Tutorial 10: Statistics

The Statistics tab works the same way as the Processes tab, except that you have to define two sets of files instead of one. Its objective is to compare conditions / files / subjects. This tool will be presented very briefly now, because the TutorialCTF protocol do not contain enough data to perform any interesting statistical test.

You can realize three kinds of operations with the Statistics tab:

• Binary processes: Difference or average file by file.

• Student's t-test: Get the signicant differences between two conditions.

  • Within one subject: possible if you have many trials for both conditions
  • Across subjects: if you have enough subjects and the everage response for both condition and all subjects.

• Permutation tests

Binary processes

• You are going to compute the difference between the recordings of the two conditions Left and Right. This has absolutely no scientific interest, it is purely to show how to use the interface.

• Files selection:

  • Drag'n'drop the StimRightThumb condition in "Samples A" list, and the StimLeftThumb condition in "Samples B" list.

  • Select the "recordings" button.
    

• Process selection:

  • Click on Run.
  • Select "A - B" in the list, to compute a "file by file" difference between files A and files B.

  • Click on Run.
    

• Results: there where 2 x 2 files in input, there are 2 files in output, in a newly created condition "StimRightThumb - StimLeftThumb".
  

• Warning: Same problem as illustrated in the previous tutorial.

  • We should not have done this substration because it combines recordings that were acquired in two different runs, with different head positions.
  • The runs should have been registered together to have the same head position.

  • Brainstorm created a channel file in the new condition by averaging the positions from the channel files of the two initial condition. Remember that this is not wrong, because the head moved between the two recording sessions.

  • If you want to avoid the problems related with the position of the head: work in source space only.

Statistical tests

We do not have enough data to illustrate those tests, it will be done soon in another tutorial, based on single trials instead of average recordings.

Its works exactly the same way with many files or many subjects: put all the files from condition A in the first list, and all the files from condition B in second list, no matter if they belong to the same subject or not.

In the multiple subjects case, the exploration mode "Functional data (by condition)" is usually useful. To switch to this mode, click on the third button over the database tree in Brainstorm main window.

Next

Last important topic:

?time-frequency.