Hi Anna,
we used 6 secs video mini-blocks for PLoS ONE 7(11): e49451.
doi:10.1371/journal.pone.0049451 .
We’ve had two experimental factors that were manipulated.
Our choice for video transitions of 6 secs duration was based on initial
tests revealing that this seemed to evoke the „strongest“ cognitive
response. 3 secs was too fast and subjects would be stressed out by
viewing the vids, 10 secs was too slow and people would loose interest /
attention.
The 15-20 secs optimal block length doctrine is based on the HRF but if 6
secs is best to evoke the best response / compliance I would go for that.
Depending on what you are after precisely, you may (randomly) alternate
between experimental and control videos or assemble these into longer
blocks.
In any case, I don’t think that your second option per se poses a problem
- as long as you can model (or in one way or the other identify) what you
are after.
Cheers,
Andreas
Von: FSL - FMRIB's Software Library <[log in to unmask]> on behalf of
Anna van 't Veer <[log in to unmask]>
Antworten an: FSL - FMRIB's Software Library <[log in to unmask]>
Datum: Mittwoch, 5. Juli 2017 um 15:27
An: <[log in to unmask]>
Betreff: Re: [FSL] optimal length of block/event (using video stimuli)
Hi Chirag and other fsl users,
Thanks so much for your thorough reply. We have some follow-up questions.
Our goal is to find a difference in the overall response to experimental
vs control videos (we are not interested in estimating shapes for
individual videos per se). We could go with several options, which would
be the most powerful?
1. we could compile several videos to make a block length of 18 seconds
(so three videos), or 24, or 30 but we would like to stay on the short
side in order to prevent habituation. This would be like a traditional
block design that generally has greater power than an event-related design
(although we still would get habituation because the experimental and
control videos are not presented randomly, thus creating predictability).
2. we could present the videos individually, this could be seen as a short
6 second block or a long event, however as we are not interested in
estimating the shape of the HRF response, it may not be a problem that our
trials are somewhat longer. And by presenting the videos this way we could
randomise their presentation and add jitter, adding to power.
3. we could edit out videos to be 4 seconds, and make it an event-related
design (again with randomisation and jitter).
We would love to hear from anybody who has thoughts on this (does anybody
have experience with video stimuli lasting 4 or 6 seconds?),
Greatly appreciate it,
Thanks,
Dr. Anna van 't Veer
\ Child and Family Studies
\ Leiden Institute for Brain and Cognition (LIBC)
\ Social and Behavioural Sciences
\ Leiden University
On 29 Jun 2017, at 18:59, Chirag Limbachia <[log in to unmask]>
wrote:
Hi Anna,
For block design, I would suggest you to keep the epoch lengths at least
20 secs long. In your case, 30 secs will be best as 6*5 = 30 sec. The
epoch length for a block design is usually 20-60 secs. Also, to get
highest optimality for your block design, mask sure you distribute your
task epochs and rest epochs evenly throughout the fMRI run. Best way to do
this is to keep 30 secs for task and 30 secs for rest (or a different
task).
For event-related design, it is advisable to have trails (events) no more
than 4 secs because longer trial duration give a block design like effect
to your event-related design. Your randomized ITIs look fine. Also, for
highest optimality of event-related design, make sure your task and rest
duration is evenly distributed for the entire length of the event-related
experiment.
To make a choice between an event-related and block design really depends
on how you plan to assess activation: by 1) estimating the overall
amplitude of the hemodynamic response to the task (rise of BOLD signal
from baseline) or by 2) estimating the shape of the hemodynamic response
to every individual trail?
For 1), block is more appropriate.
For 2), event-related is more appropriate.
Please feel free to ask any question you may have.
Thank you
Chirag
On Wed, Jun 28, 2017 at 9:34 AM, Anna van 't Veer <[log in to unmask]>
wrote:
Dear FSL users,
Could you kindly advise me on a question regarding the optimal length of
epochs in our task design?
We would like to design our task using (experimental and control) videos
that have a length of 6 seconds.
For block designs I can find documentation about optimal block lengths of
15-20 seconds, and for event related designs lengths seem to vary between
2 and 4 seconds. Our videos seem to fit in neither category.
Would you (A) advise to present our videos—comparable to an event related
design—as individual trials of 6 seconds (with truncated exponential ITI’s
with minimum, maximum and average ITI of 3, 8 and 4.5 respectively), or
(B) would you advise to present several videos of 6 seconds together,
comparable to a block design (if yes, how many videos in one block)?
Very curious to hear your thoughts!
Cheers,
Dr. Anna van 't Veer
\ Child and Family Studies
\ Leiden Institute for Brain and Cognition (LIBC)
\ Social and Behavioural Sciences
\ Leiden University
|