Week 1 Lecture - Size Perception PDF

**Size perception**   Key words **Size:** visual angle, spatial frequency, perceived size **Size constancy**: size distance scaling, Emmert\'s law **Illusions**: Ponzo, Tichner, Ames Room **Theories**: Constructivist, Direct   Perceiving size: \"the problem\" - Two objects that differ in...

**Size perception**   Key words **Size:** visual angle, spatial frequency, perceived size **Size constancy**: size distance scaling, Emmert\'s law **Illusions**: Ponzo, Tichner, Ames Room **Theories**: Constructivist, Direct   Perceiving size: \"the problem\" - Two objects that differ in size can subtend the same visual angle if they are placed at the appropriate distance - Visual angle: a measure of image size on the retina, corresponding to the number of degrees the image subtends from its extremes to the focal point of the eye   Visual angle - a = 2 arctan - A= visual angle, d= object distance, h= object height   What\'s the problem? - We want to know about size in the world (distal stimulus) but the visual system only has size in the image (proximal stimulus) to work with   Spatial frequency -  - \"fat bars\" -\> low spatial frequency   Physiology - Receptive fields are (retinal image) size specific - E.g. centre-surround ganglion cells, requires lateral interactions, varying scales (sizes) - Size specific threshold adaptation reflects retinal spatial frequency but we are better at judging object size - i.e. we are consciously aware of object size rather than retinal image size   Perceiving size as visual information changes - Holway and Boring\'s (1941) experiment: Observer sits at intersection of 2 hallways - Comparison luminous rings presented at a set distance - Test luminous rings presented at different distances but scaled so as to always project the same visual angle - Test distance \~3-36 m (10 to 120 feet) - A luminous comparison circle is 10ft away from the observer - Luminous test circles are presented at distances ranging from 10-120ft - Scaled to always have the same visual angle (1°) - Experimental design - Varied distance of standard stimulus - Task: say when the size of the comparison stimulus matches the standard stimulus - IV: Available depth information - Binocular viewing - Monocular viewing - Monocular through artificial pupil - Monocular through artificial pupil + tunnel   Theoretical accounts 1. Constructivist: \"take account\" of distance a. Unconsciously infer size from retinal image and apparent distance b. Size distance scaling 2. Direct perception: Size invariants a. Texture element occlusion b. Horizon ratio c. Relative size   Size-distance scaling - S= kRD - Where S is the perceived size, R is the retinal image size, and D is the perceived distance between the observer and the object and k is a constant   Emmert\'s Law - \"For a given retinal image size, perceived size is proportional to perceived distance\" - i.e. perceived size corresponds to retinal image size (the size of the after effect remains constant) scaled by perceived distance   Ponzo Illusion - A person standing next to a train track Description automatically generated - Same size orange lines look different size - Normally illustrated with railway tracks - Constructivists would explain it in terms of inferred depth and size distance relationships   Theory 2: Direct perception - James J Gibson - Emphasises the possible sources of information that are available - Invariant ratio: ratio is independent of size and distance. Works for any size object (so long as it cuts the horizon) at any distance   Ponzo Illusion - A Gibsonian might explain this illusion in terms of the invariant of the amount of texture covered by the different \"men\"   Ames\' Room -  - What does the Ames Room tell us about size and distance perception? - Perceived distance, not familiar size, determines perceived size (also true of Beuchet chair) - Size constancy breaks down and judgements reflect retinal image size/ visual angle - Emmert\'s Law: After images seen on the facing wall of the room do not look different sizes   **Lecture 2**: Depth Perception 1   Keywords - Depth: egocentric, exocentric, absolute, relative, ordinal, slant, curvature - Oculomotor: convergence, accommodation - Pictorial: occlusion, relative size, linear perspective, blur - Perspective: Classical (Pavlovian), acquisition - Binocular stereopsis: Binocular disparities, crossed - Motion: accretion, deletion - Ambiguity: 2D/3D, line-of-sight, convex, concave   Meanings of \"depth\" - Which is closer? - Ordinal depth - How much closer? - Relative depth - How many meters away? - Absolute egocentric distance   Three-dimensional shape perception - Slant and curvature - 1st and 2nd derivatives of depth respectively - Important for perceiving the 3D shape of objects independent of their distance from us   The \"problem\" - How do we see a three-dimensional world from a two dimensional image - Two dimensional image e.g., a painting - Points in the world have three dimensions - A diagram of a projection Description automatically generated   Another way of expressing \"the problem\" - Any points in the image corresponds to a single line-of-sight in three-dimensional space - Infinitely many points along the same line of sight project to the same point on the retina   Perceiving size and depth - Two objects that differ in size can subtend the same visual angle if they are placed at the appropriate distance Perceiving size and slant - Infinitely many objects that differ in size can subtend the same visual angle if they are placed at the appropriate distance and angle   Theories 1. Constructivism a. Perception is like a process of testing perceptual hypotheses about what is out there against the available sense data (Gregory) b. Unconscious inference (Helmholtz) 2. Direct perception a. Invariants in the optic array b. JJ Gibson 3. Information processing a. Primal sketch -\> 2.5D -\> 3D b. Algorithms implemented by neurons. Modular. Parallel c. David Marr   Types of distance/ depth \"cues\" 1. Oculomotor cues 2. Pictorial cues 3. Stereoscopic cues 4. Motion cues   Classification - Ocular/ optical? - Monocular/ binocular - Static/ dynamic - Ordinal/ relative/ absolute? - Range: near/ far?   1. Oculomotor cues a. Two main oculomotor cues i. Angle of convergence between the two eyes ii. Amount of accommodation of the lens b. Both based on information from the eye muscles c. Both can potentially provide absolute egocentric distance at close distances d. Convergence i. When fixating an object you move your eyes inwards or outwards to bring the object\'s images onto your foveae ii. Simple geometry shows that the angle of convergence required to fixate an object is inversely related to its physical distance from the observer e. Accommodation i. To focus your vision sharply onto a close object you have changed the shape of your lens f. Psychophysical evidence i. Convergence 1. Used at close distances when image cue poor ii. Accommodation 1. Coarse, ordinal information at best 2. Pictorial cues to depth a. Called pictorial because they are present in pictures b. All these cues are static, monocular and optical c. Familiar size i. Under certain conditions,, knowledge of an object\'s true size can influence our perception of its distance from us ii. Can potentially provide absolute depth 1. Whether it does or not is an \"empirical question\" iii. Familiar size has been used historically as a range finder for artillery iv. Ames room: A combination of relative size and foreshortening - Perspective a. Both converging parallels and compression gradients create an impression of depth i. Converging parallels reported to be more effective ii. But this depends on task and other factors - Foreshortening a. Does foreshortening tell you about depth? i. Orientation is the first derivative of depth 1. The rate of change of depth ii. Curvature is the second derivative of depth 1. The rate of change of orientation - Texture gradients a. Equally spaced texture elements of equal size (e.g., blades of grass) will appear to be packed closer and closer together as distance increases b. These texture elements might be used as a scale to judge both distance and size c. Properties of texture gradients i. An objects of equal size will cover an equal number of texture elements ii. An object that is twice as far away will have twice as many texture elements between it and the observer