These days it is not hard to come across a variety of shocking images and I can imagine that exposure to those could be correlated with the sensation seeking thing, making the images much more ordinary and leaving other considerations as stronger effects.

For some of us, the images themselves would be quite disturbing; just the description of them (that I have heard before) was enough to provoke a fairly strong reaction for me. I might have a somewhat unusually strong reaction, but I would have guessed that it was a fairly small percentage of people who would have little or no reaction. OTOH, a lot of things would make more sense if that percentage was quite a bit higher than I would think.

Also, fMRI descriptions always make it sound like the brain is at rest until it is being activated, but that is not how it works. Obviously the imaging tries to take into account how it actually works, but there can still be a number of issues. I don't know enough to have any idea how likely it is that there are technical issues here, but functional imaging is an area where major issues have gone undetected for surprisingly long periods of time so it should always be considered a plausable theory that the imaging isn't actually showing what we would like it to be showing.