Transcript Modification of brain circuits as a result of experience

Modification of brain circuits as a
result of experience
(Purves, chapter 23)
aka ‘the plastic people of the
universe’
Half a Brain is Enough: The Story of Nico (2001). Antonio M. Battro (Series: Cambridge Studies in Cognitive and Perceptual Development, No. 5). CUP
“Man with tiny brain shocks doctors”
New Scientist, 20 July 2007
Ref: Feuillet et al. (2007) The Lancet, vol 370, p 262
Yu, F., Jiang, Q. J., Sun, X. Y., & Zhang, R. W. (2014). A new case of complete primary
cerebellar agenesis: clinical and imaging findings in a living patient. Brain, awu239.
“Despite at least 2 decades of restored sight, every individual had slightly impaired
vision” Chatterjee, R. (2015). Out of the darkness. Science, 350(6259), 372-375.
https://www.sciencemag.org/content/350/6259/372.full
Plasticity in development is a story about
how development is organised. It is a not
a story about variability, but about how
consistent developmental outcomes are
achieved and where the constraints are
built into the system (see Clark &
Thompson, 1997)
Contiguity of learning and development
Moficationism vs Constructionism
- is there a ‘genetic blueprint’?
(see Elman et al, 1996)
•
•
•
•
Maps
Activity dependent development
Critical periods
Competitive development
– Encroachment
Ocular dominance columns in macaque
striate cortex (from Swindale 1980)
Orientation contours
(Ocular dominance boundaries)
Orientation...
1
2
around point
singularity
in linear
region
3
at saddle
point
4
along
fracture
•Continuity: the feature value is encoded
as far as possible in a smooth way but with
some....
•Discontinuities: perfect continuity is not
possible leading to map fractures and
isolated point discontinuities.
•Diversity: All feature values are
represented
•Global disorder: maps are not perfectly
regular
•Map Coordination: Maps for different
stimulus features cooperate to equalise
resources locally within each part of the
visual field.
(Carreira-Perpinan & Goodhill, 2002).
Modelling self-organising feature maps
repeated exposure to inputs + hebb rule +
shrinking neighbourhoods -> SOFMs
= The ‘Kohonen algorithm’ (Kohonen, 1984; see
Gurney, 1997)
A set of principles which can explain map
developmen in different domains, and normal
and abnormal outcomes. E.g….
Present input pattern
Change weights according to hebb rule for winning unit
….and all within its neighbourhood
(renormalise weights)
(reduce neighbourhood size)
Essential Components of the
Kohonen Algorithm
• Repeated exposure –> training by
experience/the environment
• Hebb Rule -> selectivity (+
renormalisation)
• Neighbourhoods -> topography
Swindale (1980) modelling results
Obermayer et al, 1990
Hypothesis:
Test:
(After Penfield, 1950)
Farah (1998)
Stafford & Wilson (2007)
(From Farah, 1998)
Normally the two
halves of the frog’s
visual area (the two
tecta) are innervated
separately, each by
one eye
What happens when
you implant a third
eye in a frog, so that
two eyes are forced to
innervate the same
tectum?
Law & Constantine-Paton (1981)
So, is there a blueprint for the
developmental outcome?
A moral from the modelling is that you can get
organised outcomes without full prespecification
(see also Linsker, 1986)
Essential Components of the
Kohonen Algorithm
• Repeated exposure –> training by
experience/the environment
• Hebb Rule -> selectivity (+
renormalisation)
• Neighbourhoods -> topography
A single computational principle which can
explain diverse outcomes (by varying inputs)
Bonus Material #1:
Experiments on
activity based
development of
orientation columns
23.4 Effect of early closure of one eye on
the distribution of cortical neurons. (Part 1)
23.4 Effect of early closure of one eye on
the distribution of cortical neurons. (Part 2)
23.5 The consequences of a short period of
monocular deprivation at the critical period in the
cat.
23.9 Ocular dominance histograms
obtained by electrophysiological recordings
in cats.
• Deprivation is only disruptive during the critical
period. Not after, and not before
• What is happening before? Probably
development driven by spontaneous activity
• Stryker (1986) injected the activity-blocking
agent tetrodoxin (TTX) into small areas of visual
cortex which subsequently failed to develop
ocular dominance columns.
•
See also Chapman and Stryker (1993), Sengpiel et al (1999)
Evidence for selectivity *before*
birth…
• Orientation selectivity
– Crair et al (1998)
•
see Crowley & Katz (2002) for a similar story regarding ODCs
selectivity index
‘Contralateral’ and ‘ipselateral’compared
(normal only)
age (days)
selectivity index
BD and normal compared
(Contralateral only)
age (days)
selectivity index
‘Contralateral’ and ‘ipselateral’compared
(BD only)
age (days)
• Contralateral connections ‘coach’
ipselateral connections
• Compare to the recent experience of Scott
Adams (creator of Dilbert) :
• http://web.archive.org/web/200611071200
53/http://dilbertblog.typepad.com/the_dilbe
rt_blog/2006/10/good_news_day.html
Bonus Material #2:
The instability of the
hebb rule, plasticity of
critical periods &
plasticity due to adult
experience
An aside on the instability of the
Hebb Rule
• The Hebb Rule is a positive feedback rule
– winners win even more
• This is unstable, leading to explosion of
activity, and, additionally, shouldn’t allow
phenomena such as those Crair et al
present
• There is a problem with how hebb learning
is regulated
–
See also Abbott & Nelson (2000) and Turrigiano (1999)
What determines the length of the
critical period?
• Normally, dark
rearing can be
used to delay
critical period
closure in the
visual system…
• Bartoletti et al
(2004). Nature
Neuroscience,
7(3), 215-26
Merabet, L. B., Hamilton, R., Schlaug, G., Swisher, J. D., Kiriakopoulos, E.
T., Pitskel, N. B., et al. (2008). Rapid and Reversible Recruitment of Early
Visual Cortex for Touch. PLoS ONE, 3(8).
Also TMS to
occipital cortex
increases
errors on tactic
task for
blindfolded
group only
Taxi drivers have
bigger (posterior)
hippocampi
Maguire et al, 2000
Bangert & Schlaug (2006), see also Elbert et al (1995); Munte et al (2002)
Westerman & Buckley, increased retinal nerve fibre thickness in deaf
individuals (ongoing work, cf Bavelier et al, 2006)
Tom Stafford
A deaf individual
REFERENCES for PSY6306 Lecture by Tom Stafford on 'Modification of brain circuits as a result of experience'
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http://brain.oxfordjournals.org/content/early/2014/08/22/brain.awu239.extract