# [FieldTrip] Issues with ft_math function

Marion Vincent marion.vincent at univ-lille.fr
Mon Jan 20 17:02:01 CET 2020

```Dear Fieldtrip users,

I'm runing into soe issues when using the ft_math function.

%% ** Remove baseline
active_LF_corrected = cell(1,4);
active_HF_corrected = cell(1,4);
for cond=numCond
cfg = [];
cfg.parameter = 'powspctrm';
%cfg.operation = 'x1./x2 - 1'; % this will express the power as a relative change, i.e. a percentage deviating from 0.
cfg.operation = '10*log10(x1./x2)';
active_LF_corrected{cond-4} = ft_math(cfg, active_LF{cond-4}, baseline_LF{cond-4});
active_HF_corrected{cond-4} = ft_math(cfg, active_HF{cond-4}, baseline_HF{cond-4});
end

I try to remove a baseline (baseline_LF) from my trial (active_LF), in the time frequency domain.
When I proces my data, for certain active and baseline matrices, the script works (see below).
However in some cases, my active_LF_corrected ends up empty for the frequencies, and thus for the power spectrum.
For example, I get the following structure:

active_LF_corrected{1}=
label: {128×1 cell}
freq: [1×0double]
time: [1×201 double]
powspctrm: [128×0×201 double]
cfg: [1×1 struct]
dimord: 'chan_freq_time'

And the saved matrix is :

active_LF_corrected{1}=
label: {128×1 cell}
freq: []
time: [1×201 double]
powspctrm: []
cfg: [1×1 struct]
dimord: 'chan_freq_time'

I've checked the active_LF and baseline_LF structures, and they are ok: the freq dimension is [1x15 double] as it should be (and so is the powspctrm variable) .

Do you have any idea why the freq and powspctrm are empty in some cases only ? Where could it comes from ?

Thanks,
Marion VINCENT

```