Graphical conjunction techniques can be used to support group level inferences concerning the number of participants in an experiment who exhibited activity above some specified critical threshold. Using this approach the critical threshold is applied to the individual subject maps, resulting in a group of binary images that can then be summed. The summed image is then overlayed on an anatomical template, allowing convenient visualization of how many participants showed the effects of interest at each brain location. The figure below shows a graphical group conjunction for a gestural imitation task with a critical t threshold of 4.0 in a sample of 16 participants. The color scale at the top of the figure shows the number of participants who exceeded the threshold at each brain location.

We demonstrate the procedure for constructing graphical group conjunction images using MRIcron.

1. Generate the individual participant t-statistic images in MNI space.

Begin by using SPM to preprocess the functional imaging time series and then compute the first-level statistical model for each participant. This will result in the generation of a set of spmT images for each specified contrast. Examine the resulting spmT images to determine a critical t threshold for the group conjunction. The appropriate t threshold will vary depending on a number of factors, including the experimental effect size, the length of the time series and the general quality of the data. For most experimental designs, a value between 2.0 and 6.0 will be adequate. After determining a usable t-statistic threshold, copy the spmT images from each participant into a group analysis directory.

2. Threshold the individual t-statistic images and convert them to binary masks.

Now start MRIcron and convert each of the spmT images to binary images by repeating the following steps for each participant:

File->Open->'spmT_???'

Change the display lower value to the critical t threshold

Change the display upper value to twice the lower value

Draw->Advanced->Brain Mask

3. Generate a group image by summing the individual binary masks.

Draw->Statistics->Create overlap images

4. Overlay the resulting group conjunction image on an anatomical template.

File->Open templates->ch2bet.nii.gz

Overlay->Add->'name of image generated in Step 3'

Change the display lower value to 1

Change the display upper value to the number of participants in the study

Select the 'x_rain' LUT from the pull-down menu

Following this procedure should result in the sort of group conjunction image shown below. The color scale at the top of the center image shows the mapping between color and the number of subjects who exceeded the critical threshold at each brain location.

It is possible to interrogate the results volume by moving the cursor to a desired position and then pressing the left mouse button. This causes the MNI coordinates, grayscale value for the template image and the number of participants having suprathreshold activity to appear In the lower left of the display. In this example the cursor was positioned over primary motor cortex at MNI coordinates -37, -28, +57 and 14 of the 16 participants in the group had suprathreshold activity at that location.

There are a number of alternative approaches to conjunction analysis using statistical modeling, rather than the sort of graphical overlay approach described above. The papers describing the various SPM implementations can be found here.

Zeffiro 18AUG09