Fill-in and perceptual completion In this series of visual perceptual exercises, we will briefly explore the phenomenon of fill-in and perceptual completion, first in the classic example of the blind spot, and then among aspects of visual processing dealing with surface property estimation (such as color and texture). Fill-in or perceptual completion is a general phenomenon whereby our brain seeks to automatically provide an apparently complete sensory experience, even if it does not have the sensory information to do so, i.e. gaps or holes present in the input are “filled-in”. This process hints at the notion that your perception of the sensory (outside) world is a construct or model that the brain creates, based on what input the brain gets from your senses, as well as its best guess based on experience. The model allows you to navigate and interact with the world. It doesn’t have holes, even though the input it was based on might and probably does have a whole lot of missing information. Consider that you can look and attend to a very small region of the sensory world at any given time and many things may be happening to alter that world or space without you knowing it. Yet you do not generally have a sense of the narrowness of information you truly have. Instead, once you “scope out” a room or a place, you have the sense of knowing what is where, even if you can really only confirm a very small portion of that model at any given point in time.

Nasal Temporal Left Human Retina The optic disk (optic nerve head) is about 12 degrees out (eccentric) nasal to the fovea and about 8 degrees in diameter. That is where you will find your blind spot (but of course temporal to your point of gaze, since your lens inverts the image).

Blind Spot (Optic Disk) Close your right eye and look directly at the number 3. Can you see the yellow spot in your peripheral vision? Now slowly move towards or away from the screen. At some point, the yellow spot will disappear. Note how far you are from the screen when the yellow spot vanishes. Repeat the experiment looking at a larger and then again at a smaller number. Did you notice the difference in distance from the video screen when the spot disappeared?

Blind Spot Explanation Form a triangle using the distance between the optic disc and the fovea as one side. You can see that as the distance between the yellow spot and the retina changes the angle (a) changes. It gets larger as the distance gets shorter. Consequently, the closer you are to the screen the nearer you need to fixate to the yellow spot in order to make it disappear. The angle (a) will be about 12 degrees when you have found your blind spot.

Yet another blind spot demo. Close your left eye and with a viewing distance of about 12 inches, start reading the letters with your right eye. Notice what happens to the black dot as you read.

The same demo but with a colored background. Notice that the blind spot fills in with the color of the background, not black or gray…

Now stare at the red dot with your right eye from 12 inches, covering your left eye with your left hand. Notice that the gap in the blue bar fills in (completes). Move your left hand to unblock your left eye and the gap re-appears.

Laser photocoagulation is a standard treatment for a variety of eye disorders and diseases, such as retinal detachment. Think about how many “blind spots” such patients have and what their visual experience is, i.e. just what do they see ? Do you think they see a lot of holes in visual world ? How might they know what has been done to their retina ?

Brightness fill-in. At a distance of about 12 inches, cover one eye and stare at the center dot with the other eye, keeping very still. You may notice after 10 seconds or so that either the center dot or the gray shading starts to fade, shrink and may entirely disappear.

Color fill-in. Similarly for color (brightness and color are perceptual cousins). At a distance of about 12 inches, cover one eye and stare at the center dot with the other eye, keeping very still. You may notice after 10 seconds or so that the center dot starts to fade, shrink and may entirely disappear.

Neon spreading. At each of the junctions there appears to be a cloud or luminous spot of color, but in fact the only thing that is really there is a cross made from colored lines. Your color system, a part of the surface estimation system, is seeking to invade surrounding territory and fill it in.

More Neon/color/brightness spreading. In each of these four figures, notice a region, cloud, or portion of the figure in which you see apparent shading but it isn’t really there.