Analyzing Brain Signals by Combinatorial Optimization
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Transcript of Analyzing Brain Signals by Combinatorial Optimization
Analyzing Brain Signals
by Combinatorial Optimization
Justin DauwelsLIDS, MIT
Amari Research Unit, Brain Science Institute, RIKEN
September 25, 2008
Topics• Mathematical problem Similarity of Multiple Point Processes
• Motivation/Application Diagnosis of Alzheimer’s disease from EEG signals
CollaboratorsFrançois Vialatte*, Theophane Weber+, and Andrzej Cichocki* (*RIKEN, +MIT)
Financial Support
Alzheimer's disease
• Mild (early stage)- becomes less energetic or spontaneous- noticeable cognitive deficits- still independent (able to compensate)
• Moderate (middle stage)- Mental abilities decline- personality changes- become dependent on caregivers
• Severe (late stage)- complete deterioration of the personality- loss of control over bodily functions- total dependence on caregivers
Apathy
Memory(forgettingrelatives)
Evolution of the disease (stages)One disease, many symptoms
Video sources: Alzheimer society
• 2 to 5 years before- mild cognitive impairment (MCI)- 6 to 25 % progress to Alzheimer‘s
memory, language, executive functions, apraxia, apathy, agnosia, etc…
• 2% to 5% of people over 65 years old• up to 20% of people over 80 Jeong 2004 (Nature)
EEG data
GOAL: Diagnosis of MCI based on EEG
• EEG is relatively simple and inexpensive technology• Early diagnosis: medication more effective, more time to prepare future care of patient, etc.
Overview
Alzheimer’s Disease (AD) decrease in EEG synchrony Similarity of Point Processes
Two 1-D point processesTwo multi-D point processesMultiple multi-D point processes
Numerical Results Conclusion
Alzheimer's diseaseInside glimpse: abnormal EEG
• AD vs. MCI (Hogan et al. 203; Jiang et al., 2005)• AD vs. Control (Hermann, Demilrap, 2005, Yagyu et al. 1997; Stam et al., 2002; Babiloni et al. 2006)• MCI vs. mildAD (Babiloni et al., 2006).
Decrease of synchrony
Images: www.cerebromente.org.br
EEG system: inexpensive, mobile, useful for screening
Spontaneous (scalp) EEG
Fourier power
f (Hz)
t (sec)
ampl
itude
Fourier |X(f)|2
EEG x(t)
Time-frequency |X(t,f)|2(wavelet transform)
Time-frequency patterns(“bumps”)
Sparse representation: bump model
Bumps
Sparse representation
F. Vialatte et al. “A machine learning approach to the analysis of time-frequency maps and its application to neural dynamics”, Neural Networks (2007).
104- 105 coefficients
about 102 parameters
t (sec)
f(Hz)
f(Hz)
t (sec)
f(Hz)
t (sec)
Assumptions:
1. time-frequency map is suitable representation
2. oscillatory bursts (“bumps”) convey key information
Similarity of bump models
How “similar” are n ≥ 2 bump models?
