Free will/accuracy of perception
It is important to have accurate input in order to be able to fully exercise free will. If an actor's perception of reality is distorted, it may cause the actor to react in ways the actor would not have intended, violating the intent of free will (being able to act in ways that are optimized to meet one's goals); if one's perception of reality is being deliberately manipulated, then others may be able to use those distortions to control one's actions -- a clearer violation of free will.
There is some grey area here, since nobody has perfect perception or a completely accurate model of reality; we all must model reality through senses that are both limited and distorted in various ways, using processing that is capable of serious error.
However, if our perceptive ability as an individual is not significantly worse than average. we have a lot of cultural background on which to draw in order to make mental corrections for the inevitable errors we make. "Nobody is perfect" is something we learn as children; as adults, we may learn about various aspects of the scientific method which are designed to further overcome bias and yield still more accurate models of reality. For people with no unusual perceptive or cognitive defects, then, the constraints imposed by our imperfect perception of reality are just part of life that everyone has to deal with. Our "free will" may be constrained somewhat by them, but no more than it is constrained by our inability to grow wings or to breathe water. Some distortion of reality is a known constraint.
On the other hand, if an individual has some unusual cognitive defect of which they are unaware, they would have no reason to realize that they would need to compensate -- to adjust their thinking whenever this particular deficit might have had some influence -- more than most people do in that particular area. In this kind of circumstance, then, an actor is dealing with an invisible constraint which may well be depriving them of the exercise of some significant portion of their free will. If you see the train coming, you can use your free will to get out of the way (or not) -- but if you hallucinate that the train is your best friend, you are not exercising free will if you (quite reasonably) stay in place with arms extended hoping to receive a hug.