[Eeglablist] Regarding .fdt files

[Delorme, Arnaud]

Dear Gladys,

> A couple of weeks back, I added and updated some plugins to EEGlab. It seems like after this, the .fdt files are not automatically generated after I save a file – it only saves a .set file. And I was wondering if this is usual, or there might be some issues that I should look into?

EEGLAB datasets can be saved as one file (.set file which is simply a Matlab file with a the extension changed) that embeds the data, or the raw data can be saved as .fdt (raw binary floating point precision file) and a small .set file containing the metadata. The data is the same. The reason for doing so was/is disk access when processing groups of files. It is faster to have 2 files, because the metadata can be loaded faster.

Which plugin do you think modify EEGLAB behavior. The behavior can be changed using menu item EEGLAB > Preferences (or in your version EEGLAB > Memory options).

> In addition, runica also gives a different message. Before the additions/updates to the plugins, runica has been returning the same number of components as per channels (it shows a message: “Fixing rank computation inconsistency (60 vs 59) most likely because running under Linux 64-bit Matlab”).
> 
> However, runica now shows this pop-up “attempting to convert data matrix to double precision for more accurate ICA results. Data rank (59) is smaller than no. of channels (60).” and eventually computes 59 ICs. My preprocessing steps before ICA remained similar, and hence I was wondering why this message appears now?

If you compute average reference, then the rank of the data is being decreased by 1 (so 59 instead of 60). If you do not compute average reference, and the reference is not included in the list of channels, then the rank does not need to be decreased. Rank is a hard thing to calculate and there are sometimes numerical instabilities, hence the message you are seeing.

Best wishes,

Arno