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.

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 macaquestriate cortex (from Swindale 1980)

Orientation...

Orientation contours(Ocular dominance boundaries)

around pointsingularity 1

in linear region2

along fracture4

at saddlepoint

3

• 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

(After Penfield, 1950) (From Farah, 1998)

Hypothesis: Farah (1998)Test: Stafford & Wilson (2007)

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

‘Contralateral’ and ‘ipselateral’compared (normal only)

sele

ctiv

ity in

dex

age (days)

sele

ctiv

ity in

dex

age (days)

BD and normal compared (Contralateral only)

‘Contralateral’ and ‘ipselateral’compared (BD only)

sele

ctiv

ity in

dex

age (days)

• Contralateral connections ‘coach’ ipselateral connections

• Compare to the recent experience of Scott Adams (creator of Dilbert) :

• http://web.archive.org/web/20061107120053/http://dilbertblog.typepad.com/the_dilbert_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) hippocampiMaguire 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