Space Perception and Binocular Vision

Space Perception and Binocular Vision Space Perception Monocular Cues to Three-Dimensional Space Binocular Vision and Stereopsis Combining Depth Cues 9/30/2008 1 Introduction to Space Perception Realism: The external world exists Positivists: The world depends on the evidence of the senses; it could be a hallucination! 9/30/2008 2 1

9/30/2008 3 Introduction to Space Perception (cont d) Euclidian geometry: Parallel lines remain parallel as they are extended in space Objects maintain the same size and shape as they move around in space Internal angles of a triangle always add to 180 degrees, etc. 9/30/2008 4 2

Euclidean Geometry and the Retina 9/30/2008 5 Introduction to Space Perception (cont d) Binocular summation: An advantage in detecting a stimulus that is afforded by having two eyes rather than just one 9/30/2008 6 3

Introduction to Space Perception (cont d) The two retinal images of a threedimensional world are not the same! 9/30/2008 7 Introduction to Space Perception (cont d) Binocular disparity The differences between the two retinal images of the same scene. It is the basis of stereopsis; a vivid perception of the three-dimensionality of the world that is not available with monocular vision. 9/30/2008 8 4

Introduction to Space Perception (cont d) Monocular depth cues vs. Binocular depth cues: One eye vs. two eyes Binocular depth cues provide: Convergence Stereopsis Ability of two eyes to see more of an object than one eye 9/30/2008 9 Monocular Cues to Three-Dimensional Space Occlusion A cue to relative depth order when, for example, one object obstructs the view of part of another object 9/30/2008 10 5

Monocular Cues to Three- Dimensional Space (cont d) Nonmetrical depth cue vs. Metrical depth cue Nonmetrical: Provides information about the depth order (relative depth) but not depth magnitude (e.g., his nose is in front of his face) Metrical: Provides quantitative information about distance in the third dimension 9/30/2008 11 Monocular Cues to Three- Dimensional Space (cont d) Size and position cues: Relative Size: A comparison of size between items without knowing the absolute size of either one Texture Gradient: A depth cue based on the geometric fact that items of the same size form smaller images when they are farther away Relative Height: Objects at different distances from the viewer on the ground plane will form images at different heights in the retinal image 9/30/2008 12 6

Relative Size 9/30/2008 13 Texture Gradient 9/30/2008 14 7

Texture Gradient without Relative Height Cues 9/30/2008 15 Relative Height 9/30/2008 16 8

Height Illusion 9/30/2008 17 Monocular Cues to Three- Dimensional Space (cont d) Aerial perspective: A depth cue that is based on the implicit understanding that light is scattered by the atmosphere Example: Haze 9/30/2008 18 9

Haze (or Aerial Perspective) 9/30/2008 19 Real-World Aerial Perspective 9/30/2008 20 10

Monocular Cues to Three- Dimensional Space (cont d) Linear perspective: A depth cue based on the fact that lines that are parallel in the three-dimensional world will appear to converge in a two-dimensional image 9/30/2008 21 Monocular Cues to Three- Dimensional Space (cont d) Vanishing point: The apparent point at which parallel lines receding in depth converge 9/30/2008 22 11

Monocular Cues to Three- Dimensional Space (cont d) The previous were all pictorial depth cues (a cue to distance or depth used by artists to depict threedimensional depth in two-dimensional pictures) other monocular cues are based on motion Motion cues: Motion parallax: Based on head movement; geometric information obtained from two eyes in different positions in the head the same time 9/30/2008 23 Motion Parallax 9/30/2008 24 12

Monocular Cues to Three- Dimensional Space (cont d) Accommodation and convergence help eyes perceive depth: Accommodation: Eye changes its focus Convergence: Ability of the two eyes to turn inward; reduces the disparity of the feature to zero, or near zero Divergence: Ability of the two eyes to turn outward; reduces the disparity of the feature to zero, or near zero 9/30/2008 25 Vergence 9/30/2008 26 13

Binocular Vision and Stereopsis Binocular disparity: Differences between the images falling on the two retinas 9/30/2008 27 Disparity Horopter: When you look an an object at some depth, you position our eyes so that the image it produces on both retinas correspond (i.e., zero disparity). The horopter is the imaginary 3D surface that extends from this object to include all other points at which the images fall onto corresponding places in both eyes. 9/30/2008 28 14

Stereopsis from Binocular Disparity 9/30/2008 29 The Vieth-Müller Circle 9/30/2008 30 15

Relative Disparity 9/30/2008 31 Crossed vs. Uncrossed Disparity 9/30/2008 32 16

9/30/2008 33 Binocular Vision and Stereopsis (cont d) Stereopsis: Popping out in depth Most humans are able to see this way How exactly does this translation from stimulus attribute to perception take place? 9/30/2008 34 17

Binocular Vision and Stereopsis (cont d) Stereoscope: A device for presenting one image to one eye and another image to the other eye, creating a single, threedimensional design 9/30/2008 35 Wheatstone s Stereoscope 9/30/2008 36 18

A Stereo Photo 9/30/2008 37 Binocular Vision and Stereopsis (cont d) Free fusion: The technique of converging (crossing) or diverging the eyes in order to view a stereogram without a stereoscope 9/30/2008 38 19

Free Fusion 9/30/2008 39 Binocular Vision and Stereopsis (cont d) Some people do not experience stereoscopic depth perception because they have stereoblindness An inability to make use of binocular disparity as a depth cue Can result from a childhood visual disorder, such as strabismus, in which the two eyes are misaligned 9/30/2008 40 20

Binocular Vision and Stereopsis (cont d) Random dot stereograms can only be seen with binocular cues; they contain no monocular depth cues Use cyclopean stimuli 9/30/2008 41 Random Dot Stereogram 9/30/2008 42 21

Binocular Vision and Stereopsis (cont d) Correspondence problem: Figuring out which bit of the image in the left eye should be matched with which bit in the right eye 9/30/2008 43 Stereovision in the brain Binocular neurons The neurons in the brain that are responsible for figuring out disparity must receive inputs from both eyes. So, we know that retinal cells (photoreceptors, retinal ganglion cells) cannot perform this. Likewise, LGN neurons are organized in alternating layers of eye-of-origin. And, although we talked about ocular-dominance columns in V1, there are also some binocular neurons: neurons that receive input from both eyes. After V1, all visual neurons are binocular. 9/30/2008 44 22