Perceptual Issues
• 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
– 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
129 128 125
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% 4%
Backgroundconstant
Circleconstant
Just noticeable difference (JND) at 2%
Contrast Sensitivity
0% 1% 2% 3% 4%
Backgroundconstant
Circleconstant
Just noticeable difference (JND) at 2%
Contrast Sensitivity
0% 1% 2% 3% 4%
Backgroundsame as
right half
Backgrounddifferent thenboth halves
Just noticeable difference (JND): 4% (top) and 2% (bottom)
Contrast Sensitivity
0% 1% 2% 3% 4%
Backgroundsame as
right half
Backgrounddifferent thenboth halves
Just noticeable difference (JND): 4% (top) and 2% (bottom)
Light 1 Light 2
Intensity = 10 W Intensity = 10 W
Light 1 Light 2
Intensity = 10 W Intensity = 10 W
Brightness versus intensity
• standard light at fixed intensity• test light with adjustable intensity• adjust power of test until just begins to differ• just noticeable difference: JND
Light 1 Light 2
Intensity = 10 W Intensity = 10 W
Light 1 Light 2
Intensity = 10 W Intensity = 11 W
Light 1 Light 2
Intensity = 10 W Intensity = 12 W
Light 1 Light 2
Intensity = 10 W Intensity = 13 W
Standard Test
A just noticeable difference (JND) at 11W
1 W above standard
Brightness versus intensity
Intensity = 10 W Intensity = 15 W
Forced-choice Response
• A bit more rigorous• Q: brighter light on left or right?
Intensity = 10 W Intensity = 15 W
Forced-choice Response
• A bit more rigorous• Q: brighter light on left or right?
Intensity = 13 W Intensity = 10 W
Forced-choice Response
• A bit more rigorous• Q: brighter light on left or right?
Intensity = 13 W Intensity = 10 W
Forced-choice Response
• A bit more rigorous• Q: brighter light on left or right?
Intensity = 11 W Intensity = 10 W
Forced-choice Response
• A bit more rigorous• Q: brighter light on left or right?
Intensity = 11 W Intensity = 10 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?
• Analyse accuracy of response versus intensity of lights
Intensity = 11 W Intensity = 10 W
Light 1 Light 2
Power = 10 W Power = 10 W
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
Light 1 Light 2
Power = 10 W Power = 6 W
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
Light 1 Light 2
Power = 10 W Power = 4 W
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
Light 1 Light 2
Power = 10 W Power = 10 W
Light 1 Light 2
Power = 10 W Power = 6 W
Light 1 Light 2
Power = 10 W Power = 4 W
Light 1 Light 2
Power = 10 W Power = 3 W
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
Simultaneous brightness contrast:two squares of the same intensity
Simultaneous brightness contrast:left one looks brighter
Simultaneous brightness contrast:pattern increases difference
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