Coherence of random data

[Laurens Hogeweg]

Hello,

When I pass random data in FieldTrip and calculate coherence I get some
problematic results. I shall try to explain what I did. I used the trial
definition of a real experiment and just replaced the segmented data in the
trial structure with this script (and checked with a plot if the data was
really random):

for trial=1:210
        data.trial{trial} = randn(1001,24)'; %pass random data for 24
channels
        randn('state',sum(100*clock)); %reset randon number generator state
End

The data structure passed was:

data =

      trial: {1x210 cell}
      label: {24x1 cell}
       time: {1x210 cell}
    fsample: 2000
        cfg: [1x1 struct]

Then I calculated spectra with the following parameters:

freqcfg =

        output: 'powandcsd'
        method: 'mtmfft'
        foilim: [6 100]
     tapsmofrq: 4
    channelcmb: {'all'  'FDI'}
           pad: 'maxperlen'
       channel: 'all'

And at last coherence with no further configuration. Confidence limit of
coherence = 0.0218. Nevertheless I get coherences with a mean of about 0.05
in the whole frequency range I calculated (6-100) but with peaks to even
0.16 (!).
Actually I did this because I got much higher coherences with FieldTrip than
with Neurospec (www.neurospec.org, David Halliday), with exactly the same
data before frequency and coherence analysis was started.

I'd appreciate some help in figuring out what's the difference between
FieldTrip's coherence method and Neurospec's (maybe I'm just doing something
bluntly wrong).

Lauerns Hogeweg