Claudia Nohn NohnPlusUltra at GMX.DE
Thu Apr 20 13:04:46 CEST 2006

thanks a lot.

> --- Ursprüngliche Nachricht ---
> Von: Eric Maris <maris at NICI.RU.NL>
> Betreff: Re: [FIELDTRIP] clusterrandanalysis
> Datum: Thu, 20 Apr 2006 12:13:44 +0200
> Dear Claudia,
> > i just began learning fieldtrip and am momentarilly busy with
> > clusterrandanalysis. perhaps someone can answer some of my questions:
> > which hypothesis is actually tested by the t-tests concerning the
> > channel-time-frequency-triplets?
> Clusterrandanalysis does not test hypothesis at the level of
> channel-time-frequency triplets (also called "samples"). Instead, it tests
> the null hypothesis that the data in the two (or more) experimental
> conditions come from the same probability distribution. The
> sample-specific
> t-statistics are used to construct clusters, namely by thresholding them,
> followed by clustering on the basis of spatial, temporal, and spectral
> adjacency. From a theoretical perspective, the t-statistic is not
> important
> at all. One can also use the Honolulu-statistic (which does not exist) if
> there is a sensible way of thresholding it. Clusterrandanalysis controls
> the
> false alarm rate by calculating a randomization p-value for the maximum
> cluster-level statistic. No parametric sampling distribution (such as the
> T-distribution) is required to control the false alarm rate.
> and why are´nt all triplets included using
> > an alphtresh-value of 1.0?
> If you set alphathresh at 1.0, you want all samples to be joined in a
> single
> cluster, regardless of the size of the effect at the sample-level. This
> does
> not make sense. Leave alphathresh at its default value 0.05. If you are
> just
> starting with clusterrandanalysis, you will not gain anything by playing
> with it.
> > does neighbourdist mean: MAXIMAL distance from neighbours?
> Yes, neighbourdist is the maximal distance between two sensors that are
> considered as neighbours.
> > how do i know for which triplet one specific
> > p-value/cluster-level-statistic
> > etc. stands?
> p-values are computed for clusters, not for samples (unless you specify
> cfg.makeclusters='no').
> > are only significant effects plotted by topoplotTFR?
> The effects you plot by topoplotTFR depend on what you ask topoplotTFR to
> plot for you. This is specified by the cfg.zparam field. In the
> clusterrandanalysis tutorial on the Fieldtrip homepage, I used masking to
> select the significant clusters.
> > for a two-sided test: which direction do the negative or the positive
> > clusters have? is the power of the first ore the second data bigger or
> > smaller?
> A positive cluster means that the samples in this cluster have a larger
> value in condition 1 than in condition 2. For negative clusters, the
> reverse
> holds.
> > how are more than two conditions compared (i have two independent
> > variables,
> > each having two levels, and two groups who are sensitized at the left
> and
> > right side of the body, repectively. so i´m comparing up to eight
> > channel-time-frequency-arrays.)
> Clusterrandanalysis can only be used to test main effects.
> Clusterrandanalysis can perform sample-specific comparisons for more than
> two levels (eight, in your case) by means of an F-statistic. However, I
> strongly advise you to first analyze your data for every independent
> variable separately, each time ignoring the other two independent
> variables.
> This amounts to using an independent samples t-statistic (if the
> independent
> variable is between-subjects) or a paired samples t-statistic (if the
> independent variable is within-subjects).
> good luck,
> Eric Maris

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