
In order to perform classification on a functional brain scan it first
undergoes many preprocessing steps. One of these steps is the transformation from
the timeseries output of the fMRI scan, transforming a $m{\times}n$ (where $ m $ is
the number of timepoints recorded and $ n $ is the number of brain regions used)
to an $n{\times}n$ matrix of similiarity values (called a connectome). This
similarity value is a measure of the neural synchronization between the 2 regions.

So how do we quantify the similarity of 2 different timeseries? This blog post will
explore the common ways of quantifying time series similarity in a neuroscientific
setting. Before we get into the actual methods used to calculate time series similarity,
we need to cover the corner stone of almost all of the methods we are about to
explore - covariance.

## Covariance

**Covariance** is simply a measure of how two random variables change together.
Below is the formula for calculating covariance:

$$
\operatorname{\Sigma} = \operatorname{E}\left[\left(X-\operatorname{E}[X]\right ) \left (X-\operatorname{E}[X]\right)^\mathrm{T}\right]
$$

In the case of fMRI, we have a multivariate random variable, allowing us to use
Maximum Likelihood Estimation to estimate the covariance matrix. Below is a toy
example of our estimated covariance matrix.

$$
\begin{bmatrix}
var(X) & cov(X,Y) & cov(X,Z) \\
cov(X,Y) & var(Y) & cov(X,Y) \\
cov(X,Z) & cov(X,Y) & var(Z)
\end{bmatrix}
$$

This estimated covariance matrix is called the empirical covariance matrix. In
practice we don't use the covariance matrix for our fMRI analyis.
This for a number of reasons:

-   Covariance is bound between $ -\infty $ and $ \infty $ making it less
    suitable for downstream classifiers.
-   Covariance coefficients are not standardized and cannot be used to quantify the
    strength of the relationship.
-   Symmetric Positive semi-definite (SPD) matricies do not naturally form a Euclidean space (this will be
    important later).
-   When the number of features is large relative to the number of observations,
    the sample/empirical covariance matrix has excessive estimation error.

Many of the following approaches will aim to address some or all of the above
drawbacks with empirical covariance.

To address the excessive estimation error, it is common to perform a
transformation to the covariance coefficients, known as **"shrinkage"**. In
their seminal paper, "Honey, I shrunk the Sample Covariance Matrix" \[[1](http://www.ledoit.net/honey.pdf)\], Ledoit \& Wolf
proposed using shrinkage to regularize the sample covariance matrix.
"Shrinkage" as the name implies, pulls the most extreme covariance coefficients
towards more central values. This not only resolves our excessive estimation
error, it can also make the matrix easily invertable by encouraging numerical
stability. (The interested reader should consult the scikit-learn docs
\[[2](https://scikit-learn.org/stable/auto_examples/covariance/plot_covariance_estimation.html)\] )

Now that we have a well conditioned covariance matrix, we can attempt to address
some of the other identified drawbacks.

## Canonical Approaches

**Pearson's correlation coefficient** (Pearson's $ R $) or simply correlation, is
the most commonly used method to quantify similarity between 2 fMRI timeseries.
Correlation is a linear metric computed from the covariance of the 2 timeseries.
Below is the mathematical formula to compute correlation for a pair of random
variables:

$$
{\displaystyle \rho _{X,Y}={\frac {\operatorname {cov} (X,Y)}{\sigma _{X}\sigma _{Y}}}}
$$

Correlation is widely used in neuroscience as it has long statistical history
and is bound between -1 and 1. However, correlation does have some disadvantages.
The below figure should demonstrate one clearly:

![Correlation Decay](/corr.gif "Correlation Decay")

Due to correlations linear nature, the same timeseries being slightly out of
phase causes a huge decrease in the correlation value. Additionally, correlation
provides no distinction between whether 2 regions are directly
connected or indirectly connected via another region. To account for this, we
can use partial correlation!

&nbsp;

**Partial correlation** is a variant of PCC that attempts to address
distinguishing between direct and indirect connections. This is done by
computing correlation between regions after regressing all other timeseries.
Partial correlation is computed from the inverse of the covariance matrix
(this is where the shrinkage comes in handy), also known as the precision matrix.
Below is the mathematical formula for computing partial correlation for a pair
of random variables:

$$
\rho = -{\frac {p_{ij}}{\sqrt {p_{ii} p_{jj}}}}
$$

where $p$ is the respective precision matrix (obtained through inversion of our
well conditioned covariance matrix).

## Novel Approaches

**Tangent space parameterization** is a recently proposed solution to the problematic
nature of SPD matricies, in that they don't naturally form Euclidean spaces.
Varoquax et al. \[[3](https://arxiv.org/abs/1008.5070)\] proposed a solution to this
sticky problem. Whilst I won't profess to be an expert on differential
manifolds or Riemmenian geometry, here is how I understand it.

