We talked about eight principles of fMRI design,
including all of these factors here.
And now we're going to unpack each of these principles one by one and
explain it in a little bit more depth.
So first principle is sample size.
Number of subjects is usually the rate-limiting factor.
And so larger sample sizes increase power and often dramatically,
especially if your sample sizes are small, up to 30 or 40 participants.
Secondly, scan time.
More time on task is helpful.
That can mean scanning for longer within a person but
it can also mean maximizing the time in which you're driving up the signal
of interest for each task type or each progressor of interest.
And you have to also balance this against psychological effects of fatigue,
habituation, head movement, and other factors.
So, in terms of within-person scan time, maximize the time on task.
So, what we'll see now is a simple simulation in which we're generating event
related designs with evenly spaced events.
We'll go from very close together events to very far apart events.
So now events are very close together.
And now they're getting farther and farther apart.
Once every 20 seconds and now we go to once every 30 seconds.
As you'll see, as the design gets sparser,
then we've got more efficiency and there is an optimal level.
Now we'll do this again with longer duration blocks, five second blocks, and
now we'll do it again with ten second blocks.
So now we've got ten seconds on, ten seconds off.
And as you'll the see, the efficiency can reach an even higher level.