@Logic: As I said, I also have books, including Jung's "Psychological Types" and "Gifts Differing" and other books. It's been seven books total. I've taken the "actual" indicator two times. It's not just a few resources on the web, there are numerous threads about this, where numerous opinions are mentioned.
Where do you get YOUR information?
The issue is one of choice and preference. Someone can prefer to be compassionate or impersonal in the very same hour.
When I think, it is deep inside where I plot product improvement, logistics, breeding, and quality. I like to quietly work out new systems alone. Okay, maybe that is "Te", but it is definitely thinking inside. I also spend quite a bit of time playing strategy games like chess or civilization. I took math which was not needed for my degree. I test as a weak (almost in the center) INTP the first time.
Later, when I was in a very religious period, involved in missions in graduate school, I took it again and got INFP. Ah, of course I can skew it either way.
In activities, I'm often quite impersonal, almost to the point of having aspergers. I don't look at the clerk at the store or make eye contact. Other times, I'm quite caring, and I've gone into prisons to counsel murders.
Fi, from my personal experience is visceral. It's like a light in the soul or an ocean. It involves the sensation of the body, mostly in the gut. Reasoning accompanies it or not. Much of my Fi "thought" is about images. I have pictographical language I use as shorthand, and these symbols are echoed in view of nature. However, it's not "logically" verified concepts, but accepted ones like feng shui or viewing time as a river. Sometimes I just luxuriate in feeling and the flow of images. Of course, it also involves a great deal of thought about people, and planning future conversations with them and laughing. Maybe thats Fe, but it is INTERNAL, INTROVERTED FEELING.
Ti, from my personal experience is like a fascination in the mind itself. It just about ignores the whole body in its purest form. This is how I may feel after reading something, like "Einstein's Universe" or some book. Like I said, I may spend long hours thinking about things. Once I managed data, and I'd plan macros and VBA programming for long time at night in my head. If that's Te, well, so be it. Te is just a label after all. IT IS INTERNAL, INTROVERTED THINKING.
I would define INFP as someone who uses Fi more than Ti, and the converse for someone who is INTP.
I like the analogy of left and right handed, which is used in some of the books and resources. This is probably a biological hard wiring you describe, and though I can develop it somewhat, writing with my left hand is basically difficult. Yet, there are true ambidextrous people. Science once denied the existence of left handed people, much less ambidextrous ones. Children were forced to write with their right hand.
Not in my school!
I don't disagree that things could be wired in Biological sense. I just think there are more options for wiring. Take computers. Some people have linux on their PC and some have Windows. But some have both, and they can switch back and forth. I'm not saying that is EXACTLY how it's wired in the brain; it's just an example.
Let's say it is biological. Based on your consideration, Fi and Ti users would not have an actual physical piece of their brain that the other person has. Or if they have a physical piece, there would be no wiring or connections to that piece. Ooooh, I'd love to discover that. But personally I think this is absurd. I propose that they both have a the SAME physical pieces in their brain, but the connections for their preference are thicker from use. I propose that they both can have the same wiring, but the lipids are built up on the most used function.
Which is more probable, the structure is missing, or the structure is less used? Think about it.
Since I believe Fi is visceral and often in the guts (or in "the heart"), those connections in the brain that monitor the physical aspect would be most used or illuminated.
Since Ti is more in logical reasoning, then I think those connections would be lit up.
I cannot...without PROFOUND EVIDENCE believe that Fi and Ti people have completely different pieces and wiring. In fact, I believe that studies would show they use the same circuits, and that in physical anatomy...biological hardwiring...there is no difference except to the extend that certain areas are used.
I was looking at National Geographic issue on the mind. It was showing that Buddhist meditation caused the illuminated area to move. I'd have to find the article, but the brain area for the far right would be harmony and happiness. Meditation would improve function in this area (Fi

) for ANYONE who used meditation.
Please, tell us your actual, physical, biological research!