Help_about 'time-frequency represengtations of power'

[Weina Li]

hello!
   I am a new learner in MEG signal processing by using fieldtrip. And I 
need some help in understanding the tutorials of time-frequency 
represengtations of power.

  In the tutorials of calculating TFRs with frequency dependent window 
length,there is a sentence "The analysis is best done by first selecting 
the numbers of cycles per time window which will be the same for all 
frequencies"; and "When choosing this parameter it is important that a 
full number of cycles fit within the time-window for a given frequency".
---so what is 'cycles' mean? How can explain it? 

and "cfg.t_ftimwin    = 7./cfg.foi;  % 7 cycles per time window"
---then does the cycles has any relation to the frequency of interest?  
How should I select the number of cycles, is there a rule?


    Thanks!
           
                                                                           
                    Weina Li
                                                                           
                  Chongqing,China

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