[Eeglablist] Forcing Double Precision in Makotos Preprocessing Pipeline

Malte Anders malteanders at gmail.com
Thu May 13 12:13:01 PDT 2021

Dear list,

Makoto states in his preprocessing pipeline that one should force EEGLAB to
use double set precision. I am wondering if this is a smart choice?

I am importing EEG files with a 24 bit resolution (I hope this is even
relevant info). I noticed that a 1hr EEG file with fs=512 Hz in the
manufacturers .hdf5 format is roughly 500 Mb and stays approximately that
size when importing it into EEGLAB with single set precision (the .set file
is also ~500 mb). Even when forcing double set precision, EEG.data is still
stored as a single after importing.

Only after performing an EEGLAB operation such as filtering or ASR,
EEG.data is then converted to "double". The .set file is then also double
the size (in this case, 1 Gb). In my opinion, this creates information out
of thin air, as when the original file was 32 bit (or 24 bit in this case),
filtering does not magically add more information to it that needs double
the disk space.
On top of that, it is mentioned in quite a few places that if double
precision is necessary for operations such as ICA, this is done
automatically in the process.

Thus, why should I perform the very first step in Makotos preprocessing
pipeline and change the options to force "double precision"? If my simple
thinking is correct, this only wastes space on the hard drive (in my case,
approx. 50 Gb). Are there any specific steps that really need this
checkbox? On top of that, the checkbox is even hidden in EEGLAB 2021 by
default so you really have to want to force EEGLAB to double set

Best wishes!

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