I have been told that the only truly instinctive responses are to the groin and eyes (flinching).
Also, when you watch Ueshiba in demo's he very often points with his hand or flicks his hand around before the attack. I'm not sure if he is actually saying what kind of attack he wants in mnay cases, but in some situations it seems more like a leading or distracting action.
Notice the difference in any blending exercises or techniques between grabbing nage softly or hard - with a hard grab nage often directs their focus to the hand and therefore has difficulty blending effectively.
I think mis-direction is a major part of aikido. Since it is (mostly) non-competitive, most people who attack will not have a clue what you are doing (just as most people can't detect pick-pockets).
This is to me a major part of aikido which is not fully explored.
(P.S. the techique where Ueshiba bends down in front of the attacker into a very prone position - notice he always raises himself up first, to raise uke onto their toes).