Dear Xiuqin Jia,
> I have a question about what is the event referred to.
> For example, the whole trial goes as follows,
> At the beginning, a fixation appears; then the addition task (e.g., 3+5=?)
> appears and during this period subject can response as quickly as him
> produces the answer; finally an answer selection appears (e.g., A. 5; B. 6)
> and during this period subject should select one of the answers.
> During this trial, what is the full event? What is a specific point in the
> events? What is a subset of the events?
Perhaps the simplest way to think about an "event" is as "a discrete
period of neural activity". The exact terminology (full event vs.
specific point vs. subset) really doesn't matter; what is important as
far as determining an analysis is basically: (a) what is the neural
activity you are interested in identifying? (b) how can this be
accurately captured in a design matrix?
Intuitively, I might say that each trial as you've described contains
several events (time periods when I would anticipate different sorts
of cognitive operations going on):
1) Fixation
2) Addition task
3) Response
4) Selection task
It may be that you are just interested in one of these operations
(say, addition). In this case, the trick is to see whether the
addition task is sufficiently independent from the other tasks. If
they tend to occur with the same relative timing (e.g., the regressors
are highly correlated), it will be difficult to identify any unique
neural activity associated with one or the other of the tasks. You
can already get a sense of this by thinking about your design and
whether the timing varies at all across trial, but you can also look
at the design matrix (using the "review" button), and then select
design > design orthogonality. This will give you a sense of the
degree to which your regressors are correlated. There are a number of
previous posts on this, and how to adjust a design to de-correlate
portions of trials, in the archives.
Hope this helps!
Jonathan
--
Dr. Jonathan Peelle
Department of Neurology
University of Pennsylvania
3 West Gates
3400 Spruce Street
Philadelphia, PA 19104
USA
http://jonathanpeelle.net/
|