Perceptual Issues Humans can discriminate about ½ a minute of arc –At fovea, so only in center of...

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

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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?

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%

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


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


Light 1 Light 2


Light 1 Light 2


Light 1 Light 2


Standard Test

A just noticeable difference (JND) at 11W

1 W above standard

Brightness versus intensity


Forced-choice Response

• A bit more rigorous• Q: brighter light on left or right?



• Analyse accuracy of response versus intensity of lights


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





Light 1 Light 2


Light 1 Light 2


Light 1 Light 2


Light 1 Light 2





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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