Similarity of multiple multi-dimensional point processes
with and
“point” / ”event”
Overview
Alzheimer’s Disease (AD) decrease in EEG synchrony Similarity of Point Processes
Two 1-D point processesTwo multi-D point processesMultiple multi-D point processes
Numerical Results Conclusion
Two one-dimensional point processes
tx
x’
0
0 t
Generative model
v0 T0
T0
T0
0
0 T0
x
x’
0
0 T0
-δt /2
δt /2
non-coincident
non-coincident
x
x
Stochastic event synchrony (SES): delay δt , jitter st , non-coincidence ρ
Generative model
v0 T0
T0
T0
0
0 T0
x
x’
0
0 T0
-δt /2
δt /2
geometric prior for lenght
events i.u.d. in [0,T0]
Gaussian offsets withmean -δt /2 and variance st /2
Gaussian offsets withmean δt /2 and variance st /2
i.i.d. deletions with prob pd
i.i.d. deletions with prob pd
non-coincident
non-coincident
x
x
Marginalizing over v:
Generative model (2)
Cost function
unit cost of non-coincident event
unit cost of coincident pair
Model
Probabilistic inference
DYNAMIC PROGRAMMINGPARAMETER ESTIMATION
PROBLEM: Given 2 point processes x and x’, compute ρ and θ = δt , st
APPROACH: (j*, j’*,θ*) = argmaxj,j’,θ log p(x, x’, j, j’,θ)
SOLUTION: Coordinate descent
(j(i+1) , j’(i+1) ) = argmaxj,j’ log p(x, x’, j , j’ , θ(i)) θ(i+1) = argmaxx log p(x, x’, j(i+1) , j’(i+1) , θ)
0 x1 x2 x3 x4 x5 x6
0
x’1
x’2
x’3
x’4
x’5
x’6
xk non-coincident x’k’ non-coincident (xk x’k’ ) coincident pair
Application: spike trains
High reliabilityLarge timing dispersion
Low reliabilitySmall timing dispersion
jitter st = (15ms)2, non-coincidence ρ = 3% jitter st = (3ms)2, non-coincidence ρ = 27%
Overview
Alzheimer’s Disease (AD) decrease in EEG synchrony Similarity of Point Processes
Two 1-D point processesTwo multi-D point processesMultiple multi-D point processes
Numerical Results Conclusion
Similarity of two bump models...
... by matching bumps• Bumps in one model, but NOT in other → fraction of “non-coincident” bumps ρ
• Bumps in both models, but with offset → Average time offset δt (delay) → Timing jitter with variance st
→ Average frequency offset δf → Frequency jitter with variance sf
PROBLEM: Given two bump models, compute (ρ, δt, st, δf, sf )
Stochastic Event Synchrony (SES) = (ρ, δt, st, δf, sf )
Generative modelGenerate bump model (hidden)
• geometric prior for number of bumps
• bumps are uniformly distributed in rectangle
• amplitude, width (in t and f) all i.i.d.
Generate two “noisy” observations
• offset between hidden and observed bump = Gaussian random vector with mean ( ±δt /2, ±δf /2) covariance diag(st/2, sf /2)
• amplitude, width (in t and f) all i.i.d.
• “deletion” with probability pd
yhidden
y y’
( -δt /2, -δf /2)
( δt /2, δf /2)
• Binary variables ckk’ : ckk’ = 1 if k and k’ are observations of same hidden bump, else ckk’ = 0• Constraints: sums Σk’ ckk’ and Σk ckk’ are binary (“matching constraints”)
Probabilistic inference
MATCHINGPOINT ESTIMATION
PROBLEM: Given two bump models, compute (ρ, δt, st, δf, sf )
APPROACH: (c*,θ*) = argmaxc,θ log p(y, y’, c, θ)
θ
SOLUTION: Coordinate descent
c(i+1) = argmaxc log p(y, y’, c, θ(i) ) θ(i+1) = argmaxx log p(y, y’, c(i+1) ,θ )
MATCHING: c(i+1) = argmaxc log p(y, y’, c, θ(i) )
ALGORITHMS
• Polynomial-time algorithms gives optimal solution(s) (Edmond-Karp and Auction algorithm)• Linear programming relaxation: gives optimal solution if unique [Sanghavi (2007)]• Max-product algorithm gives optimal solution if unique [Bayati et al. (2005), Sanghavi (2007)]
EQUIVALENT to (imperfect) bipartite max-weight matching problem
c(i+1) = argmaxc log p(y, y’, c, θ(i) ) = argmaxc Σkk’ wkk’(i) ckk’
s.t. Σk’ ckk’ ≤ 1 and Σk ckk’ ≤ 1 and ckk’ 2 {0,1}
Probabilistic inference (2)
not necessarily perfectfind heaviest set of disjoint edges
