Sensation: Overview
• Psychophysical approach
– Just noticeable difference
– Signal detection theory
• Top-down influence
Sensation
What is the relationship between the physical stimulus and our
psychological experience of that input?
Distal stimulus:
Proximal stimulus:
-
actual physical object in the world not directly available to the brain
-
For example, 3-D object
-
the pattern of sensory
stimulations received by the
sensory receptors in our brains
stimulation of a 2-D surface (retina)
Sensation
What is the relationship between the physical stimulus and our
subjective experience of that input?
Distal stimulus:
Proximal stimulus:
-
-
For example, 3-D object
stimulation of a 2-D surface (retina)
This is the reason why it’s possible
to experience 3-D in pixar movies
Psychophysics: Just Noticeable Difference
What’s the minimal difference in physical intensity to produce a
psychological difference? Just Noticeable Difference (JND)
• Does one bulb emit more light? (Physical stimulus)
– Independent variable
• Is one light brighter? (Subjective experience):
– Dependent variable
‘Same’
Psychophysics: JDN
What’s the minimal difference in physical intensity to produce a
psychological difference? Just Noticeable Difference (JND)
• Is one line longer? (Physical stimulus)
– Independent variable
• Does one line looks longer? (Subjective experience):
– Dependent variable
‘Same’
Psychophysics: Weber’s law
Same or different?
Size
% difference
0.2
50.0%
0.3
4.0
2.5%
4.1
Weber’s law:
• The sensory system is sensitive to percentage change, not absolute change
• The absolute difference between stimuli doesn’t matter, the relative
difference does matter
True also for sound and for weight
Signal detection
- Is this a tumor? (mammogram)
- Is there a light on? (experiment)
- Is there a fire? (smoke detector)
Your response
(what you thought):
Present
or
Absent
True state of the world
Present
or
Absent
Signal detection theory
High Hits but also high false alarms
Low false alarms but low hits
Ideally, we want HIGH HITS and LOW FALSE ALARMS (Increased Sensitivity)
Problems in Perception
(1) Perceptual stability: Proximal stimulation is hugely varied from
instance to instance
(2) The percept is not veridical
(3) A given object (e.g., tepid water) will be perceived differently at
different times (e.g., after hot or cold water). It is not consistent
(4) It is affected by knowledge (top-down processing)
Shape Constancy
•Perception of an object remains the same even when
our proximal sensation of the distal object changes
Copyright © Allyn & Bacon 2009
Size Constancy:
We ‘assume’ the train is of the same size, despite different retinal stimulation
Top-down influence
•
Naïve realism: the world is as we see it (1st day of class we
showed this is not true)
• Constructed reality: when interpreting the visual world,
we rely on expectations and past knowledge (dalmatian dog)
The brain assumes
light comes from above
Information the visual system has:
• Same retinal image
• Railroad track context makes
us think that one is further
away than the other,
Ponzo illusion
So the visual system concludes
…
• Top line must be longer
• Ames room
Gestalt Principles of Perception
Copyright © Allyn & Bacon 2009
How Well Can You Judge Relative Size?
Copyright © Allyn & Bacon 2009
• examples from youtube
• Chaplin face (schacter’s book, shading & face)
4.2 Principles of Perceptual
Organization
• Before object
recognition can
occur, grouping of
images must occur
• Gestalt perceptual
grouping rules
23
4.2 Principles of Perceptual
Organization
• Grouping involves
visually separating
an object from its
surroundings
• Separating “figure”
from “ground”
– size
– edge assignment
24
4.2 Perceiving Depth and Size
• Monocular cues to
depth
– Linear perspective
(a)
– Texture gradient (b)
– Interposition (c)
– Relative height in the
image (d)
25
4.2 Perceiving Depth and Size
• Binocular depth cues
– binocular disparity
– having space between the
eyes means that each eye
registers a slightly different
view of the world
– the difference in these views
provides the brain with
important and direct
information about depth
26
4.2 Motion-Based Depth Cues
• Motion parallax
• Optic flow
27
4.2 Illusions of Depth and Size
• Ames Room
• Moon illusion
28