Muller - Lyer Illusion

Muller - Lyer Illusion

Muller-Lyer Illusion

• What theory does Gregory take up?

Muller-Lyer Illusion

• What theory does Gregory take up?– Thiery / Woodworth : Muller-Lyer figure

“suggests perspective”

Muller-Lyer Illusion

• What theory does Gregory take up?– Thiery / Woodworth : Muller-Lyer figure

“suggests perspective”

– Is there something about the presence of pictorial depth cues in an image that distorts size?

Depth Cues and Size Distortion

The Ponzo Illusion:

Depth Cues and Size Distortion

The Ponzo Illusion:

Depth Cues and Size Distortion

• The illusion of depth in an image seems to distort the perception of size in the image

• This is due to misapplication of Size - Constancy

Pictures are Weird

• What does Gregory find paradoxical about pictures?

Pictures are Weird

• What does Gregory find paradoxical about pictures?

Pictures convey depth yet also appear flat

Why?

Pictures are Weird

• When you view a picture there are some depth cues telling you to perceive depth, but other cues telling you that the image is on a flat surface

• …the surface matters!

• Retinal image size

Pictures are Weird

• Retinal image size

Pictures are Weird

The Apparatus

• What does Gregory do to remove the paradox so that the Muller-Lyer figure no longer looks flat?

• Remove the background surface

The Apparatus

Removing the Background

• With background removed, Muller-Lyer figure actually looks 3D

Removing the Background

• What prediction can you make about how the perceived size and the perceived distance of the center line are related?

Removing the Background

• What prediction can you make about how the perceived size and the perceived distance of the center line are related?

When the centre line appears close, it should also appear small

When the centre line appears far, it should also appear large

Removing the Background

• How did Gregory test that?

The Apparatus

Removing the Background• Perceived size and perceived distance are tightly

related• inward arrowheads make line look closer / outward

arrowheads make line look farther

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”