7. Distance Perception - Eye Movements & Binocular Vision Summer 2026

Who are patients who lack stereopsis?

Strabs

Amblyopics

Anisometropes

Monocular individuals

Is a patient who lacks stereopsis able to perceive distance?

Yes -- remarkably well

To facilitate reasonable speed of cortical processing, what does visual perception assume?

Some aspects of the world are constant

How are we able to assume some aspects of the world are constant?

This comes with experience and interaction of the real world

What are the CONSTANCIES in the real world that cortical processing is able to assume?

-Shape constancy

-Size constancy

-Color constancy

-Lightness constancy

Are we able to assume speed constancy?

No

What is shape constancy?

cognitive mechanism that allows you to perceive an object's actual, rigid shape despite changes in your viewing angle or orientation

What is size constancy?

our brain's ability to perceive an object as its actual, stable size, even though its image on our retina shrinks as it moves farther away

True or False:

Sometimes prior experience is so common that we will distort perception to make constancies apply

true

Size vs Shape (Pic)

**we assume that the room MUST be square so the people in it are different sizes

Size vs Shape (Pic)

What are the 2 components of spatial localization?

1) distance perception

2) direction perception

What is relative distance perception?

distance between objects (depth)

What is absolute distance perception?

distance from body (egocenter)

What are the 2 subsets of direction perception?

1) oculocentric

2) egocentric

True or False:

Stereoscopic relative distance perception is excellent for far distances

false -- excellent for short distances

Is absolute distance perception very accurate?

No -- and perception is easily fooled

Distance Perception is (short/long) range

short

The sun, moon, stars appear _______

equidistant

Do size and distance perception interact?

Yes

REVIEW: What is Emmert's Law?

the relationship of perceived size (S) and perceived distance (D)

Emmert's Law is a _____ law

perceived

What is the equation for Emmert's Law?

S = kD

Is size directly proportional to distance?

Yes

What is the law of visual angle?

the relationship of visual angle (Φ) to physical size (s) and physical distance (d)

The Law of Visual Angle is a ____ law

physical

What is the equation for the Law of Visual Angle?

Φ = s / d

What is linear perspective based upon?

on relationship between angular size and distance

Objects that are farther away will subtend a (larger/smaller) visual angle

smaller

Objects that are closer away will subtend a (larger/smaller) visual angle

larger

How do artists use linear perspective?

-The artist draws more distant objects with a smaller angular subtense to represent greater distance

-This follows the Law of Visual Angle

When observers look at art that portrays Linear Perspective, what do they notice? D/t what Law?

The observer perceives the representation having a smaller angular subtense as more distant and larger d/t Emmert's Law

What is size constancy?

The ability to perceive the correct size of an object despite changes to its retinal image size

How Size and Distance Interact (Pic)

How Size and Distance Interact (Pic)

How Size and Distance Interact (Pic)

How Size and Distance Interact (Pic)

Abuse of Perspective (Pic)

**Once the rules of perspective are known, they can be deliberately misapplied to create impossible images

Abuse of Perspective (Pic)

Are oculomotor cues to distance good?

Not great

Oculomotor sources of information (convergence and accommodation) (are/are not) strong cues for distance

are not

What is overlay as a pictorial representation of depth?

nearer objects occlude more distant ones

How do light and shadow give pictorial representations to depth?

-Relative depth is cued by the position of highlights and shadows on objects.

-We interpret shadows as if the light was coming from above

____ information is provided by shadows

spatial

True or False:

Changing the position of a shadow will change the position of the object itself that the viewers observes

true

What is Aerial perspective?

a blue haze obscures more distant objects because of atmospheric scatter

More distant objects are (more/less) distinct with aerial perspective

less

How does the relative height of an object relate to its perceived distance?

