is your largest organ (2m^2)
- Thickness varies from 0.5mm on eyelids to >4mm on palms/soles of
feet. - Can be around 16% of body weight.
- Not just receptors: > 2 million sweat glands. > 5 million hairs.
cutaneous somatosensory receptors in mammals
more info in notes
examples
hair follicles
messier corpuscle
pacinian corpuscle
merkel cell-neurite complex
Ruffini corpuscle
c-fibre LTM
mechanical-noiceptor polymodal nociceptor
Ascending Pathways –
The Dorsal Column
Pathway For Fine
Touch
10 Afferents
* Gracile Fasiculus (fibres entering
below the midthoracic level)
* Cuneate Fasiculus (upper thoracic
and cervical levels)
20 Neurons
* Nuclei in Gracile and Cuneate
nuclei.
* Majority of medial lemniscal fibres
terminate in the venterior
posterior lateral (VPL) nucleus of
the thalamus.
30 Neurons
* Thalamocortical projections to
Primary Somatosensory Cortex
(S1 – Brodmann’s areas 1, 2 3a
0 = degree
diagram in notes
Central Somatotopical Organization
diagram in notes
We have multiple maps of the surface
Map of left side of body in right side of brain and vice versa
Map Plasticity in the Primate Somatosensory System
1978 - static
1984 - plasticity after peripheral trauma
1988 - plasticity after use-dependent change
diagrams in notes
Map Plasticity in the Primate Somatosensory
System
- First work to demonstrate that
there are two topographic
representations of the
body surface within
cytoarchitectonic areas 3b and 1
in adult monkeys. - Showed the fine detail of maps
in Areas 3b and 1. 3b and 1 are
mirrored representations of each
Other
1978 - static
Map Plasticity in the Primate Somatosensory System
- Demonstrates that alterations of
input can change map structure
in ADULT animals. - Does not create ‘silent’ zone: rfs
of amputed zone in S1 now
driven by adjacent fingers.
diagrams in notes
Map Plasticity in the Primate Somatosensory System
- Inducement of experimental
syndactyly caused by
surgically joining 2 digits in
adult monkey. - Causes an increase in
temporally-coincident
input to the two digits. - This drives – or correlates
with – the emergence of
new, dual digit rfs at the
border of the two digit
representations
1988 - plastic after use-dependent change
diagram in notes
