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Brain Code and Brain Reading. Prof.dr. Jaap Murre University of Maastricht University of Amsterdam jaap@murre.com http://neuromod.org. What principle should it use? Storage capacity Resistance to damage Access speed Serve necessary calculations. What constraints can be identified?

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Brain code and brain reading
Brain Code and Brain Reading

Prof.dr. Jaap Murre

University of Maastricht

University of Amsterdam

jaap@murre.com

http://neuromod.org


How does the brain represent information

What principle should it use?

Storage capacity

Resistance to damage

Access speed

Serve necessary calculations

What constraints can be identified?

Neurons are very noisy

They can do only simple calculations

The are severely limited in how they can communicate

How does the brain represent information?


Localized versus distributed coding
Localized versus distributed coding

  • Distributed coding

    • 1010111000101100110101000110111000

  • Extremely localized coding

    • 0000000000000000010000000000000000

  • Sparse (or semi-distributed) coding

    • 0000100000100000010000000010000000


Distributed coding
Distributed coding

  • Maximizes storage capacity

  • Resistant to damage

  • Able to implement complex calculations

  • May need high metabolism (energy, O2)

  • Overlap of codes can cause severe interference in processing and learning


Localized coding
Localized coding

  • Is able to store only a few representations but also forces category formation

  • Sensitive to loss (causes very specific disorders)

  • Mainly useful for theoretical purposes


How can codes come into existence example competitive learning
How can codes come into existence? Example: Competitive learning

  • Competitive learning is a form of unsupervised learning

  • Needs a large degree of localization (or very sparse codes) to work

  • Forms categories on the basis of regularities (is also called regularity learning)

  • These networks are able to signal novelty


Example of competitive learning stimulus at is presented
Example of competitive learning: learningStimulus ‘at’ is presented

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2

a

t

o


Example of competitive learning competition starts at category level
Example of competitive learning: learningCompetition starts at category level

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2

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t

o


Example of competitive learning competition resolves
Example of competitive learning: learningCompetition resolves

1

2

a

t

o


Example of competitive learning hebbian learning takes place
Example of competitive learning: learningHebbian learning takes place

1

2

a

t

o

Category node 2 now represents ‘at’






Category 1 is established through hebbian learning as well
Category 1 is established through Hebbian learning as well learning

1

2

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Category node 1 now represents ‘to’


Kohonen self organizing map
Kohonen self-organizing map learning

  • Winner-take-all competition

  • Network has 2D (or 3D etc.) layout

  • Weights of neighbors of the winner are updated with the winner

  • The map tries to mirror the input space

  • This works even with semantic spaces



Kohonen map winner take all1
Kohonen map: winner-take-all learning

Initial weights are random

Initial winners will be random as well

As time goes by, winners to similar inputs will tend to be located close together


Weights are updated to the winner and to its neighbors
Weights are updated to the winner and to its neighbors learning

Weights are moved towards input

For example:

Inputs are: .9 .9 .7

Weights are: .5 .3 .9

New weight are: .6 .4 .8

.9 .9 .7


Kohonen self organizing map1
Kohonen self-organizing map learning

  • The size and shape of the neighborhood determines the final layout

  • The size of the neighborhood is usually diminished with training

  • The learning rate is typically also slowly diminished

  • Training may take a long time


Example 2d map tries to capture 3d color space by simon lucas
Example: 2D map tries to capture 3D color space (by learningSimon Lucas)

  • Input vectors are drawn randomly from RGB (red, green, blue) color space

  • Node color reflects the node’s incoming weights (it will respond best to that color)

  • Depending on the learning rate and neighborhood different organizations emerge

  • No single fixed organization is best in this case

  • Other example:


Sparse coding
Sparse coding learning

  • Forms a good middle ground between fully distributed and extremely localized coding

  • Is computationally sound in that it allows very large numbers of representations with a small number of units


Brain codes in the motor system
Brain codes in the motor system learning

Codes for action and movement



Medial view
Medial view learning


Brain code and brain reading

Schematic learning

overview of the

motor system


Basic questions regarding motor control can nowadays be answered
Basic questions regarding motor control can nowadays be answered

  • How are motor movements represented in the brain?

  • How are they used in the production of movement?

  • Which brain areas are involved?