Max-product algorithmMATCHING: c(i+1) = argmaxc log p(y, y’, c, θ(i) )
μ↑μ↑
μ↓ μ↓
• At convergence, compute marginals p(ckk’) = μ↓(ckk’) μ↓(ckk’) μ↑(ckk’)• Decisions: c*kk’ = argmaxckk’
p(ckk’) (optimal if solution unique)
Summary
MATCHING → max-productESTIMATION → closed-form
PROBLEM: Given two bump models, compute (ρ, δt, st, δf, sf )
APPROACH: (c*,θ*) = argmaxc,θ log p(y, y’, c, θ)
θ
SOLUTION: Coordinate descent
c(i+1) = argmaxc log p(y, y’, c, θ(i) ) θ(i+1) = argmaxx log p(y, y’, c(i+1) ,θ )
Average synchrony
3. SES for each pair of models4. Average the SES parameters
1. Group electrodes in regions2. Bump model for each region
Overview
Alzheimer’s Disease (AD) decrease in EEG synchrony Similarity of Point Processes
Two 1-D point processesTwo multi-D point processesMultiple multi-D point processes
Numerical Results Conclusion
Beyond pairwise interactions...Pairwise similarity Multi-variate similarity
Similarity of multiple bump modelsy1 y2 y3 y4 y5
y1 y2 y3 y4 y5
Constraint: in each cluster at most one bump from each signal
Models similar if• few deletions/large clusters• little jitter
pc (i) = p(cluster size = i |y) (i = 1,2,…,M)
Parameters: θ = δt,m , δf,m , st,m , sf,m, pc
Generative modelGenerate bump model (hidden)
• geometric prior for number n of bumps
• bumps are uniformly distributed in rectangle
• amplitude, width (in t and f) all i.i.d.
Generate M “noisy” observations
• offset between hidden and observed bump = Gaussian random vector with mean ( δt,m /2, δf,m /2) covariance diag(st,m/2, sf,m /2)
• amplitude, width (in t and f) all i.i.d.
• “deletion” with probability pd
yhidden
y1 y2 y3 y4 y5
Parameters: θ = δt,m , δf,m , st,m , sf,m, pc
pc (i) = p(cluster size = i |y) (i = 1,2,…,M)
Exemplar-based formulationyhidden
y1 y2 y3 y4 y5
• Exemplars = identical copies of hidden bumps = cluster “center”• Other bumps in cluster = non-identical copies of exemplars
• Is event an exemplar?• If not, which exemplar is it associated with?• Several constraints
Integer program
Exemplar-based formulation: IPBinary Variables
Integer Program: LINEAR objective function/constraints
Equivalent to k-dim matching: for k = 2: in P but for k > 2: NP-hard!
Probabilistic inference
CLUSTERING (Integer Program)ESTIMATION OF PARAMETERS
PROBLEM: Given M bump models, compute θ = δt,m , δf,m , st,m , sf,m, pc
APPROACH: (b*,θ*) = argmaxb,θ log p(y, y’, b, θ)
SOLUTION: Coordinate descent
b(i+1) = argmaxc log p(y, y’, b, θ(i) ) θ(i+1) = argmaxx log p(y, y’, b(i+1) ,θ )
Integer programming methods (e.g., LP relaxation)• IP with 10.000 variables solved in about 1s• CPLEX: commercial toolbox for solving IPs (combines several algorithms)
NOTE: Max-product algorithm SUBOPTIMAL sometimes converged to “bad” solutions (how to fix??)
SummarySimilarity of multiple multi-dimensional point processes
Step 1: TWO ONE-dimensional point processes
Step 2: TWO MULTI-dimensional point processes
Step 3: MULTIPLE MULTI-dimensional point processes
Dynamic programming
Max-product/LP relaxation/Edmund-Karp
Integer Programming
Overview
Alzheimer’s Disease (AD) decrease in EEG synchrony Similarity of Point Processes
Two 1-D point processesTwo multi-D point processesMultiple multi-D point processes
Numerical Results Conclusion
EEG Data
EEG data provided by Prof. T. Musha
• EEG of 22 Mild Cognitive Impairment (MCI) patients and 38 age-matched control subjects (CTR) recorded while in rest with closed eyes → spontaneous EEG
• All 22 MCI patients suffered from Alzheimer’s disease (AD) later on
• Electrodes located on 21 sites according to 10-20 international system
• Electrodes grouped into 5 zones (reduces number of pairs) 1 bump model per zone
• Band pass filtered between 4 and 30 Hz
Sensitivity (average synchrony)
Granger
Info. Theor.
