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Chapter 6

PSYC 363 Chapter Notes - Chapter 6: Extraocular Muscles, Binocular Disparity, Ciliary Muscle


Department
Psychology
Course Code
PSYC 363
Professor
Michael Lantz
Chapter
6

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Chapter 6: Perceiving Depth
Goal of Perceiving Depth
1. The goal is the accurately perceive a 3-D world using two 2-D retinal images,
one in each eye.
2. The representation of 3-D space in the 2-D retinal
image is many-to-one:
a) Many different 3-D scenes can produce one
and the same retinal image:
i. Example: Objects of different shapes and
sized and at different distances produce the
same retinal image because of the way the objects are oriented.
Depth Cues
1. We perceive depth based on integrating information gained form a wide variety of cues:
a) Oculomotor cues:
i. Provide information from the muscles that move the eyes and the muscles that
change the shape of the lens:
- Accommodation.
- Convergence.
b) Cues based on retinal images (“the rest of the cues”):
i. Binocular disparity:
- Information derived from the differences in the retinal images in the two eyes:
oBinocular disparity.
ii. Monocular cues:
- Information that could be derived from the retinal image in just on eye.
- Can be divided into:
oStatic cues:
Point-Based cues:
1) Partial occlusion.
2) Relative height.
Size-Based cues:
1) Familiar size.
2) Relative size.
3) Texture gradients.
4) Linear perspective.
Lighting-Based cues:
1) Atmospheric
perspective.
2) Shading.
3) Cast shadows.
oDynamic cues:
Motion parallax.
Optic flow.
Deletion and accretion.
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Chapter 6: Perceiving Depth
OCULOMOTOR DEPTH CUES
Oculomotor Depth Cues
1. Cues that are based on feedback from the oculomotor muscles controlling the shape of
the lens and the position of the eyes.
Accommodation
1. Accommodation:
a) The shape of the lens adjusts to focus an image sharply on the retina:
i. If the object is far we need a flat lens, so the ciliary muscles relax.
ii. If the object is close we need a round lens, so the ciliary muscles contract.
b) Only provides reliable depth information for objects up to 2 meters away.
Convergence
1. Our eyes must converge (“cross”) to focus on an object moving closer and closer to us:
a) The convergence angle gets bigger and bigger.
b) Medial rectus muscles pull inward (tighten).
2. Focusing on objects at a distance of 4 meters or more requires very little (or
no) convergence:
a) Convergence angle is less than 1 degree.
3. Serves as a depth cue for only a fairly short distance in front of your eyes.
MONOCULAR DEPTH CUES
Monocular Depth Cues
1. Depth cues based on the retinal image are more important than the oculomotors cues
(accommodation and convergence) because:
a) They operate across a much greater range of distance.
2. Monocular (mono: one; ocular: eye) depth cues:
a) Cues that are based on the retinal image and that provide information about depth
even with only eye open.
3. Two types:
a) Static Monocular (Pictorial) cues:
i. Seen in motionless 2-D depictions of 3-D scenes:
- Paintings, photographs, etc.
b) Dynamic Monocular Cues:
i. Involves motion.
- Provides information about depth when you’re walking through a forest or
watching a movie.
Static Monocular Depth Cues (Position, Size, and Lighting in the Retinal Image)
1. Cues that provide information about depth based on:
a) The position of the objects in the retinal image (position-based depth cues):
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Chapter 6: Perceiving Depth
i. Partial Occlusion:
- In scenes where one object partially hides (occludes) another object, the occlusion
indicates that the former is closer than the latter:
oT-junctions are used to imply which object is covering which:
It is unlikely that the red square lines up perfectly with a potential
missing space on the blue square, therefore we assume it
continues behind the red square.
ii. Relative Height:
- The relative height of the objects in the retinal image with respect to the horizon
(or with respect to eye level if there is no visible
horizon) provides information about the objects’
relative distance from the observer:
oBelow the horizon or below eye level:
Objects situated lower in the image are
closer to the observer.
oAbove the horizon or above eye level:
Objects situated higher in the image are
father to the observer.
- People who were shown two isolated lights at
eye level in a pitch dark room perceive the
higher one as farther away.
b) The size of the retinal image (size-based depth cues):
i. Size-Distance Relation:
- The further an object is from an observer, the smaller its retinal image is:
oThe retinal image size of an object can be measured in terms of its
visual angle (the angle occupied by the object in the visual field).
- When a scene’s size-distance relation is apparent, it is said to contain the
depth cue of size perspective (the further an object is, the smaller its
retinal image is).
ii. Familiar Size:
- Knowing the retinal image size of a familiar object at a familiar distance
lets us use its retinal image size to gauge its distance:
oIf a basket ball, a golf ball, and a baseball all had the same retinal
size, we’d be able to tell which ball is closest or furthest to us
(assuming we’re familiar with all three of these balls).
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