fMRI experiment design help

Hello everyone,

I’m designing a BLOCK-designed FMRI experiment. but I’m running into some tricky issues.

I want to record the time to execute or imagine a finger-tapping task, but the duration varies per trial.

The experiment is a two within-individual factors design. Total of four conditions (The duration of the task varies for different conditions, exp. the duration of condition one maybe 2±1s, …, condition four maybe 30±5s，). I set up 30 trials for each condition, so how should I proceed with my experiment?

• assumption one is 30 trials for condition one, 30 trials for condition two, then three and four, each condition was balanced between subjects using the Latin square. So there is only one block per condition. The problem is that the block duration of condition one is normal, but the block duration of condition four is too long even more than 1,000 seconds. There must be something wrong here.

• assumption two is each block contains less trials. Each block has a fixed number of trials, maybe 10 trials of each? Although this will result in the durations of block between different condition not lasting the same length of time. Or 15 trials per block of condition one because of shorter duration, 5 trials per block of condition four because of longer duration. This way the number of blocks per condition is different, how to control each block is randomly presented. If the latter is better, how to properly set the duration of block? Is there some empirical criteria for this?

Finally, In the block design, do the intervals between trials need to be randomized? And how long is the rest set between block and block?

I am very appreciate that if there have any answers！

Yang

Maybe the best advice I can give is to maintain block duration short enough (say <=10s). You’ll have trouble measuring meaningful BOLD signal changes for long durations.
Then assuming that you have enough trials for each condition (30 is fine), I think that both solutions are acceptable. I would advise to use sol. 1.
HTH,
Bertrand