State Space
Phase
SES
Corr/Coh
Mann-Whitney test: small p value suggests large difference in statistics of both groups
Significant differences for ffDTF and SES (more unmatched bumps, but same amount of jitter)
Classification (bi-SES)
• Clear separation, but not yet useful as diagnostic tool• Additional indicators needed (fMRI, MEG, DTI, ...)• Can be used for screening population (inexpensive, simple, fast)
ffDTF
± 85% correctly classified
Overview
Alzheimer’s Disease (AD) decrease in EEG synchrony Similarity of Point Processes
Two 1-D point processesTwo multi-D point processesMultiple multi-D point processes
Numerical Results Conclusion
Conclusions
Measure for similarity of point processes
Key idea: matching of events
Application: EEG synchrony of MCI patients
About 85% correctly classified perhaps useful for screening a large population
Future work: Combination with other modalities (MEG, fMRI,...) Alternative inference techniques (variations on max-product, Monte-Carlo) More sophisticated models (e.g., interaction between events)
Analyzing Brain Signals
by Combinatorial Optimization
Justin DauwelsLIDS, MIT
Amari Research Unit, Brain Science Institute, RIKEN
September 25, 2008
References + softwareReferences
Quantifying Statistical Interdependence by Message Passing on Graphs: Algorithms and Application to Neural Signals, Neural Computation (under revision)
A Comparative Study of Synchrony Measures for the Early Diagnosis of Alzheimer's Disease Based on EEG, NeuroImage (under revision)
Measuring Neural Synchrony by Message Passing, NIPS 2007
Quantifying the Similarity of Multiple Multi-Dimensional Point Processes by Integer Programming with Application to Early Diagnosis of Alzheimer's Disease from EEG, EMBC 2008 (submitted)
Software
MATLAB implementation of the synchrony measures
Estimation
Deltas: average offset Sigmas: var of offset
...where
Simple closed form expressions
artificial observations (conjugate prior)
Large-scale synchrony
Apparently, all brain regions affected...
Alzheimer's disease
1980 1990 2000 2010 2020 2030 2040 20500
2
4
6
8
10
12
14
Outside glimpse: the future (prevalence)
USA (Hebert et al. 2003)
2000 2030 20500
20
40
60
80
100
120
Developped countriesDevelopping countries
World (Wimo et al. 2003)
Mill
ion
of s
uffe
rers
Mill
ion
of s
uffe
rers
• 2% to 5% of people over 65 years old
• Up to 20% of people over 80
Jeong 2004 (Nature)
Ongoing and future workApplications
alternative inference techniques (e.g., MCMC, linear programming) time dependent (Gaussian processes) multivariate (T.Weber)
Fluctuations of EEG synchrony Caused by auditory stimuli and music (T. Rutkowski) Caused by visual stimuli (F. Vialatte) Yoga professionals (F. Vialatte) Professional shogi players (RIKEN & Fujitsu) Brain-Computer Interfaces (T. Rutkowski)
Spike data from interacting monkeys (N. Fujii) Calcium propagation in gliacells (N. Nakata) Neural growth (Y. Tsukada & Y. Sakumura) ...
Algorithms
Fitting bump models
Signal
Bump
Initialisation After adaptationAdaptation
gradient method
F. Vialatte et al. “A machine learning approach to the analysis of time-frequency maps and its application to neural dynamics”, Neural Networks (2007).
Boxplots
SURPRISE!No increase in jitter, but significantly less matched activity!
Physiological interpretation• neural assemblies more localized?• harder to establish large-scale synchrony?
Generative modelGenerate bump model (hidden)
• geometric prior for number n of bumps p(n) = (1- λ S) (λ S)-n
• bumps are uniformly distributed in rectangle
• amplitude, width (in t and f) all i.i.d.
