2 3D Space PerceptionThe flat retinal image problem: How do we reconstruct 3D-space from 2D image? What information is available to support this process?Interaction between Perceived Size and Perceived Distance (both depend upon “scaling”)
3 Size ConstancyPerceived size is not slavishly linked to retinal size; otherwise your car would appear to be smaller when observed at increasing distances. Instead, perceived size tends to remain invariant across observation distance…a phenomenon known as size constancy. Perceived size depends upon the psychological scaling of retinal size relative to perceived distance. Hence, size, distance and 3D visual perception are all based upon a more complex process known as spatial scaling.Corollary:Perceived Speed = retinal velocity x scaled distance
5 Failures of Size Constancy The Buechet ChairClick here for more
6 The Ames Room (Iowa State University at Ames, IA) Failures of Size ConstancyFigure 12.17Horizontal line at the bottom appears smaller than the one above, even though both are of identical length.So-called Ponzo illusion arises because the two segments are taken to be at different relative distances by the pictorial cue of the converging lines.Upper line segment is perceived to be larger in order to satisfy the size-distance relationship.The Ames Room (Iowa State University at Ames, IA)
8 Oculomotor Information State of AccommodationState of Vergence
9 AccommodationIn theory, the efferent signal driving the ciliary muscles (and/or afferent feedback from stretch sensors in the ciliary muscles) could be used by higher-order visual processes to help scale 3D space and/or visual distance.There is little evidence to support this hypothetical role of accommodation.
10 Vergence Eye Movements Support for the role of efferent/afferent 3D information from vergence eye movements comes from:“Tower Speed Illusion”andBotox Treatment of Rectus Muscles in Strabismus Surgery
11 Static Monocular Sources of 3D Information OcclusionFamiliar Size (Relative Size)Texture GradientsLinear PerspectiveAerial Perspective (Atmospheric extinction)Shadow Casting
12 OcclusionNear objectsblock visual access to far objects
13 Linear Perspective Parallel lines on the visual plane converge toward the “vanishingpoint” with increasingobservation distanceThis law of projectivegeometry provides astrong cue about distanceand 3D space.
14 Linear Perspective Figure 12.17 Horizontal line at the bottom appears smaller than the one above, even though both are of identical length.So-called Ponzo illusion arises because the two segments are taken to be at different relative distances by the pictorial cue of the converging lines.Upper line segment is perceived to be larger in order to satisfy the size-distance relationship.
16 Familiar Size/Relative Size Objects of the same physical sizeproject different size retinal images depending upon the observation distance.This knowledge and prior experience contribute to 3D space perception.What is the height of thissculpture in feet?
17 Familiar Size CueNovel objects can be psychologically scaled given visual referencesof known size. For example…Easter Island Sculpturewith Familiar Size CueEaster Island Sculpturewithout Familiar Size Cue
18 Texture Gradients An extended surface with uniform spatial texture will project a retinal image with a non-uniform texture gradient that increases in spatial frequency as observation distance increases.
19 Aerial PerspectiveParticulate matter in the atmosphere scatters light; reducing contrast and intensity of the retinal image.The light from distant objects must pass through more atmosphere than the light from near objects.
20 Shadow CastingJust as occlusion of objects serves as a powerful cue for depth…occlusion of the illuminant (sun) forms shadows which provide a powerful source of information for extracting 3D representations from a 2D retinal image.
21 Identify the Monocular Depth Cues A Rainy Day in ParisGustav Caillebotte ( )
22 Identify the Monocular Depth Cues Linear PerspectiveOcclusionTexture GradientAerial PerspectiveShadow CastingFamiliar SizeA Rainy Day in ParisGustav Caillebotte ( )
23 Dynamic Monocular Sources of 3D Information Motion ParallaxRelative Angular VelocityRadial Expansion/LoomingMoving Shadows
24 Motion ParallaxMotion parallax occurs when an observer fixates a point at intermediate distance and then rotates their head.Objects in the distance appear to move WITH head motion; while objects closer than the fixation plane appear to move AGAINST the rotation of the head.Motion Parallax and DynamicShadow Casting Demo
25 Optic Flow: Radial Expansion Optic Flow and Driving Demo
28 Advantages of Binocularity Redundancy (survival value)Stereopsis (Predators) Large Field-of-View (Prey)Binocular summation improves sensitivity by √2 (signal:noise ratio sampling theory) Binocular acuity better than monocular; Same for CSF and many other functions
29 Binocular Vision (cont.) Binocular rivalry (Role of the “dominant” eye)Autokinesis phenomenon
30 StereopsisAbility to use binocular retinal disparity information to extract relative depth information from the retinal image pairsRetinal “mismatch” can be used to reconstruct much of the missing 3rd dimension from the flat retinal images
31 Retinal Disparity Understanding begins with a consideration of the geometry of the horopter
32 Horopter (Corresponding Retinal Images) The HOROPTER is an imaginarysurface whose points are all at thesame distance as the fixation point.Points on the horopter project tocorresponding locations on the temporal and nasal retinas,respectively.These corresponding locationsexhibit zero retinal disparityi.e.,D = dtemporal – dnasal = 0
33 “Crossed” and Uncrossed” Retinal Disparity The corresponding locations for the“closer” green stimulus exhibitspositive retinal disparityD = dtemporal – dnasal > 0 (or “crossed” disparity)“farther” red stimulus exhibitsnegative retinal disparityD = dtemporal – dnasal < 0 (or “uncrossed” disparity)
35 Panum’s Fusion Area Figure 12.27 Finite range of disparities encoded by binocular neurons means that only a limited range of relative depths can be captured by the visual system.Panum's fusional area defines the depth range in front of and behind the horopter within which an object is visually fused and capable of generating stereoscopic depth perception.Objects outside this area are seen as double (diplopic).
36 Nativists v. Empiricists “Debate” Nativist position The CNS is capable of processing many environmental invariants at birth – giving rise to direct perception (e.g., James Gibson)Empiricist position Sensory information is too impoverished to explain perceptual experience without recourse to knowledge about the world; it is based upon “unconscious inference” (e.g., Bishop Berkeley)
37 Support for NativismEleanor Gibson’s Visual Cliff Experiment (and HRD replication studies)Bela Julesz’s Random Dot Stereogram paradigm
38 Random Dot Stereograms Can retinal disparity yield perception of depth independent of knowledge about the nature of the world?Nativist vs. Empiricist DebateBela Julesz
39 Red-Blue Anaglyph Technique (black background) Anaglyph glasses transmit RED and MAGENTA dots to the left eye; and, the BLUE and MAGENTA dots to the right eye.Demo stimulus from USD’s PSYC 301 Lab