a large-scale model of the functioning brain - seminar ... · architecture uni ed network"...

The Plan The article How does this thing work ? Discussion

A Large-scale Model of the Functioning Brain

Seminar presentation

Ardi Tampuu

February

The Plan The article How does this thing work ? Discussion

What's going to happen today ?

Today's presentation will consist roughly in 3 parts :

Description of the behaviour and capacities of the model

General introduction to the methods applied

Discussion about this type of models

The Plan The article How does this thing work ? Discussion

A large scale model of the functioning Brain

The model presented in this article is called "Semantic PointerArchitecture Uni�ed Network" (SPAUN).

2.5 million neurons

20 brain structures and the functional connectivity betweenthem

Input 28X28 pixels results in behavioural output of moving arobotic arm

Solves 8 di�erent tasks, for which it needs memory,contingency detection, reasoning

The Plan The article How does this thing work ? Discussion

The diversity of tasks

The 8 tasks are :A0 : copy drawing (needs extraction of details from images)

A1 : image recognition (extracting a concept from an image)- 94%

A2 : reinforcement learning (needs modi�cation of connection weights)

A3 : remember sequences (needs memorizing long sequences)

A4 : counting (adding two numbers) - 400ms per position counted

A5 : P and K questions (extracting information from a sequence)

A6 : rapid variable creation (�nding regularities)

A7 : �uid reasoning (determining the operator/transformation to be applied) -88%

The Plan The article How does this thing work ? Discussion

The connectivity

The Plan The article How does this thing work ? Discussion

The functional organization

The Plan The article How does this thing work ? Discussion

Results and predictions

Accurate copy drawing (A0) and e�cient RL (A2)

The Plan The article How does this thing work ? Discussion

Results and predictions

The model uses serial WM and predicts how correlations in �ringpatterns decrease when more items are added :

The Plan The article How does this thing work ? Discussion

Results and predictions

In A4(counting), reaction times are similar to human behaviour.In A5 we see the e�ects of primacy and recency.

The Plan The article How does this thing work ? Discussion

Yeah, cool, but how does it work ?

The Plan The article How does this thing work ? Discussion

The basic concepts

The model is based on NEF(representations, transformations) andSPA (linking areas with each other).

The Plan The article How does this thing work ? Discussion

The basic concepts

The �ring pattern in working memory is a function of all of itsinputs. The resulting �ring pattern can later be decoded from thememory.

MemoryTrace = Position1⊗ Item1+ Position2⊗ Item2+ ...where :

Position1 = Base

Position2 = Position1⊗ Base

and

One = Base

Two = One ⊗ AddOne

The Plan The article How does this thing work ? Discussion

How does the system learn to do this ?

Actual learning only takes place in RL task. In other cases networkconnectivity does not change during tasks, only the state of thesystem does.

Learning during RL does not in�uence the performance in othertasks.

The Plan The article How does this thing work ? Discussion

How does the system learn to do this ?

It seems an instance of Spaun is tuned beforehand to bestsegregate between di�erent states and di�erent inputs.

Visual system is trained using RBM, not spiking neurons

Are sub-systems taught one by one or all together ?

What types of neural networks and learning rules are applied ?

Source code is accessible

The Plan The article How does this thing work ? Discussion

Working memory

The serial memorizing of inputs in the WM is illustrated below :

The Plan The article How does this thing work ? Discussion

Restricted Boltzmann Machine

The visual system uses a type of Restriced Boltzmann Machine :

Named after the Boltzmann Distribution of probabilities

Layered structure with visible and hidden units, no recurrentconnections

Hidden units can be seen as detecting patterns in the activityof VU

Activity of the hidden units can be used as input to anotherlayer of RBM

The Plan The article How does this thing work ? Discussion

Can we trust these guys ?

Is Spaun an "arti�cial intelligence" or is it simply a cleverlydesigned computer program ?

+ can switch between tasks by itself

+ does not respond to falsely posed questions

+ can be adapted to solve di�erent tasks

+ involves spiking neurons with randomized parameters

− is limited to using numbers from 0 to 9

− not capable of learning completely new tasks

− in NEF connection weights are de�ned mathematically, not byRL or spike-based rules

The Plan The article How does this thing work ? Discussion

Everything that glitters is not gold ?

Sophisticated models often make you believe they a very close tosolving the "big questions".

nobody wants to discuss the limitations of their work

it's more interesting to discuss the parts the model learned byitself rather than what was hard-coded to the system

why is the training procedure or the "life-cycle" of Spaun notdescribed ?

The Plan The article How does this thing work ? Discussion

Are such models reproducible ?

The article and the SI are not su�cient to reproduce the model,but source code is provided.

it is very hard to explain complex algorithms in a few words inthe article

most readers are not interested in details

no standards exist for model description

The Plan The article How does this thing work ? Discussion

Other models

Brain-like systems capable of learning complex behaviour are verymuch in fashion :

DeepMind model learning to play computer games(http ://arxiv.org/pdf/1312.5602v1.pdf)

Mostly adaptations of RBM are used, not biologically precise