Ain't no such thing. The brain scientists know this, they've written about it, and the film people should stop copying-and-pasting the myth into their papers, blogs, articles and textbooks. It doesn't exist, it isn't important to film, and it is an unnecessary and incorrect start to a discussion of film ideas.
Don't trust me. Go read this complete article:
The Myth of Persistence of Vision Revisited
Several years ago we wrote an article entitled "The Myth of Persistence of Vision" which appeared in the Journal of the University Film Association in the fall of 1978 (Anderson and Fisher). In it we offered a considerable volume of evidence that the concept "persistence of vision" was an inaccurate and inadequate explanation of the apparent motion found in a motion picture. At the time we thought the article had laid the matter to rest. We had pronounced persistence of vision dead. And frankly, we expected never again to hear the term, other than in an historical context.
Now, more than a decade later, we are drawn once more to the myth of persistence of vision. Why? Because it is still with us.  We read a student paper, and we cringe. We attend the lecture of a seasoned film scholar, and we cringe. We cringe not only because they have chosen to perpetuate the notion of persistence of vision, but because they apparently, even at this late date, do not understand its implications. By this time most film scholars seem to have heard of the inadaquacy of the term "persistence of vision." Some have mistakenly substituted the generally misunderstood term "phi phenomenon" as an explanation of filmic motion, and many still cling to the myth.