Perceptual ConstancyModule 19

Perceptual Constancy• Perceiving objects as stable or constant

– having consistent lightness, color, shape, and size even as illumination and images on our retina change

• Example: as a person walks away from you their retinal image decreases in size

• Example: A car in the distance is still known to be the same color and size as it was when it was driving past you.

• Important function of the perceptual system is to represent constancy in our environment even when the retinal image varies

Perceptual Constancy: Lightness Constancy

Color & Light (Brightness) Constancy• Brightness Constancy - The ability to see an object as having a

constant level of brightness no matter how the lighting conditions change

• Color Constancy – color remains the same regardless of how it looks in different levels of light.

– See this clip from Brain Games for an example

• If you look at a sheet of paper in bright sunlight it looks blazingly white. View the same sheet in a dimly lit room, and it appears gray.– Has the paper changed? Of course not!

– We know the paper stays “white” no matter what the lighting conditions are.

• This remains true only if background or context remains the same! Change the surrounding area and the color may actually change to your mind

Brightness Contrast

• Are These Squares the SAME Color?

See example in Brain Games

Brightness Contrast• Perceived lightness stays

roughly constant as long as the context or surroundings stay the SAME.

• When context changes you may perceive color as changing.

• Although the squares are in fact identical, we perceive the one as lighter or darker because of the contrast with its surroundings.

•Want to see more of these perceptions? Goto http://www.skidmore.edu/~hfoley/Perc4.htm#lightcon or

•http://www.michaelbach.de/ot/lum_contrast-contrast/index.html

See example in Brain Games

Your Perception of Color Depends on its Context/Surroundings

In the example, you can see the bright sunlight streaming into the room, a lit lamp, and parts of the room that are in shadow. Superimposed on the scene are a bunch of ellipses of exactly the same shade of gray (same lightness). Do all the ellipses seem identical in lightness? Why does the ellipse on the floor in shadow seem lighter than the ellipse in the sunlight? Are your experiences consistent or inconsistent with lightness constancy?From: http://www.skidmore.edu/~hfoley/Perc4.htm#ch4demo4

Brightness Constancy Depends on Surroundings• Obviously A is a dark square

in light and B is a light square in shadow, right?

•Both A and B are identical shades of gray! Can you believe that they are sending the identical level of light energy to your retina? •The organization of the scene into lighted areas and shadowed areas must play a role in determining the perceived lightness of the squares. •If Square B (in a shadow) is reflecting the same amount of light as Square A then it must be lighter!•To understand this better click HERE or watch this video

Perceptual Constancy: Size Constancy

Size Constancy

• A person’s understanding that as an object moves further or closer to them its actual size stays the same

• As an object appears to become LARGER we realize it is getting CLOSER, not bigger.

• As an object appears to become smaller we realize it is moving farther away, not getting smaller.

• Perceptions of the world depend on our experience - Colin Turnbull’s experiment with Kenge (Famous Studies #5)

SIZE depends on Depth

• To perceive the size of objects accurately we must also perceive their distance accurately

• Illusions occur simply because a particular image lacks sufficient depth cues

This figure shows that image size depends upon both object size and distance

Retina Pupil

Image A B

Image A

Size Constancy

• People are the same size even though their image sizes differ

• The depth cues such as linear perspective and relative size help the visual system judge the size accurately

Size Distance Relationship

Size Distance Relationship

Size Distance Relationship

Illusion of the Camera• How Lord of the Rings used depth

perception and Size Constancy to create the illusion that the hobbits are smaller people. (2 min)– See more on how they did this here (7 min)

• A quick example by amateurs: (1 min)

Shape Constancy• The understanding that an object’s shape

remains the same even though the angle of view makes the shape appear changed

Shape Constancy

• It is hard to tell if the figure on the upper right is a trapezoid or a square slanted backward.

• If we add depth cues, it helps us see that it is actually a square

Illusions• When we misperceive the true characteristics of an object

or image. • Help researchers understand how sensation and

perception normally works

Ponzo Illusion• Converging lines indicate that top line is farther

away than bottom line

PONZO “MONSTER” ILLUSION: The interplay between perceived size and distance

– The monocular cues for distance make the pursuing monster look larger than the pursued. It isn’t.

– This is because experience tells us that if a more distant object can create the same-sized image as a nearer one only if it is actually larger. As a result, we perceive the bar that seems farther away as larger.

Moon Illusion• Moon appears larger when it is on the horizon than when it is

directly overhead (lacks size constancy).

• Objects on the horizon are perceived as farther away than those above us

• The moon appears to be behind those objects on the horizon (overlap). Since it is bigger than those object it is perceived as huge! (click on box below for explanation). In the sky there is nothing to compare it to so it appears smaller.

Click Below to View an Explanation:

Moon Illusion

Ames Room Illusion: Secret Revealed

We perceive the room to be as we are used to, a perfect square or rectangular.

When in fact it is a trapezoid!

Ames Room

Ghost Perceptions?• Watch the car commercial closely. What perceptual

concept causes you to see the faint image of the “ghost?”

Click Below to view:

Ghostly Car Ad