Any covariance matrix is a symmetric positive semi-definite matrix. In order to
preserve the geometry during our transformations, we need to
perform a projection into a space where our geometry is preserved.
As SPD matricies form a diffentiable manifold, we can use this fact and
Riemannian geometry to keep our geometric intutions in tact.

![Manifold](/manifold.jpg "Manifold")

<a href="https://juliahub.com/docs/PosDefManifold/vb3YZ/0.4.8/introToRiemannianGeometry/#Intro-to-Riemannian-Geometry-1" style="text-align: center;">
Image source
</a>

The above image makes it clear that if we perform Euclidean operations like
subtraction in our manifold, the distances and therefore the geometry will break
down. By defining a homomorphic (ie structure preserving) tangent space, we can
approximate the distance on the manifold using Euclidean distance in the tangent
space!

But we aren't quite out of the woods yet, since we still need to define the
reference point $ G $ on the manifold. As all of our covariance matricies need
to be projected into the same tangent space, we need to choose an appropriate
reference point. To do this, Varoquax et al. compute the geometric mean of the
matricies to define a reference space.

In their paper, Dadi et al. \[[4](https://hal.inria.fr/hal-01824205v3)\] showed
that across many fMRI classification tasks, tangent space parameterization
should be preferred over both correlation and partial correlation.

&nbsp;

**Dynamic Time Warping** (DTW) is not like the previously discussed methods, as it does
not rely on covariance matricies. DTW is one of the canonical algorithms used for measuring
similarity between timeseries. It is commonly used in scenarios where timeseries
may have a different length or be temporally offest. DTW is a non linear metric,
and calculates the optimal match between two given sequences with certain
restrictions.

DTW was first proposed for usage with fMRI timeseries by Meszlényi et al. \[[5](https://www.frontiersin.org/articles/10.3389/fnins.2017.00075/full)\].
They propose its value for fMRI timeseries analysis as it has been noted that
"dynamic switch of brain states" can cause non-stationary time lags. If there
are known or unknown mechanisms in the brain that cause regional time differences
in hemodynamic response - DTW could be a great way to account for this.

Below is a Rust implementation of windowed DTW.
The window refers to the maximal warping distance.

```rust
let m = s.len() + 1;
let n = t.len() + 1;
let mut dtw = Array::from_elem((m, n), f64::MAX);

dtw[[0, 0]] = 0.;

let max_window = i32::max(*window, i32::abs((n - m) as i32));
for si in 1..n {
    let lower_bound = i32::max(1, si as i32 - max_window);
    let upper_bound = i32::min(m as i32, si as i32 + max_window);
    for ti in lower_bound as usize..upper_bound as usize {
        let cost = distance_fn(&s[si - 1], &t[ti - 1]);
        dtw[[si, ti]] = cost + 
            f64::min(
                f64::min(dtw[[si - 1, ti]], dtw[[si, ti - 1]]), 
                dtw[[si - 1, ti - 1]]
            );
    }
}
```

Dynamic Time Warping does have some disadvantages. One particularly
troublesome one being the computational complexity - $ O(n^2) $. The paper from
Meszlényi et al. does come with a DTW implementation for fMRI data on GitHub,
however it is extremely cumbersome and difficult to get running (particularly
on HPC systems where you don't have root access.)

To address this I wrote RustDTW, a Python module backed by Rust code that works
natively within Python on numpy matricies. Check out an example of using DTW
for fMRI classification [here.](https://github.com/FL33TW00D/rustDTW/blob/master/examples/classification/ABIDE_classification.ipynb)
My basic testing shows that it should be nearly 10x faster than a mutlithreaded
Python implementation.

## Classification Performance

Whilst not meant to be an exhaustive exploration of the different connectivity
metrics explored above (and itself is not an exhaustive exploration of available
connectivity metrics, for more please see Pervaiz et al. \[[6](https://www.biorxiv.org/content/10.1101/741595v1.full.pdf)\]
I thought I would demonstrate a comparison between them using a large open source dataset.
You can check out the inner workings and run it for yourself on Colab here!

<a href="https://colab.research.google.com/github/FL33TW00D/rustDTW/blob/master/examples/classification/ABIDE_classification.ipynb">
<img src="https://colab.research.google.com/assets/colab-badge.svg"
style="max-width:20%"/>
</a>

For those just interested in the results, the below graph summarizes the
performance of each of the similarity metrics.

<img src="/accuracy.png"/>

The results are pretty interesting! We can see that both non-linear matching and
transforming the covariance matricies into a natural space can provide improvements
over standard correlation. Perhaps a future metric will incorporate
characteristics from both tangent space parameterization and DTW to improve
performance.


Hi, I'm Chris.

Posts