[Eeglablist] Problem with base line in time-frequency

[Laszlo Balazs]

Hi,
I am less then a newbie in timefreq analysis but still let me ask again. 
Does the trimmed off section necessarily have to be a real baseline or 
could it be a part of the previous trial? This way Johan could have 
overlapping epochs from -1500 to 2500 and could analyze -1000 to 2000. 
(Assuming cca 3Hz low freq and 3 cycle wavelet.) In other words could 
the activity from -1500 to -1000 influence the output after -1000 due to 
convolution with the wavelets?
Best,
Laszlo

\Johan.poncelet wrote:
> Thanks Brian for your proposition but my protocol dont permit me to try 
> with a bigger epoch than -1000 to 2000.
>
> Brian Roach a écrit :
>   
>> In addition to what has already been covered, you should know that you 
>> will need longer epochs to use -1000 to -500ms as your baseline.  
>> Depending on what you decide to examine as the lowest frequency, that 
>> will determine the necessary epoch length.  From the -126 to 1200ms 
>> window you describe, I believe you need to have epochs starting around 
>> -2000 or -1900
>> At 07:25 AM 8/7/2007, Arnaud Delorme wrote:
>>
>>     
>>>> I have epoched my data from -1000 ms to +2000ms in order to have a 
>>>>         
>>> very
>>>       
>>>> large one.
>>>> When I apply the time frequency function (study, compute ERSP), I 
>>>>         
>>> obtain
>>>       
>>>> a time freq representation from -126ms to 1200ms maxi.
>>>>
>>>> Why?
>>>>
>>>>         
>>> This is because the time-frequency decomposition trim the time window.
>>> If your lower frequency is 3 Hz and you have a 3-cycle wavelet (at 3
>>> Hertz) for instance, this wavelet duration is 1000 ms (1 s). This means
>>> that 500 ms will be trimmed on each side.
>>>
>>>       
>>>> Now i would like to do different ERSP. I would like that the [-1000,
>>>> -500] period to be my baseline. And then have a representation from 
>>>>         
>>> -500
>>>       
>>>> to +1500.  Is it possible? I have tried with the option 'baseline' in
>>>> the ERSP box but EEGlab give me error message.
>>>>
>>>>         
>>> The baseline cannot be entered yet through this interface but we have
>>> entered an bug repport for enhancement so this should be possible soon.
>>>
>>> Best regards,
>>>
>>> Arno
>>>
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>>     
>
>
>   


-- 
      Laszlo Balazs, Ph.D.          /      dr. Balázs László
 Institute for Psychology HAS       /   MTA Pszichológiai Kutatóitézet
                 P O B 398, Budapest, Hungary, H-1394
                Tel:+36(1)354-2410 | Fax:+36(1)354-2416 
                       www.cogpsyphy.hu/balazs