Size Perception and Motion Chapter 8

Size Perception

  1. Size perception - Refers to the ability to perceive the actual size of the distal stimuli in spite of wide variations in the size of the proximal stimulus

  1. The visual system has to solve two inter-related problems

  1. Accurately perceiving that distal stimuli are the same size even though the sizes of the proximal stimuli are different

  2. Accurately perceiving that distal stimuli are different sizes even though they project the same proximal size.

2. Size-distance Invariance

The visual system is able to accurately perceive an object’s size by taking both retinal size (Visual angle of the proximal stimulus) and perceived distance to the object into account

  1. Emmert’s Law 

 A computational formula describing the size-distance invariance hypothesis: Sp = K (Sr x Dp) 

  1. Size Constancy - 

The visual system is able to maintain a consistent perception of an object’s size  in spite of variations in retinal size (Emmert’s law thus explains Size Constancy

We can test the idea that size perception is determined by taking both retinal size and perceived distance into account by holding one of these variables constant and manipulating the other.

  1. The Moon Illusion

 Refers to the commonly reported experience where the horizon moon looks to be bigger than the zenith moon

  1. Flat-dome / Vaulted heavens hypothesis – refers to the idea that the sky looks somewhat flattened (Rock & Kaufman, 1962)

So, if the moon’s retinal size is the same at every location, but the horizon sky appears farther away than the zenith sky, the result is the moon illusion (moon looks bigger at the horizon than the zenith)



Motion Perception

  1. Importants:

  1. Enhances ability to interact with the environment

  2. Enhances detection of other organisms in the environment

  3. Helps segregate Figure from Ground

  4. Provides info about the 3-d shape of object

II. Two categories of motion perception: Illusory and Real

  1. Illusory motion - perceived motion but no actual motion (underlying mechanism that is in play)

  1. Apparent Motion: (Phi Phenomenon)

Rapid sequential presentation of two separate stimuli creates the illusion of one moving object (e.g., neon signs).

  1. Induced Motion:

A moving background makes a stationary object appear to move (e.g., clouds moving past the moon).

  1. Autokinetic Motion:

A stable point of light in a completely dark environment appears to move erratically due to microsaccades (e.g., Ouchi Illusion).

  1. Motion after-effects:

Extended viewing of motion in one direction creates an illusion of motion in the opposite direction upon looking at a stationary object.

B. Real motion:
Refers to our ability to perceive actual motion. 

  1. Factors Affecting Real Motion Perception:

  1. Background: Easier to see motion against a heterogeneous (textured) background:

i. Homogeneous threshold: 1/6 – 1/3 degree/sec

ii. Heterogeneous threshold: 1/60 – 1/30 deg/sec

  1. Size of Object:

  • Due to the application of speed constancy, smaller objects appear to move faster than larger ones at the same actual speed. 

Hmman ability to perceive speed is better when the background is homogeneous - FALSE, Better when heterogenous.

III. Explanations of Real Motion Perception

  1. Motion-sensitive cells in the visual system respond to movement across the retina.

  1. Problem: Motion can be perceived without movement across the retina (e.g., tracking an object) or vice versa (e.g., moving eyes).

B. Psychological/Cognitive Mechanisms- the visual system calculates motion using cognitive cues.

  1. Corollary Discharge Theory:

  • Corollary discharge is a signal that helps the brain compare active motor signals (like eye movement) with sensory information to prevent misinterpretation of motion.

  • A "comparator" in the brain compares motor signals sent to the eye against sensory signals sent from the eye.

  • When signals match: motion is not perceived (​​rain interprets motion across the retina as due to eye movement alone); when signals don't match: motion is perceived (motion across the retina due to object actually moving)

  • If the comparator gets both motor and sensory signals together,  then no motion is perceived. If the comparator gets either signal alone, then motion is perceived

  • Testing Theory with Curare: Paralyzing eye muscles leads to perceived motion without actual movement.

  1. Perceptual Vector Analysis- influenced by the motion of nearby objects.

  • Visual system analyzes motion into vectors (directions). 

  • Common vectors cancel, leaving remaining vector(s) that provide perceived motion (relative motion). Solid lines show actual motion; broken lines represent constituent vectors. Remaining vector determines perceived motion.