The distance of objects from an observer along the ground is INVERSELY related to the angle subtended from the horizon

An object 2x the distance along the ground will appear to be (2x/half) the size as a standard object

half

Distance from the Horizon Line in 2D Image (Pic)

Distance from the Horizon Line in 2D Image (Pic)

An object that extends from the ground to the horizon is ________ tall regardless of distance

1 eye hight (h)

True or False:

Size can be estimated by the angle objects subtend relative to the ground horizon distance

true

What is linear perspective, again?

The relative sizes of retinal images provide information about the relative distances of the objects

A 1/2 size pencil (see pic) will look (twice/half) as far away

twice

We make assumptions about object size. Similar looking objects are assumed to have what?

identical sizes (telephone poles, fence posts, chairs, pencils)

We make assumptions about object size. Continuous objects are assumed to be ______ in size

unchanging (roads, railroad tracks, floor tiles)

We make assumptions about object size. Familiar objects are assumed to be what?

their expected size (coins, ruler)

Linear Perspective (Pic)

Linear Perspective (Pic)

Forced Perspective (Pic)

Forced Perspective (Pic)

Forced Perspective -- Cinderella Castle (Pic)

**It is only 185 ft tall but appears to be 350 ft tall because of forced perspective. The bricks and other details get smaller as the building gets taller

Forced Perspective -- Cinderella Castle (Pic)

Geometry in Perspective

Two pencils are separated by 1m. The nearest pencil is at 1m. This is the station point.

What is the size ratio of the 2 pencils?

-Farthest pencil is at 2m

-Closest pencil is at 1m

-2:1 ratio

Geometry in Perspective

Two pencils are separated by 1m. The nearest pencil is at 4m. This is the station point.

What is the size ratio of the 2 pencils?

-Farthest pencil is at 5m

-Closest pencil is at 4m

-5:4 ratio

Difference in perspective size is d/t the difference in ____ point

station

Picture (retinal image) only conveys ______ info

relative distance

If you overestimate distance of an object, separation between objects will be (expanded/shortened)

expanded

If you underestimate distance of an object, separation between objects will be (expanded/shortened)

shortened

Photographs taken from a great distance and enlarged using a long focal length lens appear to have ____ perspective

distorted

We tend to (over/under) estimate station point distance

under

Since we tend to underestimate station point distance, depth is usually (expanded/shortened)

shortened

Foreshortened depth will (decrease/increase) apparent speed of objects moving toward or away from an observer

decrease

Telephoto Picture w/ Foreshortening (Pic)

Telephoto Picture w/ Foreshortening (Pic)

A photographic portrait taken closer than expected will be perceived as "birdlike" because of perceived depth _______

expansion

Near Observation with Expanded Depth (Pic)

Near Observation with Expanded Depth (Pic)

What is motion parallax?

movement of body causes differential retinal image motion of objects at different distances

Motion is determined by what with motion parallax?

by the direction of the motion of the body relative to the scene

Horizontal Tangential Motion of an Observer (Pic)

**objects will move in opposite direction as the observer. Closer objects will appear faster. Farther objects will appear slower

Horizontal Tangential Motion of an Observer (Pic)

Moving Horizontally Towards a Scene (Pic)

**Objects that are straight ahead will not have any movement. Objects closer will move faster. Objects farther will move slower. Focus of expansion straight ahead.

Moving Horizontally Towards a Scene (Pic)

Radial Motion Towards a Near Point (Pic)

Radial Motion Towards a Near Point (Pic)

What is the focus of expansion?

point on the ground plane in which you are moving towards

Rules of Motion Parallax

Image velocity is ______ proportional to object distnce

inversely

Rules of Motion Parallax

Image velocity is ____ for the point forward which the body is moving

0

Rules of Motion Parallax

What is the point forward which the body is moving with 0 image velocity?

focus of expansion

Rules of Motion Parallax

Images of objects farther than the focus of expansion will have (with/against) motion

with

Rules of Motion Parallax

Images of objects closer than the focus of expansion will have (with/against) motion

against