[Eeglablist] crossf, timef

[Scott Makeig]

Rodrigo Pereira wrote:

> Hi all,
>
> As a begginer I have some basic questions,
>
> Using the crossf function, what does it mean when the blue line (in 
> the left pannel of upper image) is below the threshold for 
> significance (p=0.01; black-green line) but the image shows a red 
> patch of high coherence at same frequency?

The blue line averages over all times - thus it may appear 
non-significant even when some time windows are significant.

> Using the timef function, the ITC image (lower image) sometimes shows 
> red/yellow and blue patches, however the colorbar goes from green (0 
> ITC) to red (high ITC). What does the blue stand for? Below zero 
> coherences?!

This is an option ('plotphase','on') that shows the value of the ITC as 
well as the *sign* of the ITC phase - this allows counting of 
half-cycles in the significant area. Also, the sign of the ITC phase 
gives the sign of its contribution to the ERP. The negative color range 
should be documented in the color bar, however - we will see about 
adding that in the next version -- or, if anyone else has done it?? ...

You can always turn this open off - though once understood, I find it 
informative.

> I have 200 ms of pre-stim (-200) and 3000 ms post-stim (+3000) 
> recordings but my graphs only show from +120 to +3000 ms. [400 Hz 
> sampling rate; 20 epochs] Does it have to do with the FFT window size?

Yes - the results reported for an FFT window are refered to the middle 
of that window. So (both) ends of the epoch are shrunk.

> thanks a lot for any suggestions,
> Rodrigo

Scott Makeig