Chapter 8: Vision in three dimensions

1. Chapter 8: Vision in three dimensions Basic issue: How do we construct a three-dimension visual experience from two-dimensional visual input? Important terms: Allocentric direction (or location): understanding positions in space irrespective of viewers’ perspective. Egocentric direction (or location): understanding positions in space from a specific viewer’s perspective. Absolute distance: information about distances of objects from the observer Relative distance: information about distances of various objects in visual field from one another.

2. Inverting goggles: Are up and down innate or learned? • In the late 1890’s George Stratton put on inverting lenses to see if he could adapt to a world upside down. He did. Since then other studies have investigated this phenomenon. What do they indicate: • People can adapt remarkable well with time to an inverted world. • The adaptation appears to be largely motor-based, not visual. • Thus it appears that the up/down axis is “imprinted” on the brain.

3. Dept and distance cues Oculomotor: refers to those depth cues arising from the muscular adjustments of the eye to a changing visual scene. These are the only cues which give unambiguous information about absolute distances. There are two oculomotor cues. Accommodation: refers to the degree of strain exerted by the muscle controlling the shape of the lens. As objects get closer the muscle increases its strain (fattens the lens), as object moves further away the muscle decreases its strain (flattens lens). Degree of strain then, is an indicator of distance.   Problem -- muscles assume most relaxed state for an object about 10ft away or so. So variations in strain are only useful for a very limited range, and even in that range are not terribly accurate. Convergence: refers to the degree of strain exerted by the muscles extra-ocular muscles controlling the eye movements. Again, as object gets closer, muscle strain and angle of convergence increases, thus this indicates distance from eyes. Evaluation -- convergence is fairly reliable cue within about a 20ft range, after that muscle strain is not present.

4. Dept and distance cues Using two eyes to understand depth/distance: Stereopsis Binocular dept/distance cue: Retinal disparity – difference between the two eyes’ view of a single scene.

5. Retinal Disparity Disparity increases as depth difference between objects increases (left) Horopter: semi-circle of same perceived (egocentric) distance Corresponding retinal points: along horopter, where images are fused into single percept Non-corresponding: off horopter where images remain unfused(crossed – in front; uncrossed – behind)

6. But how does fusion happen? • Random dot stereograms • Fusion, not based on common features, but something even more primitive: spatial frequency

7. Binocular rivalry Different (un-fusable) images presented to corresponding retinal points (foveas usually). Results in perceptual switching over time

8. Stereoblindness: inability to use disparity as depth cue Disparity is powerful dept cue; reason why eyes of are frontally placed with overlapping visual fields. But stereoblindness does not mean complete depth blindness, other cues can be used. Common cause of stereoblindness: strabismus – misalignment of eyes such that they don’t work together effectively

9. Dept and distance cues Monocular cues: useful with only one eye (pictorial cues) 1) Linear perspective: parallel lines tend to converge as distance increases.  2) Texture gradient: elemental structure of ground tends to become more homogenous as distance increases.

10. Monocular cues 3) Interposition: the visual occlusion (or blocking out) of one object by another. 4) Retinal image size: closer object tend to cast larger retinal images, familiarity with true object size, or size relative to other familiar objects in visual scene facilitate use of this cue.

11. Monocular cues 5) Aerial perspective: haze tends to occlude objects as they get further away.   6) Motion parallax: as one moves close surfaces tend to move swiftly in the opposite direction of motion, while far objects tend to move slowly in the same direction of movement.

12. Illusions based on monocular cues Moon illusion: moon at zenith smaller than moon at horizon. Relation between retinal image size and perceived distance (Emmert’ Law). Depth cues make moon look farther away on horizon

13. Illusions based on monocular cues • Ponzo illusion: linear perspective affects perceived distance

14. Illusions based on monocular cues • Muller/Lyer illusion