[FieldTrip] Grand mean

Sarah Kirchler sarah.kirchler at gmail.com
Thu May 14 09:32:33 CEST 2020

Dear Jan-Mathijs,

thank you for your answer. I see, i expressed myself not very clear.
I'd like to make the Grand mean for the difference and the sum between two
specific frequencies I measured visually wih a SSVEP.
I'd like to do this because I'm interested in how the frequencies are
processed in the brain regarding each other and a control measurement.
I'm not sure if I can make the difference already when I make the GM or
later, when I plot the topography.
I tried to make this by building a datastructure that contains the GM of
the one and the other frequency and then usw ft_math to compare them. But
this doesn't work for me.
Maybe thats enough information for you to understand the problem better,
I'm a beginner so I often explain my problems a little bit confusing, sorry
for that.



Am Mi., 13. Mai 2020 um 15:30 Uhr schrieb Schoffelen, J.M. (Jan Mathijs) <
jan.schoffelen at donders.ru.nl>:

> Dear Sarah,
> It is not clear what error message you get, nor why you think you should
> ft_timelockanalysis/ft_timelockgrandaverage/ft_topoplotER.
> Some pointers:
> -For a mathematical operation to be applied to numeric data that resides
> within a fieldtrip-style data structure you’d use ft_math.
> -What you want to do (i.e. adding or subtracting estimates of frequency
> specific power estimated for different frequency bins) probably requires
> some manual intervention with the data, because FT will likely prevent you
> to do so, since it checks whether the frequency bins for the to-be-combined
> data structures match. I could type some recipe now, but since you are not
> too clear on the details so I’d rather not spend time on that now. Also,
> I’d like to give other people the opportunity to chime in here :).
> Best wishes,
> Jan-Mathijs
> On 11 May 2020, at 16:43, sarah.kirchler at gmail.com wrote:
> Dear Fieldtrip Users,
> thank you for all the help you’ve given me already.
> Now I am trying to make the sum and the difference of the Grand mean
> from/between two frequencies I measured with an EEG.
> I read that it is possible to make a datastructure using {} within
> ft_timelockbaseline and then use the option cfg.operation(x1-x2 and so on)
> as a operation of function ft_topoplotER to make this sum/these differences.
> Now my Problem is that I don’t need to make the baselinecorrection via
> ft_timelockbaseline because I don’t have the baseline in the datasegment
> I’m interested in. So I tried to build the datastructure for cfg.operation
> within ft_timelockanalysis but Fieldtrip doesn’t allow this and I get an
> error message.
> My questions are now:
> 1. Can I make the difference also within the ft_timelockgrandavverage
> function or do I have to make the difference lawer in ft_topoplotER?
> 2. If so, how can I or form a datastructure I can use in cfg.operation or
> make the difference differently with some other function?
> I hope you understand me,
> to all you extraordinary brains: thank you in advance,
> Sarah
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