This document discusses several optical illusions and how they relate to how our brain perceives and processes visual information: 1) The brightness illusion occurs because our brain compares the brightness of objects to their surroundings, not their actual brightness levels, causing squares of the same gray to appear different shades. 2) In the hybrid image illusion, faces seem to "swap" as you move back due to the brain processing fine details up close and coarse details from farther away. 3) Staring at the after-image pattern causes you to see an image of Jesus afterwards due to certain photoreceptor cells becoming desensitized at different rates for brighter and darker areas. 4) The Jastrow illusion makes

1. a
b

2. In this illusion, squares a and b are the same shade of gray and the two circles are
the same color. (If you don’t believe it, print it out, cut out the two squares, and place
them side by side.) This trick of the eye occurs because our brain does not directly
perceive the true colors and brightness of objects in the world but instead compares
the color and brightness of a given item with others in its vicinity. For instance, the
same gray square and colored circle will look lighter when surrounded by black than
when it is surrounded by white.
Brightness Illusion

3. Get up from your computer and walk back about 10 to 15 feet. What do you see now?

4. When we look at the world around us we see everything in both fine and coarse
detail. When we are closer to an object, the fine detail dominates and as we get
further away, the coarse detail dominates. Both of the pictures above are a
combination of both fine and coarse detail. Albert Einstein is shown in fine detail
whereas Marilyn Monroe or Harry Potter are in coarse detail. Therefore, as you
move away from the pictures the faces seem to ‘swap’ from Albert to either Marilyn
or Harry.
Hybrid Images

5. Look at the image above and stare at the central 4 dots for about 30 seconds. Then, look away at a blank
wall or white surface. Who do you see?

6. Did you see Jesus? This effect is explained by the photoreceptor cells (rods and
cones) in your eyes. The longer you look at the image the more desensitized your
cells become. In other words, they become pretty tired (overstimulated). This
happens a lot faster for the brighter parts of the picture compared to the dark. When
you look at a blank wall after staring at the following image, the less desensitized
areas are more active. This is why you see Jesus’s face.
After-Images

7. a
b

8. Is your cup o’ joe half empty or half full? It depends on your outlook—and on a little twist
on the Jastrow illusion, named after Polish-born American psychologist Joseph Jastrow.
In this classic illusion, two identical arches positioned in a certain configuration appear
to have very different lengths. In other words, the sleeves a and b appear to be of
different size, but they are the same size. Starbucks coffee sleeves have the perfect
shape for an impromptu demonstration of the Jastrow illusion, so now you can amaze
your office mates at your next coffee break.
All you need to do is align the coffee sleeves as in the accompanying photograph and—
presto!—your tall cup sleeve is now venti-sized! Your brain compares the upper arch’s
lower right corner with the lower arch’s upper right corner and concludes, incorrectly,
that the upper sleeve is shorter than the lower sleeve.
Jastrow Illusion

9. The straight lines near the central point (vanishing point) appear to curve outward. This illusion occurs
because our brains are predicting the way the underlying scene would look in the next moment if we were
moving toward the middle point.

10. When light hits your retina, about one-tenth of a second goes by before the brain translates the
signal into a visual perception of the world. We’re always one-tenth a second behind. This is
called a neural delay. To compensate, it has been proposed that our visual system has evolved
to compensate for neural delays, generating images of what will occur one-tenth of a second
into the future. That is, we are constantly making predictions about the future. That foresight
keeps our view of the world in the present. It gives you enough heads up to catch a fly ball
(instead of getting socked in the face) and maneuver smoothly through a crowd.
The Hering Illusion is a vivid example of our brains attempting to perceive the future. The Hering
Illusion tricks us into thinking we are moving forward, and thus, switches on our future-seeing
abilities. Since we aren't actually moving and the figure is static, we misperceive the straight
lines as curved ones.
“The converging lines toward a vanishing point (the spokes) are cues that trick our brains into
thinking we are moving forward — as we would in the real world, where the door frame (a pair of
vertical lines) seems to bow out as we move through it — and we try to perceive what that world
will look like in the next instant.” -Mark Changizi, Rensselaer Polytechnic Institute
Hering Illusion

12. In 1984 neurophysiologist Rudiger von der Heydt and his colleagues at Johns Hopkins
University discovered visual neurons called end-stopped cells that responded to real boundaries
but also registered illusory contours. The activity of these cells causes the brain to interpret the
illusory boundaries as real. That is, you see a square in the previous image, but the square does
not exist.
Since this initial discovery, a number of studies have shown that the brain treats Kanizsa shapes
as if they were real objects.
Kanizsa Shapes

13. The center of which circle look brighter?

14. The pupil automatically constricts in response to strong light to protect receptors in the retina
from damage. Scientists have assumed that the reflex is involuntary, as it is evident in comatose
patients, and that it is triggered by the amount of absolute luminance. Does the same thing
happen when looking at the brightness illusion shown in the two illustrations?
Although the center of each design has the same amount of physical luminance, the pattern on
the left appears subjectively brighter. Accordingly, researchers found that participants’ pupils
constricted more in response to the image on the left than to the one on the right, indicating that
a subjective experience of brightness—not actual luminance— governs this response.
Regardless of reality, the visual system interprets the apparently brighter pattern as a greater
threat to the eye.
Brightness Illusion II