Download to read offline
![Gestalt theory
Law of Symmetry
Objects symmetrical around a centre point form a group
( ) [ ] { }
( )[ ]{ } Law of Symmetry may even override
the Law of Proximity](https://image.slidesharecdn.com/ps3103lecture2perception-231210152521-3e464a2d/85/PS3103-Lecture2-Perception-pptx-35-320.jpg)
PS3103 Lecture2 Perception.pptx
- 1.
- 2. Today’s Schedule • Introductionto Perception • Psychophysics • Gestalt theory • Depth perception • Object Recognition
- 3.
- 4. Introduction “The Matrix” (1999)with Keanu Reeves Interesting scene (The Matrix - What is Real): https://www.youtube.com/watch?v=VVro5wxqh4U Is the key idea of The Matrix in principle possible?
- 5. Introduction Definition of Perception Perceptionis the process by which the cognitive system constructs an internal representation of the outside world.
- 6. Introduction Key issues inperception Perception is not an accurate representation of the reality Instead, perception is an interpretation of the sensory input Senses may deceive us One and the same input may be interpreted in different ways
- 7. Introduction One and thesame input may be interpreted in different ways
- 8. Introduction Definition of Perception Perceptionis the process by which the cognitive system constructs an internal representation of the outside world. ‘constructs’ it is an active process not like a photograph ‘representation’ it may be different to reality A bit like an artist painting
- 9. Introduction From Sensation toPerception Vision as example: Light → Eye → Retina → Rods & Cones (receptors) → neural activity → primary visual cortex (lines and edges) → successively higher visual areas (up to “face regions” and alike) this is called “bottom-up” processing
- 10. Introduction From Sensation toPerception however, previous “high-level” knowledge can influence our perception Although physically identical, the middle letter is once read as “H” and once as “A”. Knowledge of words shaped our perception.
- 11. Introduction From Sensation toPerception such influences must be considered in product design
- 12. Introduction From Sensation toPerception Influences on perception by knowledge, expectations, etc. is called “top-down” modulation “higher level” brain areas influence neural processing in lower level perceptual areas
- 13. Introduction - Summary Perceptionis the process by which the cognitive system constructs an internal representation of the outside world. Perception based on bottom-up information processing Perception influenced by top-down processes Previous knowledge Dis-ambiguation Interpretation
- 14.
- 15. Exercise Form groups of2-4 people Think of everyday examples where knowledge of perception is relevant Example: Development of fonts (typography)
- 16. Psychophysics Old psychophysical instruments (Laboratoryfor Experimental Psychology, Belgrade University)
- 17.
- 18. Psychophysics Psychophysics – Amerger of Physics and Psychology?? Psychophysics Relationship between physical stimuli and their perception
- 19. Psychophysics Three central questions Absolutethreshold (detection threshold) Relative threshold (just noticeable difference) Scaling
- 20. Psychophysics 1. Detection threshold Whatis the minimum strength of a physical stimulation so that we notice its presence? Example: absolute auditory threshold
- 21. Psychophysics 2. Relative threshold(jnd) smallest detectable difference between two stimuli also called just-noticeable difference (jnd)
- 22. Psychophysics Relative threshold (jnd) WeberLaw Ernst Weber 1795 - 1878 ∆𝐼 𝐼 = 𝑘 I = intensity stimulus 1 ∆𝐼 = difference in intensity of stimulus 2 k = constant (Weber’s constant) Weber Law: In approximation, the jnd is proportional to the stimulus intensity. The higher the intensity, the larger the jnd and vice versa.
- 23.
- 24. Psychophysics Relative threshold (jnd) Applicationin product design Are the brake lights bright enough?
- 25. Psychophysics Scaling How does theperceived intensity change with a change of the physical stimulus? or How much does the physical stimulus need to change to perceive a doubled intensity? It is not linear: For instance a light twice as intense does not lead to the subjective perception of a light twice as bright
- 26.
- 27. Demonstration Get into pairsof two people (A and B) Play “tit-for-tat” A slightly taps B B aims to feel (perceive!) the exact intensity of that tap Then B taps A with exactly the same intensity A aims to feel (perceive!) the exact intensity of that tap Then A taps B with exactly the same intensity and so forth… Do not assume that all taps have equal intensity, try to reproduce exactly what you felt Just in case: Don’t let it escalate
- 28. Demonstration What did welearn? Perception of the sensory events of an action is different for our own actions and those of others “Agency” plays a role Illustrating the interpretative nature of perception Our own actions are felt “less intense”, because we already expect its effects That’s why we (usually) can’t tickle ourselves Lack of this expectation in schizophrenics may explain why they attribute actions to an outside agent Science (2003)
- 29.
