Extra Credit for Participating in Experiments

Go to www.tatalab.ca to sign up

We are recruiting for a variety of experiments including basic

perception and ADHD

ADHD Research ProjectWe are specifically recruiting individuals who

have been diagnosed with ADHD and are currently taking medication

Contact Karla at karla.ponjavic@uleth.ca for more information or to sign up

Upcoming

• Read Gregory Article for Feb 23

• Read Pinker Article for help with stereo vision and stereo grams

• Read Land Article for March 2

Depth Cues Pictorial Depth Cues: aspects of 2D images that imply depth Physiological Depth Cues: Proprioception in ocular muscles indicates

accommodation and convergence Motion Depth Cues: foreground and background move in opposite directions Stereoscopic Depth Cues: disparity between two retinal images indicates

distance

Pictorial Depth Cues

– Retinal image size– far objects smaller than near objects

• Retinal image size– problem: big far things same as close

small things– “solved” by size constancy: Perceived size

is adjusted according to perceived distance (based on other cues)

– forms the basis for several visual illusions

Pictorial Depth Cues

• Retinal image size

Pictorial Depth Cues

• Retinal image size

Pictorial Depth Cues

• Linear perspective

Pictorial Depth Cues

• Texture gradient

Pictorial Depth Cues

• Height in the plane

Pictorial Depth Cues

More Depth Cues

• Pictorial Depth Cues• Physiological Depth Cues• Motion Parallax• Stereoscopic Depth Cues

Physiological Depth Cues

– Two Physiological Depth Cues1. accommodation2. convergence

Physiological Depth Cues

– Accommodation

Physiological Depth Cues

– Convergence

Physiological Depth Cues

– Convergence– small angle of convergence = far away– large angle of convergence = near

– What two sensory systems is the brain integrating?

– What happens to images closer or farther away from fixation point?

Physiological Depth Cues

– Convergence and accommodation are reflexively linked

Under what circumstances might this be a problem?

Motion Depth Cues

– Motion1. Parallax

Motion Depth Cues

– Parallax

Motion Depth Cues

– Parallax– points at different locations in the visual

field move at different speeds depending on their distance from fixation

Motion Depth Cues

– Parallax

Seeing in StereoSeeing in Stereo

Seeing in Stereo

It’s very hard to read words if there are multiple images on your retinaIt’s very hard to read words if there are multiple images on your retina

Seeing in Stereo

It’s very hard to read words if there are multiple images on your retinaIt’s very hard to read words if there are multiple images on your retina

But how many images are there on yourretinae?

Binocular Disparity

• Your eyes have a different image on each retina– hold pen at arms length and fixate the spot

– how many pens do you see?– which pen matches which eye?

Binocular Disparity

• Your eyes have a different image on each retina– now fixate the pen

– how many spots do you see?– which spot matches which eye?

Binocular Disparity

• Binocular disparity is the difference between the two images

Binocular Disparity

• Binocular disparity is the difference between the two images

• Disparity depends on where the object is relative to the fixation point:– objects closer than fixation project

images that “cross”– objects farther than fixation project

images that do not “cross”

Binocular Disparity

• Corresponding retinal points

Binocular Disparity

• Corresponding retinal points

Binocular Disparity

• Corresponding retinal points

Binocular Disparity

• Corresponding retinal points

Binocular Disparity

• Points in space that have corresponding retinal points define a plane called the horopter

The Horopter

Binocular Disparity

• Points not on the horopter will be disparate on the retina (they project images onto non-corresponding points)

Binocular Disparity

• Points not on the horopter will be disparate on the retina (they project images onto non-corresponding points)

• The nature of the disparity depends on where they are relative to the horopter

Binocular Disparity

• points nearer than horopter have crossed disparity

• points farther than horopter have uncrossed disparity

The Horopter

Binocular Disparity

• Why don’t we see double vision?

Binocular Disparity

• Why don’t we see double vision?

• Images with a small enough disparity are fused into a single image

Now on to Magic Eye Stereograms

Stereograms

Left Eye Right EyeDivider

• Right eye sees face to the left; left eye sees face to the right therefore:

faces present crossed disparity to the brain

• Face appears in front of square

Autostereograms

• Optically separate images aren’t needed

Autostereograms

• Optically separate images aren’t needed

• WARNING! Tricky stuff coming in the next slides

Autostereograms

• Optically separate images aren’t needed

• WARNING! Tricky stuff coming in the next slides

• Keep the definitions of convergence and disparity separate.

Autostereograms

• Convergence is on a point at the same distance as the images

• Boxes and faces are on the horopter

• How many boxes would you see?

boxes and faces are on the horopter

What would happen if you remove the divider of a stereoscope?

Autostereograms

• right-eye line of sight crosses left-eye line of sight in front of image (crossed convergence)

• each retina is now pointed at the opposite box

• How many boxes would you see?

• What would happen to the face?crossed convergence

Now cross your eyes:

Autostereograms

• There would be three boxes

• middle box: right eye sees face shifted to right; left eye sees face shifted to left therefore:

uncrossed disparity

• Face in the middle box appears behind square

crossed convergence

Autostereograms

• What would happen if you moved the faces within the box?

• What would happen if you viewed this with uncrossed convergence?

Autostereograms

Left Eye’s ImageRight Eye’s Image

Crossed convergence shifts right image to the right of the left image and vice versa:

Autostereograms

What would happen if the convergence was uncrossed?

Autostereograms

Left EyeRight Eye

Uncrossed convergence shifts right-eye’s image to the left of the left-eye image and vice versa:

Autostereograms

What would happen if the faces were switched relative to the boxes?

Autostereograms• one doesn’t even need two different

images!

RIGHT EYE LEFT EYE

Convergence tells your brain that the plane of the image is farther away than it really is

Autostereograms

Faces fuse

Autostereograms

• Uneven spacing between identical objects in a single picture can appear as disparity if the angle of convergence is inappropriate

Autostereograms

• Uneven spacing between identical objects in a single picture can appear as disparity if the angle of convergence is inappropriate

• TRICK: Seeing depth in autostereograms requires you to suppress the reflexive coordination between convergence and accommodation

RIGHT EYE LEFT EYE

If you uncross convergence, your right eye gets these faces shifted slightly to left, left eye gets them shifted to right = CROSSED DISPARITY

AutostereogramsAny repeating objects that have a spacing different from the background will have either crossed or uncrossed disparity

What would you see?

RIGHT EYE LEFT EYE

If you uncross convergence, right eye gets these faces shifted slightly to right, left eye gets them shifted to left = UNCROSSED DISPARITY

AutostereogramsAny repeating objects that have a spacing difference from the background will have either crossed or uncrossed disparity

What would you see?

Autostereograms

• by adjusting the disparity at different parts of the image (with a computer usually) one can make shapes that emerge or recede in depth

“Magic Eye” Stereograms• Usually viewed with uncrossed convergence • Imagine gazing farther than the surface (let your eyes

“relax”)• Now try to notice objects or forms in the blurriness• As you become aware of shapes, try to focus

(accommodate) the plane of the image without converging your eyes

Autostereograms

Autostereograms

Autostereograms