Generate two “noisy” observations
• offset between hidden and observed bump = Gaussian random vector with mean ( ±δt /2, ±δf /2) covariance diag(st/2, sf /2)
• amplitude, width (in t and f) all i.i.d.
• “deletion” with probability pd
yhidden
y y’
Easily extendable to more than 2 observations…
( -δt /2, -δf /2)
( δt /2, δf /2)
Probabilistic inference
MATCHINGPOINT ESTIMATION
PROBLEM: Given two bump models, compute (ρspur, δt, st, δf, sf )
APPROACH: (c*,θ*) = argmaxc,θ log p(y, y’, c, θ)
θ
SOLUTION: Coordinate descent
c(i+1) = argmaxc log p(y, y’, c, θ(i) ) θ(i+1) = argmaxx log p(y, y’, c(i+1) ,θ )
Alzheimer's diseaseInside glimpse: abnormal EEG
• AD vs. MCI (Hogan et al. 203; Jiang et al., 2005)• AD vs. Control (Hermann, Demilrap, 2005, Yagyu et al. 1997; Stam et al., 2002; Babiloni et al. 2006)• MCI vs. mildAD (Babiloni et al., 2006).
Decrease of synchrony
Brain “slow-down”slow rhythms (0.5-8 Hz) fast rhythms (8-30 Hz)
(Babiloni et al., 2004; Besthorn et al., 1997; Jelic et al. 1996, Jeong 2004; Dierks et al., 1993).
Images: www.cerebromente.org.br
EEG system: inexpensive, mobile, useful for screening
focus of this project
Comparing EEG signal rhythms ?
PROBLEM I:
Signals of 3 seconds sampled at 100 Hz ( 300 samples)Time-frequency representation of one signal = about 25 000 coefficients
2 signals
Numerous neighboring pixels
Comparing EEG signal rhythms ?(2)
One pixel
PROBLEM II:
Shifts in time-frequency!
Strong (anti-) correlations „families“ of sync measures
Correlations
Generative modelGenerate bump model (hidden)
• geometric prior for number n of bumps p(n) = (1- λ S) (λ S)-n
• bumps are uniformly distributed in rectangle
• amplitude, width (in t and f) all i.i.d.
Generate M “noisy” observations
• offset between hidden and observed bump = Gaussian random vector with mean ( δt,m /2, δf,m /2) covariance diag(st,m/2, sf,m /2)
• amplitude, width (in t and f) all i.i.d.
• “deletion” with probability pd
yhidden
y1 y2 y3 y4 y5
Parameters: θ = δt,m , δf,m , st,m , sf,m, pc
pc (i) = p(cluster size = i |y) (i = 1,2,…,M)
± 90% correctly classified
± 85% correctly classified
Average cluster size
Classification (multi-SES)
Average cluster size
Average bump freq
Average bump width
ffDTF
Similarity of bump models...
How “similar” or “synchronous” are two bump models?
