Perceptual Issues
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Humans can discriminate about ½ a minute of arc
– At fovea, so only in center of view, 20/20 vision
– At 1m, about 0.2mm (“Dot Pitch” of monitors)
– Limits the required number of pixels
Humans can discriminate about 8 bits of intensity
– “Just Noticeable Difference” experiments
129 128 125
– Limits the required depth for typical dynamic ranges
– Actually, it’s 9 bits, but 8 is far more convenient
BUT, while perception can guide resolution requirements for display, when
manipulating images much higher resolution may be required
Origins in Philosophy
• Mind-body problem – are the mind and body
the same or different?
• If they are different substances, how do they
interact or communicate?
– Dualism – mind (soul) is not governed by
physical laws but possesses free will.
– Descartes – mutual interaction.
– Animals do not possess souls and can be studied
because they are physical.
Rene Descartes (1596-1650)
British Empiricism
• Locke, Berkeley, Hume, Hartley
• Mind may follow laws and thus be modeled
just as the physical world is.
– Elements (ideas)
– Forces (associations between ideas)
• Tabula rasa – mind is a blank slate written
upon by experience.
• Mental activity may be mechanical:
– Mind as a machine
Helmholtz (1821-1894)
• Used experimental methods to study vision
and audition.
• Reaction times were used to determine the
speed of neural impulses.
– Test response-times for stimuli from the
shoulder and from the ankle.
– Nerve impulses are slow – 50 meters per sec.
• Reaction times vary considerably across
individuals and across trials – how is
precise measurement possible?
Weber (1795-1878)
• Weber studied perceptions of weight and tried
to relate these to actual physical weight.
– Weight is an objective physical property of objects.
• The greater the weight, the greater the
difference between a standard and comparison
must be to be detectable.
• Weber’s Law -- Just-noticeable difference
(jnd) is a constant across sensory modalities.
Fechner (1801-1887)
• Tried to relate physical properties to
psychological sensations:
– Related the objective to the subjective.
• Fechner’s Law – each JND corresponds to
one subjective unit of measure, with the
relationship described mathematically.
• Credited with founding psychophysics.
Wundt & Ebbinghaus
• Wundt (1832-1920) organized psychology
and helped to establish it as an independent
discipline.
– Wrote “Principles of Physiological
Psychology”
– Did not believe higher mental processes
(memory, thought, creativity) could be studied
experimentally.
• Ebbinghaus (1850-1909) demonstrated that
memory could be studied experimentally.
Stucturalism vs Functionalism
• Structuralism – focused on the contents of
mind.
– Sensations, images (ideas), affections
– Used introspection to identify basic elements.
– Introspection proved to be an unreliable
method.
• Functionalism – focused on the adaptive
function of psychological processes within a
context.
– Not much experimental work done.
The Same Color?
The Same Color?
Webber’s Law
Sensing the World Around Us
• Absolute threshold
– The smallest intensity of a
stimulus that must be present
for it to be detected
Contrast Sensitivity
0%
1%
2%
3%
Circle
constant
Background
constant
Just noticeable difference (JND) at 2%
4%
Contrast Sensitivity
0%
1%
2%
3%
Circle
constant
Background
constant
Just noticeable difference (JND) at 2%
4%
Contrast Sensitivity
0%
1%
2%
3%
4%
Background
different then
both halves
Background
same as
right half
Just noticeable difference (JND): 4% (top) and 2% (bottom)
Contrast Sensitivity
0%
1%
2%
3%
4%
Background
different then
both halves
Background
same as
right half
Just noticeable difference (JND): 4% (top) and 2% (bottom)
Brightness versus intensity
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•
•
•
standard light at fixed intensity
test light with adjustable intensity
adjust power of test until just begins to differ
just noticeable difference: JND
Brightness versus intensity
Int ensit y = 1 0 W
Int ensit y = 1 2 W
Light 1
Standard
Light 2
Test
A just noticeable difference (JND) at 11W
1 W above standard
Forced-choice Response
• A bit more rigorous
• Q: brighter light on left or right?
Forced-choice Response
• A bit more rigorous
• Q: brighter light on left or right?
Forced-choice Response
• A bit more rigorous
• Q: brighter light on left or right?
Int ensit y = 1 3 W
Int ensit y = 1 0 W
Forced-choice Response
• A bit more rigorous
• Q: brighter light on left or right?
Int ensit y = 1 3 W
Int ensit y = 1 0 W
Forced-choice Response
• A bit more rigorous
• Q: brighter light on left or right?
Forced-choice Response
• A bit more rigorous
• Q: brighter light on left or right?
Forced-choice Response
• A bit more rigorous
• Q: brighter light on left or right?
• Analyse accuracy of response versus intensity of
lights
Brightness depends on
wavelength
• Light 1: at one wavelength
• Light 2: at different wavelength
Adjust power of second light until its brightness is
the same as the first
Brightness depends on
wavelength
• Light 1: at one wavelength
• Light 2: at different wavelength
Adjust power of second light until its brightness is
the same as the first
Brightness depends on
wavelength
• Light 1: at one wavelength
• Light 2: at different wavelength
Adjust power of second light until its brightness is
the same as the first
Brightness depends on
wavelength
• Light 1: at one wavelength
• Light 2: at different wavelength
Power = 1 0 W
Power = 3 W
Light 1
Light 2
Adjust power of second light until its brightness is
the same as the first
Simultaneous brightness contrast:
two squares of the same intensity
Simultaneous brightness contrast:
left one looks brighter
Simultaneous brightness contrast:
pattern increases difference