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?
6. Introduction
Key issues in perception
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
8. Introduction
Definition of Perception
Perception is 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 to Perception
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 to Perception
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.
12. Introduction
From Sensation to Perception
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
Perception is 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
22. Psychophysics
Relative threshold (jnd)
Weber Law
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.
25. Psychophysics
Scaling
How does the perceived 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
27. Demonstration
Get into pairs of 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 we learn?
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)
30. Gestalt theory
Gestalt psychology
emerged early 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
provides a 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
33. Gestalt theory
Law of Similarity
Similar visual elements form a group
Similarity in
Color
Similarity in
Shape
34. Gestalt theory
Law of Closure
Objects are perceived as whole even if they are not complete
35. Gestalt theory
Law of Symmetry
Objects symmetrical around a centre point form a group
( ) [ ] { }
( )[ ]{ } Law of Symmetry may even override
the Law of Proximity
36. Gestalt theory
Law of Continuity
We tend to perceive objects as forming smooth continuous
patterns
37. Gestalt theory
Law of Figure-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 went wrong?
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
44. Depth Perception
Depth perception
The world 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 cue 1: 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 cue 2: 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 he allowed 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)
55. Object Recognition
Object Recognition
Object Recognition 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 does Object 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 by component 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
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
