Topic 4 - Visual Illusions Flashcards
Oculomotor cues
based on sensing the position of the eyes and muscle tension
- Ocular convergence
- Accommodation
Ocular convergence
inward movement of the eyes when we focus on nearby objects
Accommodation
change in the shape of the lens when we focus on objects at different distances
Monocular depth cues
Only come from one eye
Pictorial cues and motion-produced cues
Pictorial Cues
sources of depth information that come from 2-D images, such as pictures
Includes: Occlusion Relative height Relative size Familiar Size Perspective convergence Atmospheric perspective Texture gradient Shadows
Occlusion
When one object partially covers another
Relative height
objects below the horizon that are higher in the field of vision are more distant
Relative size
when objects are equal size, the closer one will take up more of your visual field
Familiar size
distance information based on our knowledge of object size
Perspective convergence
parallel lines appear to come together in the distance
Atmospheric perspective
distance objects are fuzzy and have a blue tint (on a clear day)
Texture gradient
equally spaced elements are more closely packed as distance increases
Shadows
indicate where objects are located
Motion parallax
close objects in direction of movement glide rapidly past but objects in the distance appear to move slowly
Deletion and accretion
objects are covered or uncovered as we move relative to them
Binocular depth cues
Two eyes cues
Includes binocular disparity and stereopsis
Stereoscopic depth perception
depth perception created by input form both eyes
Strabismus
misalignment of the eyes
Binocular disparity
difference in the images on the left and right retinas
- Corresponding retinal points - points on retina that would overlap - a surface called the horopter
- Noncorresponding points - images that are not on the horopter
- Absolute disparity - angle between noncorresponding points, angle of disparity
- Crossed disparity - eyes see object on the opposite side of visual field to side eye is on
- Uncrossed disparity - eyes see object on same side of visual field to side eye is on
- Relative disparity - difference between absolute disparity of two objects
Steropsis
depth information provided by binocular disparity (perceptual)
Stereoscope uses two pictures from slightly different viewpoints (one to each eye)
- Random-dot stereogram - has two identical patterns with one shifted in position
Correspondence problem
How does the visual system match images from the two eyes? – Matches may be made by specific features of objects
This may not work for objects like random-dot stereograms, A satisfactory answer has not yet been proposed
Depth perception in other species
Animals use same cues
Frontal eyes needed for binocular disparity
Lateral eyes provide a wider view
Experiment by Tsutsui et al.
binocular depth perception
- Monkeys matched texture gradients that were 2-D pictures and 3-D stereograms.
- Recordings from a neuron in the parietal lobe showed: >Cell responded to 2D pictorial cues, Cell also responded to 3D binocular disparity
Binocular depth cells, disparity selective cells
These cells respond best to a specific degree of absolute disparity between images on the right and left retinas (have a tuning curve)
Experiment by Blake and Hirsch
binocular disparity and depth perception
- Cats were reared by alternating vision between two eyes every other day for 6 months - no chance of binocular disparity
- Results showed that they: had few binocular neurons due to cortical plasticity and were unable to use binocular disparity to perceive depth
Experiment by DeAngelis et al.
binocular disparity and depth perception
- Monkey trained to indicate depth from disparate images
- Disparity-selective neurons were activated by this process (in V1)
- Experimenter used micro-stimulation to activate different disparity-selective neurons
- Monkey shifted depth judgment to the artificially stimulated disparity
Experiment by Holway and Boring
Perceiving size
- Observer was at the intersection of two hallways
○ A luminous test circle was in the right hallway placed from 10 to 120 feet away
○ A luminous comparison circle was in the left hallway at 10 feet away
○ On each trial the observer was to adjust the diameter of the test circle to match the comparison
○ Test stimuli all had same visual angle (angle of object relative to the observer’s eye)
○ Part 1 of the experiment provided observers with depth cues - Judgments of size were based on physical size
○ Part 2 of the experiment provided no depth information – Judgments of size were based on size of the retinal images (i.e., visual angle)
Visual angle
Angle of an object relative to the observer’s eye
Size constancy
Perception of an object’s size remains relatively constant
- This effect remains even if the size of the retinal image changes (or distance from the observer changes)
Size-distance scaling equation – S = K (R x D) - The changes in distance and retinal size balance each other
Size-distance scaling - Emmert’s Law
Retinal size of an afterimage remains constant
- Perceived size will change depending on distance of projection (if you look at something further away, the afterimage will appear larger and vice versa)
Muller-Lyer Illusion
Straight lines with inward fins appear shorter than straight lines with outward fins, lines are actually the same length
Misapplied Size Constancy Scaling - what works in 3D is misapplied in 2D
- Problems with this theory
Conflicting cues theory - perception depends on actual length of vertical lines in combination with overall length of the figure - compromise of perception
Ponzo illusion
Horizontal objects are placed over railroad tracks in a picture, upper object appears larger
- Misapplied size constancy scaling
Ames Room
Reasons:
Size-distance scaling - Observer thinks the room is normal, People would be at same distance – Person on the left has smaller visual angle (R) – Due to the perceived distance (D) being the same their perceived size (S) is smaller
Relative Size - Perception of size depends on size relative to other objects. – One woman fills the distance between the top and bottom of the room. – The other woman only fills part of the distance – Thus, the woman on the right appears taller
Moon illusion
The moon appears larger on the horizon than when it is higher in the sky
Apparent-distance theory - horizon moon is surrounded by depth cues while moon higher in the sky has none - Horizon is perceived as further away than the sky - called “flattened heavens”
Angular size-contrast theory - the moon appears smaller when surrounded by larger objects, Thus, the large expanse of the sky makes it appear smaller
