Psychology of Perception Psychology of Attention Dr. Mohammad Hussein August 2018

1. Perception

6. Perception is the an
active process
by which the cognitive
system construct an
internal representation of
outside world

7. Perception external environment
presentation outside world
Perception our personal experience of our sense pick up

8.  Perception is not accurate perception of reality
 It is interpretation of sensory input
 Senses may deceive us

9. What do we perceive with?
Senses

10. we perceive with senses
by
organ
sense
Electromagnetic waves of certain frequencies
eyes
Vision
1
Waves of air pressure; air vibrations
ears
audition
2
chemical substances
tongue
taste
3
chemical substances
nose
smell
4
Pressure of skin
skin
touch
5
Balance
ear
Equilibrioception
6
temperature
skin
thermoception
7
pain
all
nociception
8

11. Physiology of
Perception

12. Physiology of perception
Physical
input
Action potential
stimulus sensor

13. Primary visual cortex

15. From sensation To perception
Light
retina
Rods
&cons
Neural
activity
bottom up
processing

16. bottom up
processing

17. H
Expectation
& previous
knowledge
A
A
H

18. Physiology of perception, top down processing
Expectations,
Emotions
Previous knowledge
etc
“high level” brain areas ;
 Prefrontal cortex
 Superior parietal cortex
influences neural
processing in lower
perceptual areas

22. Top-down
modulation
Bottom-up
processing
Perception

23. To sum-up physiology of perception
Physical stimulus
 Receptor
 Neural activity activation of sensory brain areas
Sensory processing in neocortex
Highly organized
District areas for colors, motion, form,….
Hierarchical
Goes from simple to complex
Top-down influences
Context, Expectations, knowledge, etc. influence perception

24. Psychophysics

25. Which square is brighter?
Perception does not
mirror reality,
it may distort it.
Psychophysics:
how does perception actually capture reality,
It is the Relation between physical stimuli and their perception

26. Psychophysics
Absolute threshold
(detection threshold)
Relative threshold
(noticeable difference)

27. Psychophysics
What is minimum strength of
physical stimulation so that we notice its presence
Affected by motivation, expectation, attention,
adaptation
Absolute threshold
(detection threshold)

28. Psychophysics
Absolute threshold
(detection threshold)

29. Smallest detectable difference between two stimuli
Relative threshold
(noticeable difference)

30. Relative threshold
(noticeable difference)
Psychophysics

31. How to benefit from relative threshold
Reduce size
Reduce weight
Reduce quality
Increase price
Negative changes Positive changes
Discount
Increase size
Better package

32. Gestalt Theory

36. Gestalt Theory
Gestalt [ɡəˈʃtalt] "shape, form"
the mind forms a global whole with self-organizing tendencies

37. • Principle of totality
• One should investigate the whole percept,
i.e. taking into account all possible parts
Gestalt Theory

38. Gestalt Theory
 Law of Prägnanz;
People will perceive and interpret ambiguous or complex images as the
simplest form(s) possible
We prefer things that are simple, stable, clear and ordered.
This is the fundamental principle of gestalt
 Laws of grouping
Law of Simplicity

39. Gestalt Theory
• Law of proximity
Objects close together
form a group

40. Gestalt Theory
 Law of Similarity
Similar visual elements form a group

41. Gestalt Theory
 Law of Closure
Objects are perceived as whole even
if they are not complete

42. Gestalt Theory
 law of symmetry
Objects symmetrical around a center
point from a group
symmetry vs proximity

43. Gestalt Theory
 law of continuity
We tend to perceive objects as forming
smooth continuous patterns

44. Gestalt Theory
 law of figure and ground
segregation
We tend to structure the visual field
into parts:
A figure and ground
Bistable percepts Monostable percepts

45. the mind forms a global whole with self-organizing tendencies
Law of Prägnanz; People will perceive and interpret ambiguous or complex images as the simplest form(s) possible
Proximity Similarity Closure
Symmetry Continuity figure and ground segregation

46. Depth perception

47. Depth perception
 Localizing objects in a 3-dimentional world
 The world is projected onto the eye as
two-dimensional images (2 photo)
 But how?
How to create 3D percept from
two two-dimensional images?

48. cues
Binocular cues
(information from both eyes)
monocular cues
(information from one eyes)

49. Binocular cues
Binocular cue 1
“Binocular disparity”
 The eyes view the world from a different angle
• The main cue in 3D movies; present different
picture to the left and right eye

50. Binocular disparity
• The main cue in 3D movies; present different picture to the left and right eye
The 2 picture are colour coded
 glasses with colour filters
The 2 picture are presented alternatively;
LCD shutter glasses block the eye in the correct rhythm
The 2 picture are presented with
light of different polarization;
The glasses act as polarization
filter
Binocular cues

51. Binocular cue 2
“convergence”
 The two eyes move inward for near objects to
focus the object
 Eye muscles provide information on convergence
Binocular cues

52. cues
Binocular cues
(information from both eyes)
monocular cues
(information from one eyes)

53. monocular cue 1 Occlusion
 An object partly hidden by another object must
be behind it
Monocular cues

54. monocular cue 2 Texture gradient
 The texture of objects changes with distance
Monocular cues

55. monocular cue 3
Motion Parallax
 When moving, distant objects move slower
Monocular cues

56. Depth perception
Mind construct a 3D representation from two 2D images by using:
 Binocular cues
 Monocular cues
disparity convergence
occlusion Texture gradient Motion parallax

57. Object Recognition

58. Object Recognition
Basic stages of object recognition
 Stage 1 Processing of basic object components, such as colour, depth,
and form.
 Stage 2 These basic components are then grouped on the basis of
similarity, providing information on distinct edges to the visual form.
Subsequently, figure-ground segregation is able to take place.
 Stage 3 The visual representation is matched with structural
descriptions in memory.
 Stage 4 Semantic attributes are applied to the visual representation,
providing meaning, and thereby recognition.

59. Object Recognition
Hierarchical recognition processing
Visual recognition processing has been typically viewed as
a bottom-up hierarchy in which information is processed
sequentially with increasing complexities,
where lower-level cortical processors, such as the primary
visual cortex, are at the bottom of the processing
hierarchy and higher-level cortical processors, such as
the inferotemporal cortex (IT),
are at the top, where recognition is facilitated.
A most recognized bottom-up hierarchical
theory is David Marr's theory of vision

60. Disorders of Perception

61. Disorders of perception
 Perceptual loss
Visual agnosia (inability to recognize images with intact vision),
Alexia (inability to recognize written material with intact vision and previous
knowledge)
 Perceptual distortion
Micropsia (objects appear smaller)
macropsia (objects appear larger),
hyperacusis (sounds are louder)
hypoacusis(sounds are softer)
 Perceptual deception
Illusions: misinterpretation of existing stimuli, e.g. seeing a rope as a snake.
Hallucinations: perception in the absence of external stimuli.

63. Attention

64. The process by which we select some stimuli for
further processing while ignoring others
Attention

65. Involuntary attention
Types of Attention
Voluntary attention Spontaneous attention

66. factors attracts attention

67. factors attracts attention
External Internal

68. External factors
Pizza
Type of stimulus Position of stimulus
Contrast of stimulus
Changeability & repetition of stimulus

69. Internal factors
Mental set Biological needs

70. 2
What is OCD
Epidemiology
Nosology
Symptoms
Diagnosis
OCPD Vs. OCD
Investigations
Etiology
Treatment
Questions
Terms
‫يحبذ‬ ‫حسين‬

71. Further reading

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