Section 4 Flashcards
What does it mean for the three dimensional world to be Euclidean?
It respects a set of basic geometrical rules, such that size, shape, and relative coordinates of objects remain constant when viewed at different distances
What is Euclidian geometry?
Objects remain the same size and shape as they move around in space
What are the two main ways our visual system infers the third dimension?
- monocular clues
2. binocular cues
What are the two main monocular depth cues?
Pictorial and movement-based cues
List all the pictorial cues
- occlusion
- relative size
- familiar size
- relative height
- texture gradient
- linear perspective
- aerial perspective
- shadows
What is the difference between a non-metrical and metrical depth cue?
A non-metrical depth cue cannot tell you anything about the relative or absolute distance between two objects, whereas a metrical depth cue can
Which pictorial cue is an absolute metrical depth cue?
Familiar size
How does relative height work?
Objects that appear higher in the visual field tend to be perceived as being further away
When do we experience the greatest feeling of distance between us and an object?
When the object is parallel to the ground
How does linear perspective help us capture the third dimension?
By relying on the idea that objects become smaller and closer to each other as they recede in space
What is a vanishing point?
The apparent point at which parallel lines receding in depth converge
Which combination of pictorial cues generates linear perspective?
relative size and relative height
What is the ponzo illusion?
When two parallel lines extend into the distance, our visual system makes us think they are converging together. If you place an object at two different points along these converging lines, because the further lines look smaller and closer together it will make the object placed further look bigger
How do shadows infer the third dimension?
If the shadows of objects are at different distances in the visual field, then the objects will also look to be at different differences
How do objects appear if their shadows are aligned?
It will look like the objects differ in height but not distance
How does motion parallax work?
It triggers the perception of depth based on the perceived translational movement of objects
How does optic flow work?
Allows the perception of objects while looking at where you are headed to
What is the focus of expansion?
The fixation point at the horizon from which the world seems to expand or vanish as you go forward or backwards, respectively
What two muscle groups send sensory feedback to your brain as you adjust your focus point?
Ciliary muscles and extraocular muscles
What information do the ciliary muscles send to the brain?
informs on the level of accommodation required to focus the image on the retina
What information do the extraocular muscles send to the brain?
informs on the level of vergence
Nearing objects cause _____ (convergence/divergence) of the eyes to keep the fixated object at the center of both retinas, whereas fixating a far away object increases eye _______ (convergence/divergence)
Convergence, divergence
TRUE/FALSE
Unlike most pictorial cues, oculomotor cues provide us with unambiguous information about absolute depth
true
What is stereopsis?
The rich impression of depth that we get from seeing with both eyes
Describe an advantage and disadvantage of binocular summation
Advantage: optimizes our interactions with objects in front of us
Disadvantage: narrows down the extent of our visual field
What is binocular disparity?
The difference between the two retinal images of the same scene
What is the concept of corresponding retinal points?
States that points on the retina of each eye where the monocular retinal images of a single object are formed are at the same distance from the fovea of each eye
What is the Vieth-Muller circle?
A circle drawn based on the point of fixation, whereby points that fall on this circle will have corresponding retinal points and be seen as one image
What happens when an object does not fall on the Vieth-Muller circle?
It will have dissimilar points on the two retinas which will make you see double of the object
What is the horopter?
It is a semi circular delineation where points along this path have zero disparity between the two retinal images
What is Panum’s fusional area?
An area around the horopter where the retinal disparities are small enough for our brain to correct them, so images from each eye will still be seen as one
What is a stereothreshold?
The smallest perceptible distance between two objects before they merge into one
When does diplopia occur?
When objects lie outside of Panum’s fusional area
What happens as object move further away from the limits of Panum’s fusional area?
The greater the duplication of the objects until they start to blur with the background
What is crossed disparity and how does it occur?
It is when the position of an object in the visual field is opposite to the eye’s anatomical position (projects onto the temporal retinas) and arises when an object is located in front of the horopter
What is uncrossed disparity and how does it occur?
It is when an image will be projected on the nasal retinas and therefore will appear to be located on the same side of the visual field as the eyes anatomical position. it arises when an image is located behind the horopter
How does the visual cortex determine whether an object is near or far?
By computing the binocular coordinates of the retinal projection of objects relative to the fovea
How are binocular neurons organized?
Their organization follows a gaussian distribution where neurons that respond to zero disparity are the most numerous
What do we call binocular neurons that respond to crossed and uncrossed disparity?
tuned near and tuned far
Based on neuron abundance, why do we experience diplopia?
Neurons that respond to objects outside of Panum’s fusional area are less abundant
TRUE/FALSE
Each binocular neuron has its own specificity for the amount of disparity it prefers
TRUE
What are the two hypotheses for how our visual system knows an object seen by the left and right eye are the same?
Hypothesis 1: object recognition is a monocular process that is followed by the binocular integration of mages from which stereopsis emerges
Hypothesis 2: binocular integration happens automatically without the need for object recognition
What is a random dot stereogram?
The stimuli are composed of dots that are distributed in a random fashion. Part of the image is duplicated so that its superposition with a stereoscope gives rise to a 3D image that can only be perceived with stereopsis
Why can a random dot stereogram determine which hypothesis is true?
It can only be seen stereoscopically, which is a process that cannot be done with monocular vision - this proves hypothesis 2 is the correct pathway
What is binocular rivalry?
Occurs when the images seen from the left and right eye are non-corresponding
How does the brain deal with binocular rivalry?
If the images are different the brain will choose what it wants to perceive, whereas if they are of similar interest the perception of the two images with alternate in time over the course of minutes