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

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 ( )

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 ( ) 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 ( ) 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 ( ) 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 ( ) 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 ( ) 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% Background constant Circle constant Just noticeable difference (JND) at 2%

Contrast Sensitivity 0%1%2%3%4% Background constant Circle constant Just noticeable difference (JND) at 2%

Contrast Sensitivity 0%1%2%3%4% Background same as right half Background different then both halves Just noticeable difference (JND): 4% (top) and 2% (bottom)

Contrast Sensitivity 0%1%2%3%4% Background same as right half Background different then both halves Just noticeable difference (JND): 4% (top) and 2% (bottom)

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

StandardTest 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?

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?

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 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