[Eeglablist] High pass (FIR) filter

[Andreas Widmann]

Hi Giorgia,

> I have pre-processed my data with EEGLAB version 13.5.4b in Matlab R2014a and used the default FIR filter to filter my data, setting the lower edge to 0.5 Hz. In Makoto’s preprocessing pipeline I have read that “1-Hz cutoff in EEGLAB is actually 1-Hz pass-band edge, and actually -6dB point, which is a standard definition of 'cut off frequency', is at 0.5 Hz.”
>  
> Apologies if these might seem silly questions:
> 1.       Does this mean that the ‘cut off frequency’ is at 0.5 Hz for a 1 Hz high pass?
Yes. Note, that this only applies to the „Basic FIR filter (new, default)“ (pop_eegfiltnew). In general it is more common to specify the cut-off frequency. The user interface was kept from the deprecated old version for backward compatibility.

> 2.       What would the ‘cut off frequency’ be for the 0.5 Hz high pass filter I used in EEGLAB?
0.25 Hz.

> 3.       And what value would it be correct to report in a publication?
„0.25 Hz (-6 dB cutoff, …) highpass Hamming windowed sinc FIR filter“

Note that the cutoff is not half the edge in general. The heuristic is described in the help text of pop_eegfiltnew. In a nutshell: Cutoff is
* half the passband edge for edge <= 2 Hz,
* passband edge minus 1 Hz for edge > 2 Hz and edge <= 8 Hz, and
* passband edge minus edge / 8 for edge > 8 Hz.

Hope this helps,
Andreas

>  
> Many thanks in advance.
>  
> Best wishes,
> Giorgia
>  
>  
> Giorgia Michelini
> PhD student
> MRC Social, Genetic and Developmental Psychiatry Centre
> Institute of Psychiatry, Psychology and Neuroscience, King's College London
> De Crespigny Park, London, SE5 8AF, UK
> Email: giorgia.michelini at kcl.ac.uk
>  
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