slide 1 mathematics of the brain: a. overview of the general approach b. promising directions of...
TRANSCRIPT
Slide 1
Mathematics of the brain:
A. Overview of the general approach
B. Promising directions of research
Victor Eliashberg
Consulting professor, Stanford University, Department of Electrical Engineering
A. Overview of the general approach
Slide 2
This is our most important “personal computer” – the pinnacle of biological system engineering. What does this computer compute?
Frontal Lobe
Temporal Lobe
Parietal Lobe
Occipital Lobe
Cerebellum
Cervical Spinal Cord
Thoracic Spinal Cord
Lumbar Spinal Cord
Cauda Equina
Our brain still lives in a sea!
Dura mater
Slide 3
12 cranial nerves ; ~1010 neurons in each hemisphere
8 pairs
12 pairs
5 pairs
6 pairs
31 pairs of nerves; ~ 107 neurons
~1011 neurons
Big picture: Cognitive system (Robot,World)
B(t) is a formal representation of B at time t, where t=0 is the beginning of learning. B(0), call it Brain 0 (Brain Zero), is an “unprogrammed” brain.
BDW
Human-like robot (D,B)External world, W
External system (W,D)
Sensorimotor devices, D
Computing system, B, simulatingthe work of human nervous system
Slide 4
Slide 5
Processor
LTM
RAM
OtherW,D
Conventional computer versus the brain
Processor
LTM
E-states
OtherW,D
1 encoded (symbolic) states 2 analog (dynamic) states
There is a big intuitive difference between 1 and 2 that is difficult to express formally.
Time vs. space: computational universality
Type 1: Context-sensitive grammars
Type 4: Combinatorial machines (the lowest computing power)
Type 0
Type 1
Type 2
Type 3
Type 4
Type 0: Turing machines (the highest computing power)
Type 2: Context-free grammars (push-down automata)
Type 3: Finite-state machines
Slide 6
NOTE. We can perform, in principle, any mental computations. This means that the human brain is a system of type 0 – the limited size of our working memory is of no principle importance. Importantly, an attempt to represent a behavior of higher type in terms of a system of a lower type leads to a combinatorial explosion of the size of such an inadequate representation!
The mystery of human learning
How big is B(0)? How big is B(t)?
B(0) B(t), t >20 years
MEGABYTE(S)? TERABYTES?
learning
The main part of brain’s software must be created in the course of learning
Slide 7
General approach to learning
HYPOTHESIS. The results of learning depend only on the SMI-sequence and not on the way this sequence is produced.
NI –internal observable centers such as, e.g., the centers of emotion, etc.
Teacher
BD
NS
NM
NI
W
Concept of observable behavior
NS –sensory centers
NM –motor centers
S
M
I
S(1) S(2) . . . S(i) . . . S(n)
M(1) M(2) . . . M(i) . . . M(n)
I(1) I(2) . . . I(i) . . . I(n)
SMI – sequence or the brain’s complete experience =
Slide 8
G-states and E-states: the brain as an E-machine
where
gt+1 = fg(x*) fg: X* G (learning
procedure)
et+1 = fe(xt, et, gt ,qt) fe: X×E×G×Q E (next E-state
procedure)
yt = fy(xt, et, gt , qt) fy: X×E×G×Q Y (interpretation
procedure)
X and Y are the input and output sets, respectively, G is the set of states of
“symbolic” LTM, E is the set of states of “dynamic” STM and ITM, X* is the
set of SMI-sequences (see slide 7), t is discrete time. Q all other states.
BDW
xS
yS
xM
yM
xI yI
x =(xS,xM,xI)
y =(yS,yM,yI)
NS
NM
NI
Slide 9
Wolfgang Amadeus Mozart
Henri Poincaré John von Neumann
People who had eidetic memory
Slide 10
Savant Kim Peek Dustin Hoffman and savant Stephen Wiltshire
Eidetic memory vs. creativity
Slide 11
Working memory and mental imagery as a simulation of external system (W,D). System (W,D) is the “Teacher” for system AS
AS
D
Working memory and mental imagery
associations
NS
AM
Teacher
SM M
Motor controlW
associationsMS S
S
S
M
M
S
M
NM
Slide 12
Motor and sensory areas of the neocortex
Slide 13
Working memory, episodic memory, and mental imagery
ASAM
Motor control
Slide 14
Primary sensory and motor areas, association areas
Association fibers (neural busses)
Slide 15
Basic structure of a primitive E-machine
Slide 16
Control outputs
Association outputs
E-STATES (dynamic STM and ITM)MODULATION, NEXT E-STATE PROCEDURE
CHOICE
Data inputs to ILTM
Data inputs to OLTM
Control inputs
INPUT LONG-TERM MEMORY (ILTM)DECODING, INPUT LEARNING
OUTPUT LONG-TERM MEMORY (OLTM)ENCODING, OUTPUT LEARNING
Association inputs
Data outputs from OLTM
Modulated (biased) similarity function
Similarity function
Selected subset of active locations of OLTM
Turing’s machine as a system (Robot, World)
Slide 17
Mental computations (thinking) as an interaction between motor control and working memory (EROBOT) , (PEM)
Slide 18
~4,000 inner hair cells ~12,000 outer hair cells
~30,000 fibers~90,000 cells
~390,000 cells
~580,000 cells
~100,000,000 cells
From vectors to symbols: auditory pathways
Slide 19
Slide 20
Eye movements: oculomotor, trochlear, and abducense nerves
From symbols to vectors: voluntary movements
Slide 21
Internal signals: from emotions to symbols and vice versa
Slide 22
Slide 23
W
AS1
ASk
AM1
AMm
S1
SENSORY CORTEX
MOTOR CORTEX
SUBCORTICAL SYSTEMS
SUBCORTICAL SYSTEMS
M1
D
D
The brain as a complex E-machine
B. Promising directions of research
Slide 24
1. Investigating and simulating computational resources of a single neuron
The metaphor the brain as an E-machine promotes the hypothesis that a large portion of the brain hardware computations – especially the MODULATION and the NEXT E-STATE procedures -- is performed at the level of individual cells.
