Nardi's work does not map functions to the brain. It draws a correlation between MBTI types and regions on the brain that lit up when certain tasks were performed. There were overall patterns with MBTI types and lit patterns, but it was not an effort to decisively map each function to a pattern. Even if there was, it would not be "using" a function. It would be defaulting to one.
@Aquarian @JSRS01 and everyone else saying that you do "use" functions:
What, exactly, are you using?
Almost all of you have said in some way that you catch yourself "using" your dominant function, and that you have very little to no control over your preference stacking. If you want to think of that as "using" your dominant function even though you have little to no control over whether or not you can "use" it or not, understand that to "use" something still implies that you have the choice as to whether or not you want to do so in a given situation.
I can pick up a jackhammer right now and use it. I may not have an option as to whether or not I will use it, but I still pick it up and go. As you all have noted, you cannot "use" functions outside of your stack, but you do not have an option as to whether or not you "use" your dominant. That's because you are not using any functions to begin with. You are simply defaulting to them, to varying extents, which is the exact reason why they seem prominent or not - there's nothing that comes before the "use" to say "use this one." You simply do it.
To say that you use a function is to say that you have a choice as to whether or not it is part of your engagement with the outside, and as you have agreed, you do not have that choice. It makes no sense to say you "use" a function when you would default to the function you prefer regardless of how you have mapped which functions you "use."