BCS547 Neural Decoding

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BCS547Neural Decoding
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Nature of the problem
In response to a stimulus with unknown
orientation q, you observe a pattern of activity
A. What can you say about q given A?
Bayesian approach recover P(qA) (the posterior
distribution)
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Population Code
Tuning Curves
Pattern of activity (A)
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Estimation theory

• A common measure of decoding performance is the
  mean square error between the estimate and the
  true value
• This error can be decomposed as

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Efficient Estimators

• The smallest achievable variance for an unbiased
  estimator is known as the Cramer-Rao bound, sCR2.
• An efficient estimator is such that
• In general

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Fisher Information
Fisher information is defined as and
it is equal to
where P(A q) is the distribution of the
neuronal noise.
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Fisher Information
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Fisher Information

• For one neuron with Poisson noise
• For n independent neurons

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Fisher Information and Tuning Curves

• Fisher information is maximum where the slope is
  maximum
• This is consistent with adaptation experiments

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Fisher Information

• In 1D, Fisher information decreases with the
  width of the tuning curves
• In 2D, Fisher information does not depend on the
  width of the tuning curve
• In 3D and above, Fisher information increases
  with the width of the tuning curves.
• ATTENTION this is true for independent gaussian
  noise.

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Ideal observer

• The discrimination threshold of an ideal
  observer, dq, is proportional to the variance of
  the Cramer-Rao Bound.
• In other words, an efficient estimator is an
  ideal observer.

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• An ideal observer is an observer that can recover
  all the Fisher information in the activity (easy
  link between Fisher information and behavioral
  performance)
• If all distributions are gaussian, Fisher
  information is the same as Shannon information.

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Voting Methods

• Optimal Linear Estimator

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Voting Methods

• Optimal Linear Estimator

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Voting Methods

• Optimal Linear Estimator
• Center of Mass

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Center of Mass/Population Vector

• The center of mass is optimal (unbiased and
  efficient) iff The tuning curves are gaussian
  with a zero baseline, uniformly distributed and
  the noise follows a Poisson distribution
• In general, the center of mass has a large bias
  and a large variance

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Voting Methods

• Optimal Linear Estimator
• Center of Mass
• Population Vector

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Population Vector
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Population Vector
Typically, Population vector is not the optimal
linear estimator.
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Population Vector

• Population vector is optimal iff The tuning
  curves are cosine, uniformly distributed and the
  noise follows a normal distribution with fixed
  variance
• In most cases, the population vector is biased
  and has a large variance
• The variance of the population vector estimate
  does not reflect Fisher information

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Population Vector
Population vector
Fisher Information
Population vector should NEVER be used to
estimate information content!!!!
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Population Vector
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Maximum Likelihood
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Maximum Likelihood

• The estimate is the value of q that maximizes
  the likelihood P(Aq). Therefore, we seek such
  that

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Maximum Likelihood

• If the noise is gaussian and independent
• Therefore
• and the estimate is given by

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Maximum Likelihood
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Gradient descent for ML

• To minimize the likelihood function with respect
  to q, one can use a gradient descent technique in
  which q is updated according to

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Gaussian noise with variance proportional to the
mean

• If the noise is gaussian with variance
  proportional to the mean, the distance being
  minimized changes to

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Poisson noise
If the noise is Poisson then And

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ML and template matching

• Maximum likelihood is a template matching
  procedure BUT the metric used is not always the
  Euclidean distance, it depends on the noise
  distribution.

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Bayesian approach

• We want to recover P(qA). Using Bayes theorem,
  we have

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Bayesian approach
What is the likelihood of q, P(A q)? It is the
distribution of the noise It is the same
distribution we used for maximum likelihood.
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Bayesian approach

• The prior P(q) correspond to any knowledge we may
  have about q before we get to see any activity.
• Ex Zhang et al.

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Bayesian approach
Once we have P(qA), we can proceed in two
different ways. We can keep this distribution for
Bayesian inferences (as we would do in a Bayesian
network) or we can make a decision about q. For
instance, we can estimate q as being the value
that maximizes P(qA). This is known as the
maximum a posteriori estimate (MAP). For flat
prior, ML and MAP are equivalent.
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Using the prior Zhang et al

• For a time varying variable, one can use the
  distribution over the previous estimate as a
  prior for the next one.

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Bayesian approach
Limitations the Bayesian approach and ML require
a lot of data Alternative estimate P(qA)
directly using a nonlinear estimate.
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Bayesian approachlogistic regression
Example Decoding finger movements in M1. On each
trial, we observe 100 cells and we want to know
which one of the 5 fingers is being moved.
P(F5A)
1 0
5 categories
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2
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4
5

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100
100 input units
A
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Bayesian approachmultinomial distributions
Example Decoding finger movements in M1. Each
finger can take 3 mutually exclusive states no
movement, flexion, extension.
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Decoding time varying signals
s(t)
r(t)
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Decoding time varying signals
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Decoding time varying signals
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Decoding time varying signals
s(t)
r(t)
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