Signatures of local synchronyf (Hz)
t (sec)
Time-frequency patterns(“bumps”)
EEG stems from thousands of neuronsbump if neurons are phase-locked= local synchrony
Alzheimer's diseaseInside glimpse: brain atrophy
Video source: P. Thompson, J.Neuroscience, 2003
Images: Jannis Productions.(R. Fredenburg; S. Jannis)
amyloid plaques andneurofibrillary tangles
Video source: Alzheimer society
POINT ESTIMATION: θ(i+1) = argmaxx log p(y, y’, c(i+1) ,θ )
Uniform prior p(θ): δt, δf = average offset, st, sf = variance of offset Conjugate prior p(θ): still closed-form expressionOther kind of prior p(θ): numerical optimization (gradient method)
Probabilistic inference
MATCHING: c(i+1) = argmaxc log p(y, y’, c, θ(i) )
ALGORITHMS
• Polynomial-time algorithms gives optimal solution(s) (Edmond-Karp and Auction algorithm)• Linear programming relaxation: extreme points of LP polytope are integral• Max-product algorithm gives optimal solution if unique [Bayati et al. (2005), Sanghavi (2007)]
EQUIVALENT to (imperfect) bipartite max-weight matching problem
c(i+1) = argmaxc log p(y, y’, c, θ(i) ) = argmaxc Σkk’ wkk’(i) ckk’
s.t. Σk’ ckk’ ≤ 1 and Σk ckk’ ≤ 1 and ckk’ 2 {0,1}
Probabilistic inference
not necessarily perfectfind heaviest set of disjoint edges
p(y, y’, c, θ) / I(c) pθ(θ) Πkk’ (N(t k’ – tk ; δt ,st,kk’) N(f k’ – fk ; δf ,sf, kk’) β-2)ckk’
Max-product algorithmMATCHING: c(i+1) = argmaxc log p(y, y’, c, θ(i) )
Generative model
Max-product algorithmMATCHING: c(i+1) = argmaxc log p(y, y’, c, θ(i) )
μ↑μ↑
μ↓ μ↓
Conditioning on θ
Max-product algorithm (2)• Iteratively compute messages
• At convergence, compute marginals p(ckk’) = μ↓(ckk’) μ↓(ckk’) μ↑(ckk’)• Decisions: c*kk’ = argmaxckk’
p(ckk’)
Algorithm
MATCHING → max-productESTIMATION → closed-form
PROBLEM: Given two bump models, compute (ρspur, δt, st, δf, sf )
APPROACH: (c*,θ*) = argmaxc,θ log p(y, y’, c, θ)
θ
SOLUTION: Coordinate descent
c(i+1) = argmaxc log p(y, y’, c, θ(i) ) θ(i+1) = argmaxx log p(y, y’, c(i+1) ,θ )
Generative modelGenerate bump model (hidden)
• geometric prior for number n of bumps p(n) = (1- λ S) (λ S)-n
• bumps are uniformly distributed in rectangle
• amplitude, width (in t and f) all i.i.d.
Generate two “noisy” observations
• offset between hidden and observed bump = Gaussian random vector with mean ( ±δt /2, ±δf /2) covariance diag(st/2, sf /2)
• amplitude, width (in t and f) all i.i.d.
• “deletion” with probability pd
yhidden
y y’
Easily extendable to more than 2 observations…
( -δt /2, -δf /2)
( δt /2, δf /2)
Generative model (2)
• Binary variables ckk’
ckk’ = 1 if k and k’ are observations of same hidden bump, else ckk’ = 0 (e.g., cii’ = 1 cij’ = 0)
• Constraints: bk = Σk’ ckk’ and bk’ = Σk ckk’ are binary (“matching constraints”)
• Generative Model p(y, y’, yhidden , c, δt , δf , st , sf ) (symmetric in y and y’)
• Eliminate yhidden → offset is Gaussian RV with mean = ( δt , δf ) and covariance diag (st , sf)
• Probabilistic Inference:(c*,θ*) = argmaxc,θ log p(y, y’, c, θ)
y y’
( -δt /2, -δf /2)
( δt /2, δf /2)
i
i’ j’
p(y, y’, c, θ) = ∫ p(y, y’, yhidden , c, θ) dyhidden
θ
• Bumps in one model, but NOT in other → fraction of “spurious” bumps ρspur
• Bumps in both models, but with offset → Average time offset δt (delay) → Timing jitter with variance st
→ Average frequency offset δf → Frequency jitter with variance sf
PROBLEM: Given two bump models, compute (ρspur, δt, st, δf, sf )
APPROACH: (c*,θ*) = argmaxc,θ log p(y, y’, c, θ)θ
Summary
