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Chapter 5

PSYB51H3 Chapter Notes - Chapter 5: Intrinsically Photosensitive Retinal Ganglion Cells, Subtractive Color, Circadian Clock

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Matthias Niemeier

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Ch5- the Perception of colour
Recall that humans see wavelengths of appx 400 and 700 nanometers (nm)
Most of the light that we see is reflected light
The more light that;s absorbed, the darker the surface will appear
3 steps to colour perception
several problesm must be slved in order to go from physical wavelengths to the
perception of colour
1. detection wavelengths must be detected
2. disccriminiation we must be able to tell the difference tbn one wavelength
(or mixture of wavelengths) & another
3. appearance we want to assign perceived colours to lights and surfaces in
the world
we want those perceived colors to go w the object (that rose looks
red) and not to change dramatically as the viewing conditions change
(that rose should remain red in the sun and shadow)
- recall that we have 3 types of cone phtotrecptors
othese cones differ in the phtotpig they carry, & as a result, they differ
in their sensitivity to light of diff wavelengths
s-cones: cone that is preferentially senstiivte to short
wavelengths; “blue cone”
m-cones: cone that is preferentially sensitive to middle
wavelengths; “green cones”
l-cones: cone that is preferentially sensitive to long
wavelngths; “red cones”
- each cone tyoe is named for the location of the peak of its sensitivity on the
ocones that peak at 420nm short-wavelength cones (S- CONES)
opeak at 535 M-CONES
opeak at 565 L-CONES
- cones aren’t exculisvly sensitive to diff parts of the spectrum
oeven tho the L CONES is maximally sensitive at about 565nm, the M
CONE can detect that wavenelth as well
their spectral sensitivities overlap
= referring to the sensitivity of a cell or a device to diff
wavelengths on the electromagnetic spectrum
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the combination of sensitivities of the 3 types of cones gives us our overall ability to
detect wavelengths from about 400nm to about 700nm
recall that cones work in the daylight (phtopic) light levels
-photopic: light intensities that are bright enough to stimulate the cone
receptors and bright enough to “saturate” the rod recpetors (that is drive
them to their maximum responses)
we have one tpe of rod photoreceptor; it works in dimmer (scotopic) light & has
somewhat diff sensitivity profile, peaking at about 500nm
-scotopic: the light intensities that are bright enough to stimulate the rod
recpetors but too dim to stimulate the cone recptors
The Prinicpe of Univairance
How can we distinguish btn lights of 450, 550, and 625mn??
- we know that diff wavelengths of light give rise to diff experinces of colour,
and the varying responses of their photoreceptor to diff wavelengths could
provide a basis for colour vision
Lights of 450 and 625mn each elicit the same response (PG 125
FIG 5.3)
An equal amount of 450mn light will produce the same
response fomr this phtotrecptor that 625nm light does
This situation illustrates the principle of univariance
If we were looking at the output of the phtotorecptor, we
would wave NO WAY OF DISTINGUSIHING btn the two lights
But when we look w a normal human color vision
system, the 625mn light looks orange, and the 450nm
light looks bluish
- The graph in fig 5.3 indicat the light at either 450 or 625nm produces a
response lower than the peak response obtained at about 535mn
oIf we haf a 535mn light, we could reduce its intensity until it
prodciced exactily the same level of response from our phtotorecptor
as the 450 or 535mn light
Thus, when it comes to seeing color, the output of a single phtorecptor im
completely ambiguous!!!
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- an infinite set of diff wavelength-intensity combos can elicit exactly the same
response from a single phtorecptor; so the output of a single photoreceptor
cannot by itself tell us anything about the wavelengths stimulating it
othis is the principle of univariance
thus, oen phtorecptor type cannot make color discriminations
based on wavelength
univaraince explains the lack of color in dimle lit scenes
- rememeber that there is only ONE TYPE OF ROD PHOTORECEPTOR
oALL rods contain the same photopig molecule: rhodopsin
- Thus, they all have the same sensititvty to wavelength
oTHUS…although its possible to tell light form dark under scotopic
condition, the problem of univarince makes it impossible to
discriminate colours
W just 1 type os phtotorecptor, we cannot make
ddisciminations based on wavelength, so we cannot see colour
Our nighttime colour blindness is one hint that colour is
psychophysical and NOT PHYSICAL
oWe fial to see color under dim illuminations like moonlight, bc dim
light stimulates only the rods, & the outputs of that single variety of
photorecptrs doesn’t permit colour vision
The Trichroomatic Solution
- we can detect differences btn wavenelths or mixtures of wavlengths
precisely bc we have MORE THAN 1 kind of cone photoreceptor
when looking at the three cones’ responses to two wavelengths (450 & 625nm) we
see that….
- the M-CONE PRODUCES THE SAME responses for the 2 diff wavelengths (PG
126 fig 5.5)
- however…
othe two wavelengths produce diff outputs/responses form the L-
in fact, any wavelgnth from ~ 420 to 660nm produce a uniwue set of 3 responses
form the three ones types
- this combined signal , a triplet of numbers for each “pixel” in the visual field,
can be used as the basis for color vision
in our discussion of the inivariance proble,. We noted that we can make any
wavelength produce the same response as any other from a single cone type by
adjusting the intensity of the light
- that doesn’t happen in the 3-cone world of human color vision
find more resources at oneclass.com
find more resources at oneclass.com
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