Some tutorial Matlab programs for fMRI, pattern-based analysis and SPM
Here are some tutorial files that show
how to use Matlab for fMRI, including pattern-based analysis
(also known as multi-voxel pattern analysis, or MVPA).
Please feel more than free to use the code for teaching,
and if you do, please
mail me with comments and feedback.
The code has lots of comments in it,
which attempt to explain the concepts as explicitly
as possible. No prior knowledge of Matlab or linear algebra
is assumed.
These first two tutorials show some approaches in pattern-based fMRI
anaylsis:
classification_plane_tutorial.m - This is a very introductory tutorial,
showing how a classification task (in this case, deciding whether people
are sumo wrestlers or basketball players, based on their height and weight)
can be viewed as drawing a decision boundary in a feature space.
It shows how to plot the data, calculate the weights of a simple linear
classifier, and see how the resulting classifier carves up the feature
space into two categories.
Note to Python users: Michael Hanke
has very kindly ported this tutorial into Python and has
included it in the examples section of the
PyMVPA toolbox.
The webpage describing it is
here
and the Python script is
here.
explore_kamitani_tong_pattern_based_fMRI.m - This tutorial is a bit
more in-depth, and explores the ideas put forward in the important paper
by Kamitani & Tong (2005). The code simulates a map of orientation columns in
primary visual cortex, divides this simulated cortex into voxel-like
chunks, and counts up how much tissue there is in each voxel which is tuned
to particular orientations. It then shows how this coarse-grained measure
can be used to decode which visual orientation is being presented,
even when some noise is added to the activation map.
I should emphasise that this code is simply inspired by the Kamitani & Tong study,
and should not be thought of as in any way endorsed by those authors! You can
download the original paper from the
Tong lab website.
The remaining tutorials illustrate standard fMRI analysis, i.e. modeling
individual voxel time-courses using a General Linear Model (GLM):
hrf_tutorial.m - This program is a good place to
start. It shows the basic shape of a Haemodynamic Response Function
(HRF), how to convolve an HRF with a time-series of stimulus onsets
in order to produce a predicted fMRI signal, and how to plot the results
in Matlab. The program also shows what happens when closely-spaced stimuli
produce HRFs which overlap in time and linearly sum.
math_of_convolution.m - This program shows
how convolution works, going through all the steps of exactly
what gets added up and multiplied with what. It also explains
from the ground-up how to do for-loops in Matlab.
(By the way, if you want to see an example of how to "vectorise"
code to avoid for-loops altogether, look at the bkprop.m program
in my
Matlab for neural-nets page).
design_matrix_tutorial.m
- This program shows what the rows and columns of a design matrix
mean, how each column represents the predicted response to a given
stimulus condition, and
how the design matrix gets used to estimate the sensitivity
of a voxel to various stimulus conditions.
The program walks step-by-step through the process of
estimating the model parameters for a voxel's noisy fMRI-signal,
making plots of the results along the way.
In my recent attempts to start learning Python, I made
a Python version of this script:
design_matrix_tutorial.py.
It requires the
SciPy and
Matplotlib
modules.
Probably the easiest way to get these modules all at once
is to install the Enthought Python Distribution, which
you can get for free if you are at an academic institution,
from
here.
If you are not at an academic institution, you can get them all for
free from the individual links above.
I'd love to hear any comments or feedback about this script.
The following programs show how to use Matlab to view brain
slices and to overlay statistical maps on top of anatomical scans.
In order to run them, you need the companion .mat file that contains
the brain images:
speech_brain_images.mat (Choose "Download link to disk" with
right-click, or Control-mouseclick).
showing_brain_images_tutorial.m - Start with this
program. It doesn't assume any prior knowledge of Matlab. It shows
how to extract slices (axial, sagittal etc.) from a 3D volume
and how to plot them with the appropriate axes, colormaps,
and color-scaling.
overlaying_Tmaps_tutorial.m - This program
shows how to overlay a statistical map on top of an anatomical map.
It assumes that you've already worked through showing_brain_images_tutorial.m
You can see some more examples of showing brain images, this time
using the transparency properties of Matlab 6, in the program
put_into_same_voxel_space.m below.
The programs below are slightly more complicated, and use functions
from SPM99. The code gives commented examples of how to do things
like reading SPM images into Matlab, finding their origins,
displaying overlays, and extracting parts of SPM plots.
put_into_same_voxel_space.m - This program
lets you put one image into the same voxel space as another image.
E.g. you might want to extract a Brodmann area from the brodmann.img
file that comes with MRIcro,
and then put that set of 1x1x1mm voxels
into the same voxel space as your functional data, which probably
covers a smaller volume and has larger voxels.
Then you can use the Brodmann area
as an ROI to explore your functional results.
grab_mip.m -
This program grabs the "glass brain"
Maximum Intensity Projection (MIP) from an SPM99 results plot,
makes the voxels orange-ish instead of gray, and offers
the option of saving the separate brain images into EPS files,
which are good for distilling into PDFs or for loading directly
into Adobe Illustrator. By the way, the EPS files are large (around
192K), but they shrink into nice compact 8K PDF files.