- 30. Gestalt theory Gestalt psychology emergedearly 20th century Usual scientific approach: Reductionism If something can be broken down, break it down… …and then investigate the smallest possible parts Gestalt psychology “Principle of Totality”. One should investigate the whole percept, i.e. taking into account all possible parts “Gestalt” (German): essence or shape of an entity's complete form
- 31. Gestalt theory Gestalt psychology providesa description of perceptual principles does not explain perceptual mechanisms Principle of “Prägnanz” German: simplicity, pithiness, conciseness Principle of Prägnanz means that sensory input is processed in a way which results in a percept which is simple, stable, regular, orderly, and/or symmetric Laws (Principles) of Grouping
- 32. Gestalt theory Law ofProximity Objects close together form a group
- 33. Gestalt theory Law ofSimilarity Similar visual elements form a group Similarity in Color Similarity in Shape
- 34. Gestalt theory Law ofClosure Objects are perceived as whole even if they are not complete
- 35. Gestalt theory Law ofSymmetry Objects symmetrical around a centre point form a group ( ) [ ] { } ( )[ ]{ } Law of Symmetry may even override the Law of Proximity
- 36. Gestalt theory Law ofContinuity We tend to perceive objects as forming smooth continuous patterns
- 37. Gestalt theory Law ofFigure-Ground segregation We tend to structure the visual field into two parts: a figure and a ground Rubin Illusion (~1915) Idaho Scenic State Route shield “Sky and Water I” by Escher (1938) Bistable percepts: Stimuli which can be perceived in two different ways
- 38. Gestalt theory What wentwrong? Law of proximity? “L and I” are too close to each other Law of closure? “L and I” are seen as components to form a “U” Intended: Grouping by law of similarity (colour, boldness) Competing: Grouping by law of proximity (“S” and “H” are closer to each other than “N” and “S”, due to the ‘)
- 39. Gestalt theory -Summary Gestalt theory Describes the organising principles that guide perceptual processes Relevant, e.g. for design www.youtube.com/watch?v=LlzuJqZ797U
- 40. Gestalt theory -Summary Gestalt theory – Criticism Nicely describes the organising principles that guide perceptual processes But does not explain why these principles exist Does not provide a model of perceptual processing No reference to depth perception
- 41.
- 42.
- 43. Depth Perception Depth perception Localisingobjects in a 3-dimensional world Estimate proper distance of an approaching car. Orient and navigate in environment.
- 44. Depth Perception Depth perception Theworld is projected onto the eye as two-dimensional image How can we create a three-dimensional percept by two two- dimensional images? By “construction” and “interpretation”
- 45. Depth Perception Binocular cue1: Binocular disparity The eyes view the world from a different angle Larger object distance → Less disparity C same distance but closer objects have larger disparity
- 46. Depth Perception Binocular cue2: Convergence The closer an object, the more the eyes rotate inwards (towards the nose) to focus the object Eye muscles provide information on convergence
- 47. Depth Perception Demonstration 1:Importance of binocular cues for depth perception How could one test this? Compare mono- with binocular vision Take two coins (preferably same size) At roughly full arm’s length, make them touch each other on the edges First try with both eyes open Then with one eye closed (repeat a couple of times)
- 48. Depth Perception Is heallowed to drive a car? Why? But why then two eyes? 2 types of information binocular cues (information from both eyes) monocular cues (information from one eye)
- 49. Depth Perception Monocular cue1: Occlusion An object partly hidden by another object must be behind it
- 50. Depth Perception Monocular cue2: Texture gradient The texture of objects changes with distance
- 51. Depth Perception Monocular cue3: Motion parallax When moving, distant objects move slower
- 52.
- 53.
- 54.
- 55. Object Recognition Object Recognition ObjectRecognition is one central purpose of perception Is the object harmful? Is it useful? How can it be utilised? The identification of objects is important for selecting and performing proper actions
- 56. Object Recognition How doesObject Recognition work? Gestalt principles described it, but did not explain it Recognition by component (RBC) theory (Biederman, 1987) Irving Biederman
- 57. Object Recognition Marr’s theory(1982) 3-dimensional model hierarchical organisation of surface and volumetric primitives (allocentric) Original Image Full primal sketch Identification of contours and basic shapes (blobs,…) 2½-D Sketch Addition of depth information in egocentric representation David Marr (1945-1980)
- 58. Object Recognition Recognition bycomponent theory Every object can grossly be described by a combination of basic geometrical shapes These basic shapes are called “geons” (geometric ions) Biederman proposed 36 primitives
- 59. Object Recognition Recognition bycomponent theory How geons represent objects Same geon used in different objects
- 60. Object Recognition Recognition bycomponent theory Experimental evidence (e.g. Hayworth & Biederman, 2006) Priming experiments
- 61. Object Recognition Priming effect: I= C > DE I = Identical, C = Complement, DE = Different Exemplar In all versions, all geons can be identified. I and C have the same geons, DE different ones Priming effect: I > C = DE In all versions, different geons can be identified. I and C have different geons, I and DE have different ones “Parts” = geons
- 62. Object Recognition -Summary Object Recognition Is a key part of visual perception Works in stages, from basic to complex One stage is the identification of basic geometrical units (geons) In the brain, this stage is localised in the LOC Lateral occipital cortex (LOC) Hayworth & Biederman, 2006
- 63.
- 64.