FEAT 2 Practical

This tutorial leads you through examples of higher-level group analysis in FEAT.

Contents:

Paired t-test
Perform a group-level analysis of a repeated measure experiment, using the paired t-test.
Group analysis with multiple sessions for each subject
Perform second and third level analyses for an experiment with multiple sessions per subject.

Paired t-test

We have a group of six subjects, each scanned twice: once doing motor tasks with their left hand, and once with their right hand. This is a stroke study, and hence comparing left and right motor function is particularly interesting in this case. Within each run, subjects completed different blocks of index finger movement, sequential finger movement and random finger movement.

Research question: Is there a significant left vs right hand finger movement paired-difference, generalisable to the population from which the subjects are drawn?

To address this, we want the left − right paired mean difference within a mixed effects model, taking into account the within-subject fixed effects variances and the between-subject random effect variance. This is done as a two-level analysis with the following structure:

In FSL terminology, each contrast is represented by a COPE (contrast of parameter estimate), and it is these which we pass up to any higher-level analysis. Note that as well as the COPEs, FEAT passes the variance of these COPEs (VARCOPEs), and even the uncertainty in the variance of these COPEs (DOFs; degrees-of-freedom), between the different levels.

Higher level analysis flowchart
What do the values in a COPE image obtained from the single-session analysis represent?
Incorrect! COPE (contrast of parameter estimates) images contain combinations of parameter estimates for a certain contrast. For a simple 1 0 contrasts, these may be the same as the parameter estimates for a certain EV, but for more complex contrasts such as 1 -1 these have to be calculated by combining the EV parameter estimates in the correct proportions.
Correct! COPE (contrast of parameter estimates) images are calculated by performing simple algebra on the EV parameter estimates. For example, for the contrasts 1 -1, the contrast parameter estimate is calculated by subtracting the parameter estimate for EV 2 from the parameter estimate for EV1. The COPE image will show regions in the brain in which this difference is large.
Incorrect! The uncertainty (or variance) is contained in the VARCOPE image.

First-level analyses

Each first-level analysis contains 6 contrasts, each related to the different types of finger tapping performed in the scanner (e.g. mean response over the different conditions, index finger only, etc.). Thus there are 6 COPEs in the stats subdirectory of each first-level .feat directory. A higher-level FEAT analysis entails an independent analysis on each of these contrasts (i.e. a second-level analysis of all subjects' first-level mean contrasts, a separate second-level analysis of all first-level index contrasts, etc.). Each of these second-level analyses is performed simultaneously and will form a separate cope?.feat directory inside a newly-created .gfeat directory.

cd ~/fsl_course_data/fmri2/paired_ttest

The first-level analyses are held in 6 different directories within ~/fsl_course_data/fmri2/paired_ttest, one for each subject. The subject directories are ac at cm df dn eg. There are two first-level FEAT directories within each of these, and these have already been run for you. Have a quick look at one of the lower-level reports if you want to familiarise yourself with the study design and data.

FEAT set-up

Open FEAT (Feat & or [Feat_gui & on a mac]) and follow the instructions below to set up the higher-level analysis.

First, change First-level analysis to Higher-level analysis in the drop down box at the top.

Data

Here we are going to set up the input data for our higher level analysis. First, change the Number of inputs to 12 (i.e. 6 subjects × 2 sessions).

Press Select FEAT directories. At this stage, you need to decide on a sensible order for the first-level analyses. You could choose to group the analyses by subject (i.e. ac/ac_left.feat, ac/ac_right.feat, at/at_left.feat, etc.), or you could group by condition (i.e. ac/ac_left.feat, at/at_left.feat, …, ac/ac_right.feat, etc.). We recommend the latter option, because this matches the example paired t-test in the FEAT manual, and also matches the way the paired t-test is set up for you if you use the Model setup wizard (explained below).

You can often avoid having to tediously hand-select each of these first-level FEAT directories separately, using the Paste button. If you press this, a new free-text window comes up, within which you can paste text (in this case the list of first-level FEAT directories) which you can copy, e.g. from a list in a terminal. Press Clear to clear the text window. Then in your terminal, making sure you are inside the paired_ttest directory, type:

ls -d1 "$PWD"/??/??_left.feat ; ls -d1 "$PWD"/??/??_right.feat

How does this command work? Answer. This should give you a complete listing of the full pathnames of the FEAT directories in the right order. You can now highlight this list with the mouse, and paste it into the FEAT paste window with the middle mouse button, or by clicking in the paste window and pressing control-y.

To save time, we will only pass the mean contrast up to the top level. Make sure that ONLY contrast 1 is selected in the Use lower-level copes boxes.

When this is done, set the Output directory to paired_ttest_ols (the full path will end up as /home/fslcourse/fsl_course_data/fmri2/paired_ttest/paired_ttest_ols.gfeat).

Stats

Select the Mixed Effects: Simple OLS option from the top drop down box. Also, make sure that the Use automatic outlier de-weighting button is NOT turned on. It is important that these two settings are chosen, otherwise the analysis will not be quick enough to be of use to you in the time that we have available for the practical. Normally, we recommend that the more accurate "Mixed Effects: FLAME 1" option is used in combination with outlier de-weighting, for the reasons outlined in the lectures. However, in the interest of speed, in this practical we choose the faster OLS option without outlier de-weighting.