Objective function
• Logarithm of model: log p(y, y’, c, θ) = Σkk’ wkk’ ckk’ + log I(c) + log pθ(θ) + γ
wkk’ = -(1/st (t k’ – tk – δt)2 + 1/sf (f k’ – fk– δf)2 ) - 2 log β
β = pd (λ/V)1/2
Euclidean distance between bump centers
• Large wkk’ if : a) bumps are close b) small pd c) few bumps per volume element
• No need to specify pd , λ, and V, they only appear through β = knob to control # matches
y y’
( -δt /2, -δf /2)
( δt /2, δf /2)
i
i’ j’
Distance measures
wkk’ = 1/st,kk’ (t k’ – tk – δt)2 + 1/sf,kk’ (f k’ – fk– δf)2 + 2 log β
st,kk’ = (Δtk + Δt’k) st sf,kk’ = (Δfk + Δf’k) sf
Scaling
Non-Euclidean
p(y, y’, c, θ) / I(c) pθ(θ) Πkk’ (N(t k’ – tk ; δt ,st,kk’) N(f k’ – fk ; δf ,sf, kk’) β-2)ckk’
Generative model
Expect bumps to appear at about same frequency, but delayed Frequency shift requires non-linear transformation, less likely than delay Conjugate priors for st and sf (scaled inverse chi-squared):
Improper prior for δt and δt : p(δt) = 1 = p(δf)
Prior for parameters
CTR
MCI
Preliminary results for multi-variate modellinear comb of pc
Probabilistic inference
MATCHINGPOINT ESTIMATION
PROBLEM: Given two bump models, compute (ρspur, δt, st, δf, sf )
APPROACH: (c*,θ*) = argmaxc,θ log p(y, y’, c, θ)
θ
SOLUTION: Coordinate descent
c(i+1) = argmaxc log p(y, y’, c, θ(i) ) θ(i+1) = argmaxx log p(y, y’, c(i+1) ,θ )
X
Y
Minx2 X, y2Y d(x,y)
Generative modelGenerate bump model (hidden)
• geometric prior for number n of bumps p(n) = (1- λ S) (λ S)-n
• bumps are uniformly distributed in rectangle
• amplitude, width (in t and f) all i.i.d.
Generate M “noisy” observations
• offset between hidden and observed bump = Gaussian random vector with mean ( δt,m /2, δf,m /2) covariance diag(st,m/2, sf,m /2)
• amplitude, width (in t and f) all i.i.d.
• “deletion” with probability pd
(other prior pc0 for cluster size)
yhidden
y1 y2 y3 y4 y5
Parameters: θ = δt,m , δf,m , st,m , sf,m, pc
pc (i) = p(cluster size = i |y) (i = 1,2,…,M)
(Hebb 1949, Fuster 1997)
Stimuli Consolidation Stimulus
Voice Face Voice
Role of local synchrony
Assembly activation Hebbian consolidationAssembly recall
Probabilistic inference
CLUSTERING (IP or MP)POINT ESTIMATION
PROBLEM: Given M bump models, compute θ = δt,m , δf,m , st,m , sf,m, pc
APPROACH: (c*,θ*) = argmaxc,θ log p(y, y’, c, θ)
SOLUTION: Coordinate descent
c(i+1) = argmaxc log p(y, y’, c, θ(i) ) θ(i+1) = argmaxx log p(y, y’, c(i+1) ,θ )
Integer program• Max-product algorithm (MP) on sparse graph• Integer programming methods (e.g., LP relaxation)
Fourier transform
High frequency
Low frequency
Frequency
1 23
2
13
Windowed Fourier transform
* =Fourier basis functions Window
function windowed basis functions
WindowedFourierTransform
t
f
Overview
Alzheimer’s Disease (AD): decrease in EEG synchrony Synchrony measure in time-frequency domain
Pairs of EEG signalsCollections of EEG signals
Numerical Results Conclusion
Average synchrony
3. SES for each pair of models4. Average the SES parameters
1. Group electrodes in regions2. Bump model for each region
Beyond pairwise interactions...Pairwise similarity Multi-variate similarity
Similarity measures• Correlation and coherence• Granger causality (linear system): DTF, ffDTF, dDTF, PDC, PC, ...
• Phase Synchrony: compare instantaneous phases (wavelet/Hilbert transform)
• State space based measures sync likelihood, S-estimator, S-H-N-indices, ...
• Information-theoretic measures KL divergence, Jensen-Shannon divergence, ...
No Phase Locking Phase Locking
TIME FREQUENCY