[FieldTrip] disconnected cluster

[Roy Cox]

hi all,

I'm worried that something is not quite right with how the channel
neighborhood configuration is used in the formation of spatial clusters.

I first noticed it when I got a significant cluster like below. Plotted are
t statistics for a between-group comparison, with the significant "cluster"
indicated with white electrodes. Notice F5 and P8 here which are
disconnected from the rest. I should also mention that I have only one
"time point" (and no frequencies), so it's not possible that these channels
are somehow connected via a "time bridge" (if that would even be
possible).[image:
Inline image 3]

I've inspected my channel layout and that looks good:

[image: Inline image 4]
I've looked at my neighborhood plot and that looks good too, although I
notice that frontal channels are plotted to the right here (not sure if
that's simply by default):

[image: Inline image 5]

Looking into the disconnected channel F5, the neighborhood structure seems
in order:

*cfg.neighbours(15)*

ans =

          label: 'F5'
    neighblabel: {'Af7'  'F3'  'F7'  'Fc5'}


I call ft_timelockstatistics, which then calls ft_statistics_montecarlo,
which, on line 165 calls

*cfg.connectivity = channelconnectivity(cfg);*

Stepping into this function, on line 20 it says:

*chans=cfg.channel;*

Checking what this variable contains, it has all my channels in *alphabetical
order*, but my electrode location order is most definitely not. I've tried
to force the orders to be the same by setting cfg.channel = cfg.label
before calling ft_timelockstatistics, but somewhere down the line my
channel order is made alphabetical again.

So I suspect that the neighborhood structure, reflecting my actual channel
order, is applied to the alphabetical channel order. That would explain why
a cluster could be scattered across the brain (and also why there's still
some spatial continuity given that alphabetically close channels are
usually close together in space).

Has anyone ever seen this? Any suggestions how to solve this (other than
making my entire channel location file alphabetical)? I'm also posting this
because it may be of relevance to others.

Roy
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