With this design you can use the Model setup wizard, which provides an easy way of setting up a few simple designs. Select two groups, paired and press Process. You will now see the design matrix that has been created for you.

To understand how this is controlled in detail, click on Full model setup.

Post-stats

Because we only use a small number of subjects in order to make it possible to run the analysis in the practical session, we will reduce the cluster threshold slightly. This will allow us to see some more results, but is NOT recommended for your own analyses. In the Thresholding box change the Z threshold to 2.3.

Go!

Press Go! The web browser that appears monitors the overall progress. This second-level analysis should take about 5 minutes. While you're waiting, either make a cup of tea (but do NOT add milk while the bag is still in the water) or familiarise yourself with the introduction to the next major section of the practical on group analyses with multiple sessions per subject.

Results

Higher-level FEAT runs produce .gfeat directories. Once the analysis has finished, explore the web report. This top-level report provides links to the previous level reports, a registration summary page and links to the separate higher-level reports.

LOOK AT YOUR DATA! In particular it is always important to check the registration summary report page very carefully, to ensure that all lower-level registrations succeeded. If any of the lower-level FEATs look like the registration has failed badly, you need to fix this before re-running the higher-level FEAT analysis. Note that field maps were not acquired with this data—you should be able to spot this on the registration page!

In the results page you get a link to the group results from running the group-level analysis on each first-level contrast. Within each contrast you get a group-level results page showing the standard post-stats output. However, note that the time course outputs in these higher-level results no longer refer to time (despite the heading). They refer to subject (or session) number. In this case that is the 12 sessions (6 subjects × 2 conditions) in the study, and it is effect size shown on the vertical axis, rather than normalised MRI signal. Have a look at this and the other parts of the results webpage and make sure you understand what is being shown.

Pre-baked analyses

We have run a full analysis for you on this data (i.e. on all the contrasts, using FLAME for statistics, and with the recommended Z-thresholds). Take a quick look at this report as well.

firefox examples/flame.gfeat/report.html &

Can you spot any major differences between the two analyses?


Group analysis with multiple sessions for each subject

It is common to split a task up into multiple short scans instead of having one long scan. This can often help to reduce subject movement in the scans, and also to keep the attention of your participant. As a result, we need to combine data across multiple scanning sessions using a three-level FEAT analysis.

The data consists of a set of subjects, each scanned twice several months apart. For simplicity's sake, we will look for a simple mean effect across subjects and sessions. Hopefully this will help you understand how this analysis can be extended to more complex questions.

We want the mean group effect, within a mixed effects model, taking into account the within-subject fixed effects variances and the between-subject random effect variance. This is done in THREE levels:

Because each subject will typically only have a handful of sessions, we do not run a mixed effects second-level analysis to get an estimation of each subject's mean response. The reason for this is that we would not be able to get a good estimation of the within-subject session-to-session variance with a limited number of sessions. Hence we choose to ignore the session-to-session variance by using a fixed effects analysis at this second level. See here for a more involved discussion of the choice of a fixed effects analysis.

In addition to this, the analysis cannot be combined into a single second-level analysis. This is tempting as a design matrix can easily be formed containing each subject's mean (across sessions) as a separate EV, and then contrasts can be formed to test the mean across all subjects. The problem with this model is that there are two separate sources of variability (session-to-session and subject-to-subject) but, within FSL, a single level cannot model more than one separate sources of variance.

First-level analyses

In both sessions, subjects performed the "Pyramids & Palm Trees" task (PPTT). Participants are presented with a target image, and are asked to select the image they most associate with the target from a pair of additional images. The canonical example is below:

Example stimulus

This is meant to be a test of semantic memory, as the task requires reasoning about the links between objects. There is also a control condition, where participants have to match abstract line drawings. We are primarily interested in the semantic > lines responses. To begin with, familiarise yourself with the first-level design and typical responses in one of the session-specific FEAT analyses we have run for you:

cd ~/fsl_course_data/fmri2/3_levels

Take a quick look at one of the web reports within the run directories of the level_1/ directory.

Second-level analysis

We will now set up the second-level (i.e. within-subject) analysis. Open FEAT (Feat & [or Feat_gui & if on a mac]) and follow the instructions below:

If something has gone wrong with your analysis, or it is taking too long to run, there is a pre-baked version available: examples/level_2.gfeat. You can use this as the input to the third level analysis too if necessary.

This analysis should only take a couple of minutes to run. Wait for the result web pages and then view them carefully. Check that the registrations are accurate, and then take a look at the results.

Third-level analysis

We are now ready to set up the third-level (i.e. between-subject) analysis. This will be valid for one of the contrasts we passed up to the second level (but it easy to repeat the analysis for the others). We will use the semantic > lines results, which corresponds to contrast 1. Reopen FEAT (Feat &) and follow the instructions below:

You can look at examples/level_3.gfeat if necessary.

Again, this analysis should only take a couple of minutes to run. Wait for the result web pages and look at the results. Do they look plausible?


The End.