Perceptual Processes Ch 6

• Depth and space are perceived and interacted with in different ways
• Two main perspectives on space/existence are Realism and Positivism

Realism vs. Positivism

• Realism states that the external world exists
• Positivism claims the world relies on the evidence of the senses, so it could simply be a hallucination

How Space is Constructed

• Euclidean geometry is a mathematical system for understanding relationships and properties of geometric shapes in space
• The rules of Euclidean geometry follow
• Parallel lines remain parallel as they extend into space
• Object shapes and sizes stay the same as they move through space
• Internal angles of a triangle always add up to 180 degrees

Problems With Euclidean Geometry

• Images projected onto the retina are non-Euclidean
• The brain constantly uses non-Euclidean geometry
• People use non-Euclidean information to interact with Euclidean space

Retinal Image

• Each eye gets distinct retinal images
• Eyeballs are in slightly different positions
• Light enters each eye from a slightly different angle

Binocular Vision

• Visual spares are always beneficial
• Probability summation increases the possibility of stimulus detection from having two or more samples
• Binocular summation combines signals from each eye to improve task performance

Predator vs. Prey Eye Placement

• Predators have eyes on the front of their head resulting in better depth judgement for hunting due to better visual field overlap
• Prey tend to have eyes on the sides of their head meaning poor depth perception and wider field of vision

Binocular vs Monocular Vision

• Binocular vision relies on the use of both eyes
• Monocular vision relies on the use of one eye

Binocular Disparity

• The differences between the two retinal images of the same scene give the ability to see 3 dimensions in space
• Disparity forms the basis for stereopsis, the vivid perception of the three-dimensionality of the world that is not available with monocular vision
• The Horizontal Lang two pencil test helps diagnose problems with stereopsis, and the task is significantly harder when relying on monocular vision

Depth Cues

• Depth cues are information about the third dimension (depth) within visual space
• Monocular depth cues are available even when the world is viewed with only one eye, they tend to act at 2 meters away or more
• Binocular depth cues rely on information collected from both eyes, and they tend to act within 2 meters

Monocular Depth Cues

• Pictorial depth cues are used by artists to depict three-dimensional depth in two-dimensional pictures

Occulsion

• Occulsion is where one object partially blocks another object
• The overlapping object is closer, and the occluded object is away
• Occulsion is the most reliable of all the depth cues
• Occulsion fails in the case of accidental viewpoints
• Occulsion does not tell any information about how far away anything is

Metrical vs. Nonmetrical Depth Cues

• Metrical depth cues provide quantitative information about distance in the third dimension
• Nonmetrical depth cues provide information about the depth order/relative depth but not depth magnitude
• Occulsion gives a nonmetrical depth cue

Relative Size and Height

• Relative size is comparison of size between items without knowing the absolute size of either one, smaller objects are assumed to be farther away than larger objects
• Relative height is where objects touching the ground that are higher in the visual field appear farther away; in the sky above the horizon, objects lower in the visual field appear farther away

Texture Gradient

• Texture gradient is a depth cue that is tied to the geometric properties where items of the same size form smaller closer, spaced images, the farther away they get
• Texture gradients result from a combination of the cues of relative size and relative height
• Texture gradients work better vertically than horizontally because it uses relative height and relative size

Relative Metrics

• Relative size and relative height both provide some metrical information
• Relative metrical depth cue could specify object A is twice as far away as object B without providing information about the absolute distance to either A or B

Familiar Size

• Familiar size is a cue based on knowledge of the typical size of objects
• The cue of familiar size often works in conjunction with the cue of relative size
• Familiar size can provide precise metrical information when the actual size of the object along with the visual angle are known

Absolute Metrical Depth Cues

• Absolute metrical depth cues provide quantifiable information about distance in the third dimension

Aerial Perspective

• Aerial perspective is a depth cue based on light being scattered by the atmosphere
• Distant objects appear fainter, bluer, and less distinct

Linear Perpective

• Linear perspective is a Monocular Depth cue
• Parallel lines in the three-dimensional world will appear to converge in a two-dimensional image as they extend into the distance
• The vanishing point is the apparent point at which parallel lines receding in depth converge

Anamorphosis

• Anamorphosis is the use of linear perspective to create a two-dimensional image so distorted that it looks correct only when viewed from a special angle or with a mirror countering the distortion