How to be precise with noisy components
How to be precise with noisy components answered

Area 5 neuron during repeated reaching movements: each

individual trial gives a rather imprecise signal


Population coding
Population coding answered

  • Population coding allows precise representations on the basis of (very) noisy or even damaged components

  • Population coding is based on the statistics of averages

  • They rely on coarse-coded neural representations


Coarse coding
Coarse coding answered

  • If a neuron’s representation responds to ‘many’ inputs, this is called coarse coding

  • The advantage is that more accurate representations can be formed by suitable combination of the coarse representations


Why coarse coding works
Why coarse coding works answered

  • If we move along a straight line, each time we cross a receptive field boundary one neurons changes its activation:

  • the representation changes.






Response competition
Response competition upon other areas


Spinal cord
Spinal cord upon other areas


Coarse maps of limb movements in the frog
Coarse maps of limb movements in the frog upon other areas

  • Spinal cord of frogs does significant motor processing

  • Frog can still ‘clean’ itself after severing of cord (dogs can also still scratch themselves)

  • The data suggest that even at a spinal level coarse coding is used

  • It is likely that similar types of coding are used in mammals



Method followed by emilio bizzi
Method followed by Emilio Bizzi treadmill

Based on the idea of ‘muscles as springs’ by Feldman



The interactions of force fields can be described by vector calculus
The interactions of force fields can be described by vector calculus

Fields A and B combined predict field <AB> (see C). When A and B are stimulated the resulting field (see D) corresponds to the theoretical field <AB>


Brain codes in the visual system
Brain codes in the visual system calculus

Is our knowledge represented in a localized or distributed fashion?


Freedman riesenhuber poggio miller science 2001

Many (395) single cell recordings in monkey pre-frontal cortex

Trying to find categories in the brain

82 out of 395 were category selective

Freedman, Riesenhuber, Poggio, & Miller (Science, 2001)


Brain code and brain reading

Cells predict the correct category cortex

(single-cell, lateral pre-frontal in monkey)


Brain reading
Brain Reading? cortex

Jim Haxby’s (2001) study


Haxby et al 2001 categories in the human brain
Haxby et al. (2001) cortexCategories in the human brain




Analysis with neural networks by hanson matsuka haxby 2004
Analysis with neural networks by other categoriesHanson, Matsuka, & Haxby, 2004

face cat house bottle scissor shoe random chair


Brain code and brain reading

Marieke van der Linden other categoriesFCDC

Miranda van Turennout FCDC

Jaap Murre UvA


Brain code and brain reading

Methods other categories

Pretest 15 subjects

Task: 1-back with feedback

Example

Subjects will be trained on categorizing birds that are indistinguishable to the untrained eye.


Brain code and brain reading

3 Training sessions on 3 consecutive days other categories

  • Two bird types will be assigned to categorization training (1-back same/different bird type) with feedback (“correct”, “false” or “too late”)

  • Two bird types will be assigned to visual training (same 1-back same/different bird type) with random feedback

  • Two bird types will not be trained, untrained

  • Different morphs than scan session (55, 65, 70, 80, 95%) to prevent pure repetition fx

  • Each task contains five blocks of 150 trials, 10 minutes per block, total: 50 min.

2 fMRI sessions: pre- and post training

Factors:Training (categorization, visual, untrained)

Morph level (60, 75, 90)

Design:

20 sec 12 sec 20 sec 12 sec 20 sec 12 sec 20 sec

….etc

Etc.

!

Each block will be repeated 6x, the order will be randomized

Total scan time: 57.6 min.



Putting it all together
Putting it all together Categories

  • Models help us understand different roles of representations in the brain

  • At different levels of analysis, different brain codes are more effecient



Visual processing in the brain palmeri gauthier 2004
Visual processing in the brain Categories (Palmeri & Gauthier, 2004)


Natural input model nim
Natural Input Model (NIM) Categories

Artificial Intelligence approach to cognitive modeling

Functional (computational) model of visual processing in V1/V2, V4, and IT.

With Eric Postma, Joyca Lacroix, and Jaap van den Herik (Universiteit Maastricht)


Natural input model nim1
Natural Input Model (NIM) Categories

  • Modeling recognition and similarity of individual images (faces)

    • Cognitive Science (in press)


Summary
Summary Categories

  • A lot is now known about motor and visual codes in the brain

  • We can understand many aspects of this circuitry in terms of ‘why this representation makes sense’

  • For example, coarse grained coding has the advantage of precise control despite noisy components


Summary continued
Summary (continued) Categories

  • The brain employs many different representations, depending on their use and constraints

  • We are now starting to unravel many of these codes


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