Slide 25
Typical neuron
Neuron is a very specialized cell. There are several types of neurons with different shapes and different types of membrane proteins. Biological neuron is a complex functional unit. Where does this complexity come from?
Slide 26
Computational machinery of a cell
Nucleus
Membrane proteins
Membrane
It took evolution much longer to create individual cells than to build systems containing many cells, including the human brain. Different cells differ by their shape and by the types of membrane proteins.
Nucleus
Membrane proteins
Membrane
18nm
3nm
Slide 27
Protein molecule as a probabilistic molecular machine (PMM)
i
Slide 28
Ensemble of PMMs (EPMM)
E-states as occupation numbers
Slide 29
EPMM as a statistical mixed-signal computer
Slide 30
2. Investigating and simulating neural networks with reciprocal inhibition
The metaphor the brain as an E- machine promotes the hypothesis that a neural layer with reciprocal inhibition can perform the procedure of random equally probable choice. It is interesting to study different possible implementations of such a procedure in the case of a large number of neurons.
Slide 31
Simple “3-neuron” associative neural network (WTA.EXE)
Slide 32
DECODING
ENCODING
RANDOM CHOICE
Input long-term memory (ILTM)
Output long-term memory (OLTM)
addressing by content
retrieval
S21(I,j)
N1(j)
S21(i,j)
3. Studying possible neural implementations of Input and output LTM in the case of a very large number of neurons.
Slide 33
Example of a possible implementation of a very big ILTM
Slide 34
Example of a possible implementation of a very big OLTM
Slide 35
5. Studying possible neural implementations of associative connections in the case of a very large number of neurons.
NOTE. It is physically impossible to have a crossbar connectivity between, say, 108 and 108 neurons – the number of required synapses is 1016. The hypothesis of hash-recoding offers a possible solution. Is this hypothesis correct? What other solutions are possible?
Slide 36
6. Studying the possibilities of primitive E-machines with different DECODING, MODULATION, CHOICE, ENCODING, NEXT E-STATE, and LEARNING procedures.
NOTE. It is particularly interesting to study the possibilities of primitive E-machines with universal learning algorithms – the algorithms that don’t lose training information.
Slide 37
7. Studying the possibilities of universal learning E-machines (type 0) consisting of two primitive E-machines
NOTE. The program EROBOT.EXE provides a simple example of such a universal learning E-machine.
Slide 38
8. Studying the possibilities of complex E-machines with hierarchical structure of associative memory
NOTE. Eliashberg 1979 contains a simple example of such an E-machine.
Slide 39
9. Using natural language analysis for reverse engineering the basic hardware mechanisms of a hypothetical brain-like E-machine.
The problem of a natural language is the most important and interesting problem that seems to be consistent with the metaphor the brain as an E-machine. Therefore, the analysis of a natural language provides a rich source of reliable psychological facts for developing this metaphor.
NOTE. So far I’ve been unable to find either psychological or neurobiological facts that would force me to reject this general metaphor. It is interesting to try to find such facts .
Slide 40
10. Understanding the possibilities of dynamically reconfigurable associative software.
This is the most promising and practically unlimited area of research. An E-machine with a given knowledge (G-state) can be dynamically reconfigured into a combinatorial number of different symbolic machines by changing the state of its dynamic STM and ITM (E-state). It is my belief that understanding the possibilities associated with such a context-dependent dynamic reconfiguration of knowledge (software) is a key to understanding the work of the human brain and to creating really intelligent autonomous robots.
It is especially promising and challenging to try to understand how increasingly complex dynamically reconfigurable associative software can be created in the course of learning.
NOTE. Interesting as they are for specific applications, traditional ANN models have a limited general level of computing power (not higher than type 3) and cannot address the problem of nontrivial brain software – more so, the problem of context-dependent dynamically reconfigurable software.
Slide 41
BIBLIOGRAPHY (slide 1 of 4)
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Eliashberg, V. (1989). Context-sensitive associative memory: "Residual excitation" in neural networks as the mechanism of STM and mental set. Proceedings of IJCNN-89, June 18-22, 1989, Washington, D.C. vol. I, 67-75.
BIBLIOGRAPHY (slide 2 of 4)
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BIBLIOGRAPHY (slide 3 of 4)
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