[FieldTrip] Kurtosis topography at channel level

[Rui Li]

Dear FieldTrip users,

Recently, I am working on the case 1 dataset of epilepsy tutorial. The
first patient got the MEG recording from Neuromag and CTF, respectively. I
encountered some problems when I tried to depict the Kurtosis topography
at channel - level.


*Question 1:* why and how does the filter affect the kurtosis?

Figure 1 and Figure 2 are kurtosis topographies at channel level without
band pass filter and with band pass filter, respectively. As we can see,
these two figures are very different. Therefore, I am wondering why and how
does the frequency filter affect the kurtosis?

The figure 1 is generated by the following program;

%% preporcessing the channel level data

dataset = 'case1.ds';

cfg = [];

cfg.dataset   = dataset;

% cfg.hpfilter  = 'yes';

% cfg.hpfreq    = 10;

% cfg.lpfilter  = 'yes';

% cfg.lpfreq    = 70;

cfg.channel   = {'MEG'};

data = ft_preprocessing(cfg);



%% compute channel-level kurtosis



datak = [];

datak.label    = data.label;

datak.dimord   = 'chan';

datak.kurtosis = kurtosis(data.trial{1}')';



cfg = [];

cfg.comment = 'computed channel-level kurtosis';

datak = ft_annotate(cfg, datak);



%% plot kurtosis topography at channel-level



cfg = [];

cfg.layout    = 'CTF275.lay';

cfg.parameter = 'kurtosis';



figure;

ft_topoplotER(cfg, datak);


​

Figure 1 kurtosis topography + no filter + case1.ds

If the band pass filter is included in the pre-processing, the kurtosis
topography is figure 2; the pre-processing matlab program is

%% preporcessing the channel level data

dataset = 'case1.ds';

cfg = [];

cfg.dataset   = dataset;

cfg.hpfilter  = 'yes';

cfg.hpfreq    = 10;

cfg.lpfilter  = 'yes';

cfg.lpfreq    = 70;

cfg.channel   = {'MEG'};

data = ft_preprocessing(cfg);



Figure 2 kurtosis topography +  [10Hz 70Hz] filter + case1.ds

*Question 2:* Why the kurtosis topographies at channel level from two
dataset case1.ds (figure 1 (no filter), figure 2 (filtered)) and
case1_cHPI_raw_trans_sss.fif (figure 3 (no filter) and figure 4 (filtered))
are not consistent? Because these two datasets are measured from the same
patient, the kurtosis topography should be consistent. Do you have any idea
about this?


​

Figure 3 kurtosis topography + no filter + case1_cHPI_raw_trans_sss.fif


​

Figure 4 kurtosis topography + [10Hz 70Hz] filter +
case1_cHPI_raw_trans_sss.fif

Regards,

Rui.
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