Johannes Textor

MotilityLab is designed to help with describing, visualizing, and quantifying tracks of moving objects. Many common measures used in physics and biology are implemented, such as mean square displacement and autocorrelation. The package also provides a flexible function to import tracks from text files.

The MotilityLab package has been developed as part of the project of the same name. On the project website (motilitylab.net), a simple GUI frontend to many functions in the package is implemented, and public datasets are available to download and analyze.

The latest development version of MotilityLab can be installed from R-forge:

   install.packages("MotilityLab",repos="http://R-Forge.R-project.org")

Then the package can be loaded as usual:

    library(MotilityLab)

Tracks are organized as lists of matrices, and have S3 class tracks. Three example datasets are provided with the package. A plot method is implemented and can be used to visualize these datasets:

    plot( TCells, col=1 )

    plot( BCells, col=2 )

To generate a simple mean square displacement plot for the example dataset `TCells', use:

    msqd <- aggregate( TCells, squareDisplacement )
    plot( msqd, type='l' )

This computes the squared displacement (MSD) for over all subtracks of the dataset, and computes the average stratified by subtrack length. To compute the MSD for non-overlapping subtracks only, use:

    msqd <- aggregate( TCells, squareDisplacement, max.overlap=0 )
    plot( msqd, type='l' )

MSD estimates can be biased in applications with a finite field of view (such as microscopy), because slower objects remain in the field of view for longer times. This can complicate comparisons between different populations. We may thus wish to restrict our comparison to subtracks of a certain (short) length. This can be done as follows:

    msqd.t <- aggregate( TCells, squareDisplacement, subtrack.length=1:5, max.overlap=0 )
    msqd.b <- aggregate( TCells, squareDisplacement, subtrack.length=1:5, max.overlap=0 )

    plot( msqd.t, type='l' )
    lines( msqd.b, col=2 )

Another common measure to analyze tracks is the autocovariance function; geometrically speaking, this is the dot product between pairs of pairs of positions a fixed distance apart. For random walks, the autocovariance decreases to 0 as the subtrack length increases. The speed of convergence gives an indication of persistence of orientation, a feature of many realistic objects. To compare persistence of the T cells and B cells datasets, we can use:

    angle.t <- aggregate( TCells, overallDot )
    angle.b <- aggregate( BCells, overallDot )
    plot( angle.t, type='l' )
    lines( angle.b, col=2 )

Many other ways to quantify tracks are implemented in the package and described in the PDF documentation. For an overview of the available commands, use the function

    help( package="MotilityLab" )