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Visual Perception

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  1. the EFFECT OF BROWSERS Visual Perception Last term we looked at: Software Design Context Design as Practiced Top Ten Mistakes in Web Design: May 1996/1999 From Jakob Nielsen’s Alertbox:

  2. Visual Perception These lectures indicated guidelines and mistakes to avoid. IS Development is often about guidelines and constraints but there are lots of opinions out there – innumerable guidelines and methodologies to fit (or not) the innumerable situations that commerce, society and government can throw up. In order to evaluate the usefulness or appropriateness of guidelines some understanding of your working environment is useful – and in ISD much of your working environment comprises people and how they work and view the world.

  3. This lecture is about perception, but most of the work on perception has been about: Visual Perception An understanding of visual perception is important because virtually everything you design and build will be viewed by your users on a VDU – engagement with systems is mostly through vision and actions resulting from interpretation of visual messages first some memory tests followed by some observations: observe the next 5 slides and remember the values and the positions of the items on the slides

  4. 29

  5. 15

  6. 6

  7. 23

  8. 51

  9. Waiting time

  10. 15 6 29 23 51

  11. 15 6 29 23 51

  12. Visual Perception Obviously, you all received the same visual stimuli but some parts of the stimuli were easier to remember than others because of the instructions given at the beginning of the test i.e. if I’d asked you to remember the COLOURS as well, you would have. memory works a lot better if we know what we are supposed to be looking at - instructions may be important to perception perception appears to be some sort of a filtering mechanism - how much is the brain involved in perception?

  13. One more simple test:


  15. observe the next set of slides which will be displayed as quickly as the machine can do it

  16. A

  17. U

  18. How many slides were there? What was on the slides?

  19. 2 slides: first showing an A Second showing a U inside a double circle Experiments with a tachistoscope have shown that there is a difference in perception time between simple forms like A, U and T and more complex ones, letters within a ring, or double ring. A U Masking: Rapid exposure to a second pattern from the same light source can cause the original pattern to be erased – effectively not seen. This is taken to mean that some sort of staging of perception exists. Whatever this actually is, it indicates that the brain isn’t just a receiver for incoming signals which results in vision.

  20. The brain is obviously involved with perception in conjunction with the eyes but equally the stimulus received is also important for understanding perception.

  21. Theories of Visual Perception by Ian E. Gordon published by John Wiley & Sons, 1989. Theories must meet certain criteria: Should offer economical accounts of a range of facts. A theory is not much use if a description of it is a long as that required to describe the relevant phenomena • Should attempt to explain phenomena, or at least suggest causal links between them Should be testable – should be stated in such a way that deductions can be derived and tested empirically

  22. There is considerable variation in the style and language of theories of visual perception The reason for this is essentially that none of them deals with exactly the same arena of perception The greater the number of regions to be included in a theory the more that theory tends to be general in form


  24. ENVIRONMENT ENVIRONMENT receptors Brain stimuli effectors ENVIRONMENT ENVIRONMENT The environment is the physical world of surfaces and objects - the ecology of the organism Incoming stimuli from objects in the world give rise to events some of which can be detected by perceivers Knowledge of the important properties of stimuli has come mainly from physics Sensory surfaces take incoming stimuli and translate them into a neural code: it is important to know the nature of this transduction, how light is absorbed by the eye

  25. ENVIRONMENT ENVIRONMENT receptors Brain stimuli effectors ENVIRONMENT ENVIRONMENT Organisms make explicit responses to stimuli in the environment Important questions concern the pathways taken by neural messages, the codes which are used to represent differences in quality, intensity and duration The brain obviously has a role to play Most behaviour depends upon brain processes but these are commonly not available to direct study and must be explored indirectly

  26. ENVIRONMENT ENVIRONMENT receptors Brain stimuli effectors ENVIRONMENT ENVIRONMENT Responses: The pupil constricts in response to light and sweat is produced by very brief exposure to taboo words This can be used as a sign that the words have been detected and that they have induced emotional responses The quickest action any human is capable of is an eye movement we move around in the world and in this way partially determine the stimulation we receive We may make eye movements which are abrupt and ballistic and also movements which are smoothly graded It has been discovered that the eye takes in much less information during an eye movement then when it is stationary what guides the selection of appropriate movements?