Triangulation Cues

• Triangulation cues provide depth information that cannot be reproduced in static 2D images and require a third kind of info that cannot be found in a single static picture
• They are made by comparing perspectives from multiple vantage points
• Can be monocular or binocular

Motion Parallax

• Motion parallax is a depth cue based on head motion where geometric information obtained from an eye in two different positions at two different points in time is similar to the information from two eyes at two different positions in the head at the same time
• Images closer to the observer move faster across the visual field, the brain uses this information to calculate the distances of objects in the environment
• Head movements and any other relative movements between observers and objects reveal motion parallax cues

Optic Flow

• Optic flow is the pattern of apparent motion of objects in a visual scene produced by relative motion between the observer and the scene
• Adds on motion to the ideas of relative height and relative size
• Objects get bigger as they move toward you and smaller as they move away

Eye and Focus

• The eyes observe a visual scene
• Accommodation is where the eye changes focus and the lens gets fatter as gaze is directed toward nearer objects
• Convergence is the ability of the two eyes to turn inward, often used to focus on nearer objects
• Divergence is the ability of the two eyes to turn outward, often used to focus on farther objects
• Vergence angle is the angle formed by the lines from each eye to the object of focus

Binocular vision & Stereopsis

• Corresponding retinal points are a geometric concept stating 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 in each eye
• This only happens for objects that lie on the horopter
• Horopter is the location of objects whose images lie on the corresponding points
• Vieth-Müller circle is the location of objects whose images fall on geometrically corresponding points in the two retinas, and is essentially the same as the horopter

Binocular Vision and Viewing

• Objects on the horopter appear as single images when viewed with both eyes
• Panum's fusional area is the region of space, in front of and behind the horopter, within which binocular single vision is possible
• Objects that are closer to or farther away from the horopter fall on noncorresponding points in the two eyes and appear as two images

Double Vision

• Diplopia is double vision
• Stimuli visible in both eyes that lie outside of Panum's fusional area will appear diplopic

Disparity

• Crossed disparity is created by objects in front of the plane of the horopter
• They appear to be displaced to the left in the right eye and to the right in the left eye
• Uncrossed disparity is created by objects behind the horopter
• They appear to be displaced to the right in the right eye and to the left in the left eye

Free Fusion vs. Stereoblindness

• Free fusion is the technique of converging (crossing) or diverging (uncrossing) the eyes in order to view a stereogram without a stereoscope
• Stereoblindness is an inability to make use of binocular disparity as a depth cue
• Stereoblindness can result from a childhood visual disorder where the two eyes are misaligned
• Most people who are stereoblind do not even realize it

Random Dot Stereogram

• Random dot stereograms (RDS) are a stereogram made of a large number of randomly placed dots that contain no monocular cues to depth
• Stimuli visible Stereoscopically in RDSs are cyclopean stimuli, or stimuli defined by binocular disparity alone

Correspondence Problem

• The correspondence problem is figuring out which bit of the image in the left eye should be matched with which bit in the right eye in binocular vision
• Images on the horopter make this easy
• The problem is challenging in images like random dot stereograms where the points do not line up geometrically on the fovea

Solving the Correspondence Problem

• The correspondence problem can be solved in several ways
• Blurring the image helps
• Leaving only the low-spatial frequency information
• Uniqueness constraint claims that a feature in the world is represented exactly once in each retinal image
• Continuity constraint states that, except at the edges of objects, neighboring points in the world lie at similar distances from the viewer

Stereopsis Processing in the Brain

• Input from two eyes must converge onto the same cell
• Many binocular neurons respond best when on corresponding points in the two retinas
• Other binocular neurons respond best when similar images occupy slightly different positions on the retinas of the two eyes
• Neurons being tuned to binocular disparity
• Stereopsis can be used as both a metrical and nonmetrical depth cue
• Some cells just code whether a feature lies in front of or behind the plane of fixation (nonmetrical depth cue)
• Other cells code the precise distance of a feature from the plane of fixation (metrical depth cue)

Combining Depth Cues

• Illusions relating to the construction of space takes into account depth cues when interpreting the size of objects

Ponzo Illusion

• The Ponzo illusion combines relative height, linear perspective, and texture gradients
• Objects higher in the image are seen as larger
• Parallel lines that seem to converge suggest distance
• Repeating textures that get smaller