[Eeglablist] about the evoked activity and induced activity

[Andrei Medvedev]

Hi All,

Let me suggest a different opinion.

First of all, evoked/induced activities are relative measures. There
is no reason to believe that RELATIVE changes in power at different
frequencies would obey the same 1/f law unless in a very special case.

Second, even if we consider absolute (not relative) power changes, it
seems to me that, in general, evoked/induced activity would not follow
the 1/f power law. Why? The 1/f law describes a relationship between
powers at different frequencies within the total spectral content of
EEG. To keep their obedience to this law, ALL frequencies should
experience SIMILAR changes after the stimulus i.e., to increase or
decrease proportionally. But this is obviously a rare case. If we
evoke/induce something in the brain by a stimulus, this something, for
example, can be a very narrow-band process i.e., an increase in power
at a particular frequency (say, 40 Hz) while all other frequencies
would not increase at all or even decrease (this happens quite often).
This means that the pre-stimulus 1/f power law would be broken after
the stimulus (for a relatively short time when we observe a RESPONSE).

Also, one should keep in mind that evoked/induced activities are not
stationary processes so any stimulus-related power modulations do
themselves change from one moment in post-stimulus time to another
(again, the concept of a RESPONSE constrained in time applies).

As for the second question, I believe the evoked change can
(theoretically!) be equal to the induced change but again, speaking
more generally, it's more likely they would change differently
depending on the underlying processes.

Best,
Andrei Medvedev
Georgetown University

On Fri, Sep 26, 2014 at 8:24 PM, Makoto Miyakoshi <mmiyakoshi at ucsd.edu> wrote:
> Dear Hui-bin,
>
>> Does the evoked activity and induced activity,which were calculated by
>> subtracting the ‍pre-stimulus power  from the post-stimulus power ‍at each
>> frequency, follow the same law?
>
> Generally speaking, yes.
>
>> Is it possible that  the magnitudes of ‍the evoked or induced activity  at
>> each frequency are equal?
>
> Yes, that is possible... I mean I can't think of any reason to exclude that
> possibility.
>
> I'm not sure if I'm answering to your question exactly.
>
> Makoto
>
> On Fri, Sep 19, 2014 at 9:12 PM, Hui-bin Jia <420247417 at qq.com> wrote:
>>
>> Hi,everyone
>>
>>         I have a question about the evoked and induced activity‍.
>>
>>         As we all know, spontaneous EEG activity follow " the 1/f law",
>> which means the spectral power of lower band is much larger than the
>> spectral power of the higher band. In rest EEG, alpha band power may be the
>> largest one.
>>
>>         Does the evoked activity and induced activity,which were
>> calculated by subtracting the ‍pre-stimulus power  from the post-stimulus
>> power ‍at each frequency, follow the same law? Is it possible that  the
>> magnitudes of ‍the evoked or induced activity  at each frequency are equal?
>>
>>         Any reply will be appreciated !‍
>>
>>
>> Yours,
>> Hui-bin jia
>>
>>
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>
>
>
> --
> Makoto Miyakoshi
> Swartz Center for Computational Neuroscience
> Institute for Neural Computation, University of California San Diego
>
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