How we perceive depth. The constructivist theory of perception
24 important questions on How we perceive depth. The constructivist theory of perception
What do we use depth cues for?
What did Herman von Helmholtz say about cues?
---> Particularly important when viewing pictures
---> A 2-D representation is made into a 3-D image by the brain
What are the 4 primary cues?
2. State of accommodation ---> Ocularmotor
3. Binocular disparity (stereopsis) ---> Binocular
4. Motion parallax (optic flow) ---> Monocular
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What are the 5 secondary cues?
Linear perspective ---> Monocular
Texture gradient ---> Monocular
Size ---> Monocular
Shadow ---> Monocular
What is meant by the vergence angle of the eyes?
---> If the object is near (left), this angle will be relatively large. If the object is far (right), the angle will be smaller
What is meant by state of accommodation?
---> can also be used to determine relative depth.
- The lens gets fatter when viewing nearby objects
What is meant by binocular disparity (stereopsis)?
---> disparity is the basis for a vivid perception of the three-dimensionality of the world that is not available with purely monocular (one-eyed) vision.
---> The brain uses binocular disparity to extract depth information from the two-dimensional retinal images in stereopsis.
--->Because our eyes are separated by about 6 cm, they provide slightly different views of the world.
---> Binocular neurons in V1 have two receptive fields, one in each eye that are nearly identical in orientation, speed and direction preference.
What is the technical term of binocular disparity?
What are binocular disparity detectors?
What happens when very different images are viewed by the two eyes?
However, rather than fusing, the two images compete for perception, such that wherever one is visible the other is suppressed.
---> When one image is dominant - the other is removed from consciousness
What is meant by motion parallax?
(because as we move about the world the position of objects changes according to their relative depths)
- objects that are closer to the point of fixation move in the opposite direction to our movement, and objects that are further away move in the same direction as our movement
What does motion parallax provide?
(p. 161 perceptual textbook)
What is a limitation of motion parallax?
(p. 161 perceptual textbook)
What is meant by occlusion (interposition)?
---> However, the amount of depth implied by interposition can only be based on the thickness of the occluding object.
Children under 5 months are bad at this: they don't understand depth or occlusion
e.g. the lecturer standing behind a lecture stand - a 5 month old will think it is a lecture stand with a head - they won't understand that a person is standing behind the lecture stand
What is the illusion of interposition?
---> The illusion is revealed when the objects are viewed from a different angle.
(see slide 3, p. 4 lecture notes)
What is the corridor illusion?
---> the three cylinders appear to be of different sizes (despite the fact they form an identical image in the eye). The illusion occurs because the perceptual system uses linear perspective to determine that the cylinders are at different distances from the observer and takes this into account when determining size.
(see slide 1, p. 5)
What is the Muller-Lyer illusion?
---> an optical illusion consisting of a stylized arrow.
- subjects perceive the upper line segment (a) to be longer than the lower line segment (b). However, a and b are identical in length length!
One explanation of the Müller-Lyer is based on the visual system assuming that the upper segment is further away than the lower segment.
What is meant by texture gradient?
What is meant by the secondary cue, size?
What is meant by the familiar size cue?
(p. 156 perceptual textbook)
Describe the experiment using coins to judge depth.
---> Based on the subjects knowledge of this size, they assumed that the larger coin was further away and the smaller coin was closest
Note: However, this is not likely to affect the perception of subjects with no
knowledge of American coins.
(see slide 2, p. 7 lecture notes)
What demonstrates the conflict between perspective (what we see) and familiar size?
In this situation, we perceive an abnormality in size rather than reevaluating the depth. This illusion is based on our assumption that all rooms are rectangular.
= It would appear that perspective cues have more of an influence on the visual system than familiar size cues.
Conflict between what we know and what we see
What is meant by the secondary cue, shadow?
What are atmospheric cues?
---> this cue is based on the implicit understanding that light is scattered by the atmosphere. More light is scattered when we look through more atmosphere. Thus, distant objects are subject to more scatter and appear fainter and less distinct (p. 157 perceptual textbook)
(why far away mountains look fuzzier than closer mountains)
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