  28. GESTALT • Wertheimer (1880-1943) • Koffka (1886-1941) • Köhler (1887-1964) "why do things look as they do?" Their statement of intent: what must be explained by perceptual theories is the stability and coherence of the world of everyday experience Pencil and hole in page and your nose

  29. GESTALT Whenever we open our eyes we see objects and surfaces, not sensations of light. We can easily distinguish between figure and ground (the figure possesses Gestaltqualität) - ground is less distinct.

  30. figure ground GESTALT distinguishing between figure and ground

  31. GESTALT distinguishing between figure and ground But is not usually a problem in a three-dimensional world Differentiation between figure and ground can be confused in a two- dimensional image, such as that on a page or a screen A white circle or a hole in the black triangle?

  32. GESTALT believed that: there is a general, underlying principle behind the numerous examples of organisation which they discovered (see next few slides) Gestalt theorists also laid much emphasis on the simple idea that the whole is greater than the sum of the parts See also: Peter Checkland’s book Systems Thinking: Systems Practice The Muller-Lyer illusion: the individual lines are objectively the same but their relationship with the arrows creates an illusion which could not be predicted from knowledge of the individual components

  33. GESTALT the whole is greater than the sum of the parts The Muller-Lyer illusion

  34. GESTALT natural organisation:

  35. grouping by columns grouping by rows equal proximity: no dominant direction of grouping

  36. grouping by similarity grouping by continuation

  37. people animals GESTALT - many of those examples demonstrated the Law of closeness: * cow boy * * bat man * * cat woman *

  38. GESTALT Law of enclosure: * cow boy * * bat man * * cat woman *

  39. The philosophy of Emmanuel KANT (1724-1804)was important to Gestalt Theorists he advanced the nativist theory GESTALT The Gestalt movement took a phenomenological approach rather than an introspective approach to perception. Their explanation of perceptual and related phenomena took the form of hypothetical brain processes these were part of a psycho-neural isomorphism this is inherently nativist in its implications concerning the origins of perception in the individual perceiver

  40. On the basis of such data, hypotheses are advanced to predict and makes sense of events in the world This chain of events is the process we call perceiving Empiricists Richard Gregory born 1923 concluded that perceiving is an activity resembling hypothesis formation and testing. Signals received by the sensory receptors trigger neural events Appropriate knowledge interacts with these inputs, which are often incomplete, to create psychological data.

  41. The main arguments are: perception allows behaviour to be generally appropriate even to non-sense object characteristics Empiricists when we “see” only 3 legs on a table


  43. The main arguments are: perception allows behaviour to be generally appropriate even to non-sense object characteristics perception can be ambiguous: if a single physical pattern can induce 2 different percepts, then perception cannot be tied to stimulation in a one-to-one manner Neckar cube Empiricists perception can mediate zero-time delay reactions when we “see” only 3 legs on a table


  45. Empiricists Conclusion: The main arguments are: perception can extract familiar objects from a cluttered background perception seems to be aided by knowledge thrushes searching image atoo talur Even “impossible designs are “rationalised”: perception can be paradoxical but stereotyped, well reinforced knowledge can refute actual perception so that even if we know a hollow mask is hollow we still perceive it as a normal face

  46. Empiricists Conclusion: We receive incomplete sets of data about the world and the visual perception system creates a representation of that world which is essentially a system of models, images or schemata. Our perception of the world is INDIRECT through the mediation of these models

  47. Probabilitistic Functionalism Egon Brunswik was part of the FUNCTIONALIST school of perception Brunswik started the idea that there was a probabilitistic functionalism involved in perception. He considered that cues arriving from the world were not only incomplete but uncertain. Appropriate use of these cues had survival value - the environment of the organism was important to understanding perception.

  48. . . He arrived at this conclusion after a series of experiments involving perceptions of a minimal set of facial characteristics: Brunswik-Reiter schematic faces: eye separation Variation in: forehead height nose length mouth height

  49. . . associations found between: apparent mood and age character, likeability and beauty intelligence and energy Brunswik-Reiter schematic faces: Experimental design: categorise according to these scales: gay-sad young-old good-bad likeable-unlikeable beautiful-ugly intelligent-unintelligent energetic-unenergetic