Hi
On 19 Jun 2007, at 21:50, Xuelin Cui wrote:
> Dear folk:
>
> I have some questions about the PCA plot in the MELODIC results
> report. In the MELODIC report folder, there is a html file showing
> the ICA analysis results. If opening the html file, one will see a
> PCA plot report with 3 curves(red, green blue) on it.
> First of all, is the x-axis and y-axis in the plot means the number
> of eigenvalues and the value of eigenvalues respectively?
Yes, though the x-axis will typically only be plotted up to the
number of Eigenvalues which jointly explain the firs 99% of the
veriation, i.e. your experiment might be 100 TRs long but the x-axis
might only go from 1-56, say, because the final 44 Eigenvalues are
<1% of the vraiance.
> Also, as the plot says, the red curve means dimention estimate and
> the green one means percentage of variance. I am just confused on
> how to understanding the red and green curves in the plot.
The y-scale is only meaningful for the green line - the other lines
(the Eigenspectrum itself and the dim estimat) have been normalised
to be in the range 0-1. It is plotted from 0-1.05 in order to avoid
clipping of the curves at the top. The estimated dimensionality is
the maximum of the red curve. From this point onwards the blue line
(the Eigenspectrum) is not significantly different from the
theoretical Eigenspectrum of random noise. The green curve simply is
the sum of the leading Eigenvalues divided by their total sum, i.e.
is the proportion of explained variance for any given dimension.
cheers
christian
> For example: the maximum y-value on my PCA plot is 1.05, but my
> projected subspace is 42-dimentional. Then, how should I explain
> the red curve in the plot, as well as the green one?
>
> Thanks alot
>
> Xuelin
> ****************************************
> Xuelin Cui
> Department of Electrical Engineering
> University of Hawaii-Manoa
> Honolulu HI 96822
>
> Tel: 1-808-349-0983
> Email: [log in to unmask]
> ****************************************
____
Christian F. Beckmann
University Research Lecturer
Oxford University Centre for Functional MRI of the Brain (FMRIB)
John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK.
[log in to unmask] http://www.fmrib.ox.ac.uk/~beckmann
tel: +44 1865 222551 fax: +44 1865 222717
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