Dear Mike,
> We are intending to run a sparse imaging study but are unsure about how
> many volumes are necessary per condition. The figure we have heard is
> 25-30, but are unsure where this figure comes from and whether it is
> relevent for our experiment.
>
> We intend to auditorily present 3 or 4 words per 12 second TR and
> subjects will perform a non-word identification task at the end of the
> stimulus presentation part of the TR. We will be using a 3T magnet.
This is a difficult question because there is no definitive answer. By
'sparse imaging study' I assume you mean an event-related study with
sparse events. As you probably know this is the least efficient design
one has to contend with. Assuming that the task requirements enforce
this design I would note that an inter-trial interval of slightly more
than 12 seconds would be more efficient (ideally about 16 seconds
assuming a fairly short TR). Our experince is that results are
obtained with as few as 16 events (provided they elicit a reasonable
response), but generally more (e.g. 32) is better. At the end of the
day I think you just have to do the experiment and see if you get
results.
I hope this helps - Karl
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