Depth Perception part 1 Flashcards
Name the 5 Mechanisms of perception
1) convergence of the eyes
2) Accomodation of the lens
3) Binocularity : Stereopsis spatial disparity fusion / Matching
4 Neural Mechanism- activation/ Inhibition Location sesitivity
5)Depth cues and cue intergration
convergence
inward movement of the eyes when we focus on nearby objects
accommodation
the term for the adaptation of the lens of the eye
the lens breaks the light wave which is emitted from the object these waves are then absorbed by the receptor cells in the retina
the closer the object the more the lens needs to be thicker. adaption of the lens can be corrected by prescribing concave or convex lenses
what is the Horopter
denotes a half circle in the visual field of the observer.
binocular depth information, objects that do not fall on the horopter fall on noncorresponding points
these points make disparate images AD and RD ?
Absolute Disparity
concerns one object the amount of disparity indicates how far an object is from the horopter
Relative Disparity
Is the difference between the absolute disparity of two objects.
stereopsis
depth information provided by binocular disparity
false fusion
occurs in near space when two pictures are presented seperately to each eye in the stereoscope
Sherrington 1906
panums limit is the distance at which fusion occurs
occlusion is over known as
over lap
interposition
always involves the violation of the contours of the overlapped object
size reduction
even in the absense of explicit viewpoint perspective the adult viewer seems - with a sequence of objects - to perceive the smallest object as furthest away
convexity and concaveity
£D perception can be produced with black andwhite shading or light which comes above or below
the variation of the objects surface is also called texture gradient
atmospheric perspective
space further away is depicted in lighter colours and contours are more blurry than those in the forground
view point perspective
discovered in the renaissance in europe by using a grid which worked like a transparent layer over reality
viewpoint perspective is created by diagonal spatial axes which converge in a viewpoint.
Motion paralax
occurs when the observer or the scene is moving.
objects nearer to the observer hae to travel further and faster across the retina.
thus we can deduce the slower the moveing object - the further away.
ISI
Inter stimulus Interval
the time between two stimuli
if very short then the obderver simply sees two dots
if the ISI is too long the observer sees one dot flash and then one dot flash
but if the ISI is Intermediate then the viewer sees APPARENT MOTION
Perception of causality
Pure percieved motion can also be interpreted in meaningful ways
HEIDER + SIMMEL 1944 made a film with geopmetric objects moving
participants were instructd to write down what happened
Anthropomorphic interpretations of movement perception
Anthropomorphic interpretations of
movement perception
Perception of causality
MICHOTTE 1946
showed people films of little blocks interacting
further research suggests that not every person would interpret these visual cues as meaning full
Ie BERRY MISOVICH ini 1994
rigidity assumption
using moving lights as cues , participants can interpret moving dots as being from a human form with as little as 6
infants can also do this
Binocular depth Cells
or disparity selective cells have been found that respond best to binocular disparity
These cells respond best to a specific degree of absolute disparity between images on the right and left retinas.
Disparity tuning curve
a graph showing the response time a neuron on the x (verticle) axis and the degree of horizontal disparity on the y axis.
this shows more spikes per second for the near objects indicating that the neuron is specialized for near objects
BLAKE AND HIRSCH and their pirate cats
in 1975
cats were reared for 6 months by alternating vision between two eyes daily
the cats were found to not develop many binocular neurons
cats were unable to use binocular disparity to percieve depth
they developed esotropic (inward) and Exotropic (outward) squints
linear size of an object ie its actual size is reffered to as S . what is the visual angle ?
V
the visual angle V determines the size of the retinal image
this it the factor that perception scientists care about
size- distance scaling equation-
s = A (RxD)
Percieved size S is the result of the angle A
real object size - R
object distance - D
the changes in distance and retinal size balance each other out
size constancy
object size estimation
- remains tied to the realobject despite various changesin distance
this breaks down when contextual depth cues are removed and poeple are only alowed a peep hole view of the object its self
HOLWAY + BORING 1941
MULLER-LYER Illusion
with the two lines with arrows pointing in and out
the two lines look different lengths but they are not
why?
Why does this length illusion occur?
Misapplied size-constancy (Gregory, 1966)
Size constancy scaling that works in 3-D is misapplied to 2-D objects.
Observers unconsciously perceive the fins like corners, as if they were corners belonging to buildings.
Gregory 1966
Misapplied size-constancy (Gregory, 1966)
Size constancy scaling that WORKS in 3-D is MISAPPLIED to 2-D objects.
Observers unconsciously perceive the fins like corners, as if they were corners belonging to buildings.
what are the problems with Gregorys explanation for the mis applied size constancy (the corner of a building makes one line seem far away, and thus bigger)
The “dumbbell” version shows the same perception even though there are no “corners.”
Still, one could argue that the circles resemble weights for weight lifting
Muller-lyer three dimensional illusion ?
The illusion ( with the line and the arrows pointing in and out) also occurs in a 3-D display with the line not actually present.
Muller-lyer illusion
potentially most plausible explanation
Another possible explanation:
Conflicting cues theory - our perception of line length depends on:
The actual length of the line
The overall length of the figure
The conflicting cues are integrated into A
COMPROMISED LENGTH PERCEPTION
the Ames room
Two people of equal size appear to be of very different in size in this room.
The room is constructed so that:
The shape looks like a normal room when viewed with one eye.
On the next slide, a photo has been taken through the peephole showing what the participants in the experiment were perceiving.
the Ames room
two explanations -
size distance scaling
Observer thinks the room is normal.
Women would be at same distance.
Woman on the left has smaller visual angle (R).
The participant computes an assumed distance (D’), but the true distance D is larger and thus the perceived size (S) is smaller
relative size - Perception of size depends on size relative to room height (spatial context)
Moon illusion
where the moon appears to be larger when it is low in the sky and close to the ground
APPARENT DISTANCE THEORY - horizon moon is surrounded by depth cues, while moon higher in the sky has none.
ANGULAR SIZE CONTRAST THEORY- the moon appears smaller when surrounded by larger objects. (the heavens!)
Actual explanation may be a combination of a number of cues such as atmospheric perspective.
