Its hard to say specifically what your instructor means by "after-image."
It could be that your teacher holds to a "tabula rosa" view of the human mind that argues against any "archetypes" or "ideals" that are universally human. His view may be that our mind only regurgitates what it observes in physical reality. The problem is, there is a "gap" to be crossed when dealing with material reality. In other words, your professor might be suggesting that I never really interact directly with reality but only with my perception of it. This perception is not, in a sense, even first hand knowledge because I am not really responding to reality but rather I am responding to the "after-image" that is left in my brain after the "gap" of perception occurs. For example, we perceive the universe as four dimensional and solid, but some would suggest that reality is far different, that really the universe is holographic.
Unfortunately, it is hard to say exactly what he means. If he is at all approachable, the best way is to find out is to try and ask him directly.