WittSem 100L: Visual Illusions (11/6/07)

In investigating the topic of patterns in nature, we need to understand the role of perception in discerning patterns. That the role of perception is complicated and important is demonstrated by the existence of visual (optical) illusions—many of which are still not well understood.

1. A few “simple” illusions to get started:

a) Müller-Lyer illusion

http://www.michaelbach.de/ot/sze_muelue/index.html

Follow the instructions. How large a distance estimation error do you make?

Try it a couple of times. Can you improve with practice, or does the illusion persist?

Read the explanation near the bottom of the page (starting “The picture on the right”). Mouse over the illustration to compare the sizes of the lines in the picture. Make sure you can explain what causes the illusion.

b) The “Gateway Arch” illusion

Look at the picture of the Gateway Arch in St Louis at

http://www.finnmoller.dk/tr-usa/mo/stlouis-gateway-arch02-da.htm

Visually (don’t measure!) compare the horizontal and vertical distances spanned by the arch.

How does the height compare to the width?

Now find the actual height and width of the Arch:

What's going on here?

Look at (and follow the instructions for) the T Illusion at

http://www.michaelbach.de/ot/sze_t-illusion/index.html

What percentage of the horizontal length did you get on your first try? (This percentage will measure the strength of the illusion—the farther it is from 100%, the stronger the illusion.)

Try different orientations of the T to see whether the effect depends on the angle.

angle percentage

0º

45º

90º

180º

With the T at 90 degrees, turn your head sideways. Does this change the strength of the effect (percentage)?

What's the relationship between the T illusion and the “Gateway Arch” illusion?

c) Motion aftereffect (waterfall illusion)

http://www.lifesci.sussex.ac.uk/home/George_Mather/Motion/MAE.HTML

OR

http://www.michaelbach.de/ot/mot_adapt/index.html

Describe the illusion.

Class discussion

2. Can we see things that aren't there—or not see things that are there?

a) Pyramid illusion

http://www.michaelbach.de/ot/lum_pyramid/index.html

Increase and decrease the number of squares. At about what number of squares do you start to see the “X” appear?

Does the effect depend on the color of the squares?

What happens if the brightness of the squares is inverted so the inner ones are darker?

Go between the squares and the stars. What happens?

b) Experience your blind spot

http://serendip.brynmawr.edu/bb/blindspot1.html

Read through this page and follow the directions.

At what distance does the dot go away?

Now try

http://serendip.brynmawr.edu/bb/blindspot2.html

What seems to take the place of the dot when it disappears?

For something really spooky, try

http://serendip.brynmawr.edu/bb/blindspot5.html

Now what seems to take the place of the dot when it disappears?

Map out your own blind spot:

http://serendip.brynmawr.edu/bb/blindspot/

Do all creatures with eyes have blind spots? See

http://faculty.washington.edu/chudler/chvision.html

Class discussion/explanation of the blind spot.

c) Motion-induced blindness

http://www.michaelbach.de/ot/mot_mib/index.html

Follow the directions! What happens?

Does there seem to be any dependence on speed? On color? On the size of the dots?

d) Afterimages

Go to

http://www.opticsforteens.org/illusions/afterimage.asp

and follow the directions.

What happens? From viewing these illusions, what colors are complementary? (We’ll explain this illusion later.)

Match the color with its complement:

Red Yellow

Green Cyan (blue-green)

Blue Magenta (purple-red)

e) Lilac chaser

http://www.michaelbach.de/ot/col_lilacChaser/index.html

Follow the directions. Give this one time—if you can keep your eyes fixed, it's a great effect.

What happens?

Can you relate this illusion to the previous three illusions?

Class discussion

3. Now some illusions in which our perception of patterns is affected:

a) http://www.michaelbach.de/ot/ang_fraser/index.html

What “pattern” do you see that’s not really there?

b) Patterns in blocks

http://www.michaelbach.de/ot/fcs_mosaic/index.html

At what level do you recognize the face with blurring? Without blurring?

Look at several of the other examples (including Dali's painting).

c) Effects of motion on patterns and vice versa

http://www.michaelbach.de/ot/mot_biomot/index.html

Follow the directions. (A Gestalt is an overall pattern that you perceive as a whole.)

What pattern becomes obvious once you start the motion?

Motion binding

http://www.michaelbach.de/ot/mot_motionBinding/index.html

How does your interpretation of the pattern change when the occluders are present?

Rotating snakes

http://www.michaelbach.de/ot/mot_rotsnake/index.html

Do you see the motion in your central vision, or in your peripheral vision?