I'm still waiting to see if Google/Blogger might recover the draft of that fourth post in the series on background physics for fMRI artifact recognition, so in the mean time I thought I'd take a closer look at the only part of the resting-state experiment that I didn't address in detail over the past few months: what we allow the subjects to do during the acquisition.
I'd like to begin with a definition change to assist in understanding the limits of "resting state" fMRI. I'll continue to refer to rs-fMRI as the act of acquiring a block of fMRI data - let's say six minutes' worth - in the absence of any specific externally presented task during the acquisition, with one small exception: we'll assume that the subject is presented with a simple fixation cross and is asked to keep his eyes open. (More on visual and auditory effects on rs-fMRI below.)
Now, though, I'd like to rename the mental activity that is happening during the rs-fMRI acquisition period. The definition of "rest" is tricky because it depends on so many state-dependent factors. What if I'm worried about an upcoming exam? What if I'm hungry and distracted by the need for food? Just because I'm awake during both doesn't make them equivalent periods of "rest," even if I am lying in the scanner staring at a fixation cross in both cases. And, because I want to distinguish between such periods without an explicit task in the discussion to follow, I'm going to term what we do today as "free-thinking state" fMRI, a term used by Cindy Lustig from WashU in a 2003 interview about some of her work.
Okay, so now we have a new definition to work with. How might this state of free thinking be manipulated without fundamentally changing the goal, which is (I assume) to map the largest networks that arise intrinsically, in the absence of explicit, externally driven, goal-directed behavior (in the form of some sort of task presentation)?
Caveats: every fMRI experiment should have some!
Before we look at intentional manipulation of a subject's brain state, let's first review the confounds and limitations that have already been unearthed when it comes to conducting free-thinking state fMRI experiments. These are effects that have been shown to change the networks detected in standard rs-fMRI experiments. (See Note 1.)