Often I receive emails, messages, or even some comments on posts challenging the non-veridical nature of visual perception. Many tend to instinctively believe that their eyes are direct windows to the physical world and find some interesting arguments to debate this point. The fact is though that even at the level of the retina, where indeed some camera-like optical principles are in play, we do not find a veridical window to the physical world. Rather, the patterns of light that are projected onto the retina are inverted, reversed, subject to significant changes in retinal receptive fields (fovea to periphery), fail to be read within a significant blind-spot, must pass through several layers of intricate neural circuitry before arriving at appropriate photoreceptors, and then subject to the shadows of the blood vessels that nourish all of that circuitry. …and all of this, again, comes before we even leave the level of the retina. (Note: the image of the apple on the right in the attached graphic is a very stylized glimpse at some of these dynamics in play at the level of the retina.)
While many are familiar with the inversions, reversals and even the blind spot—many are quite surprised at how “salient” the aforementioned blood vessels actually are. Well, here is a really cool experiment you can do quite intricately to perceive this intricate web. As shown on the attached graphic, place a tiny hole in the center of an index card or other opaque piece of paper. Close one eye, and closely look through the hole at a plain (homogeneous) brightly illuminated, or light surface. (The card should be up to your eye). Carefully jitter the card horizontally or vertically and, almost immediately, you will begin to see a grayish web of blood vessels appear. It’s pretty incredible. Give it a try and let me know if this works for you!
The reason that we don’t perceive these vessels all the time is because of a very valuable mechanism called neural adaptation (i.e., if a sensory stimulus is unchanging, we tend to stop “processing” it—like the way you don’t feel the clothes on your body after a bit.) The hole in the index card changes the way that light is entering the eye and thus begins to change the way the blood vessels cast shadows onto the retina. This allows us to perceive them for a short time.
Keep in mind that the term “accurate” in regards to visual perception can be misleading when used to to communicate something “veridical”—rather, accurate in these contexts tends to describe success in yielding appropriate responses to available visual input. In any case I hope that this is insightful for those of you still kicking this subject around.