1 - TRICHROMACY

Question 1

thomas young

Answer 1

• proposed that the three colour mixing principle tells us there are three different types of light sensitive elements in the retina
• sensitive to different wavelengths of light
• no direct physiological evidence
• based on logical reasoning
• 150 years later conclusive physiological evidence was found
• CONE RECEPTORS 👆🏼

Question 2

trichromatic visual system

Answer 2

has three kinds of photoreceptive elements with different wavelength sensitives

Question 3

dichromatic and monochromatic visual systems

Answer 3

only two / one kinds of photoreceptive elements

Question 4

human vision (day and night)

Answer 4

daytime = photopic
- trichromatic (based on the three types of cones)

nighttime = scoptopic

• monochromatic (based on rods only)
• colour blind
• cannot make discriminations between different light sources based on the wavelength they contain

Question 5

trichromatic theory of human colour vision

young-helmholtz theory 1850s

Answer 5

• extends youngs proposal by making specific proposals about the wavelength of different photoreceptors
• also how the signals produced in response to stimulation correlate to the colour sensations experienced
• colour sensation is evoked by stimulation and is directly determined by the response pattern from the three types of receptors
• different patterns produced by different wavelengths
• how stimulation by different wavelengths produces the differing colour sensations
• means that all three must be stimulated in order to see colour
• so all retina areas (for colour vision) should have all three types in roughly equal numbers

Question 6

helmholtz’s suggested photoreceptor sensitivities

Answer 6

• pg. 17 diagram
• didn’t have actual wavelengths
• done by writing the colours instead
• wrote them long>short (opposite to normal)
• top = most sensitive to long (red)
• centre = most sensitive to mid (green)
• bottom = most sensitive to short (blue/violet)
• BROAD SENSITIVITIES = sensitive to light from whole visible range - but more sensitive to some wavelengths more than others
• sensitivities overlap - so will evoke a response from all three but response will be different
• MAXIMALLY SENSITIVE = highest point on the curve (at a particular wavelength)
• different for each receptor

Question 7

trichromacys four proposals

Answer 7

1 - three receptor types
- TRUE

2 - broad, overlapping wavelength sensitivities

• CONCEPTUALLY TRUE
• sensitive to a wide range but most sensitive to parts of the visible range

3 - receptor distribution

• FALSE
• roughly equal numbers and evenly distributed

4 - colour sensations

• FALSE
• colour sensations experiences due to stimulation of a particular area of retina is due to the patterned response by the three types of photoreceptor present

Question 8

proposal 2

Answer 8

• cones do indeed have broad overlapping sensitivities
• however not like helmholtz proposed in the 1850s
• they do not have their maximum sensitivities in the red, green or blue regions
• the more light a cone absorbs, the more sensitive it is to that light
• curves are similar to the shape proposed by helmholtz but far from identical
• receptors 1 and 2 are closer together
• do not lie in the end or middle of the spectrum like he suggested

Question 9

s-cones, m-cones and l-cones

Answer 9

s-cones = short wavelength

m-cones = medium wavelength
- greenish-yellow

l-cones = long wavelength
- orangey-yellow

Question 10

response = sensitivity x light intensity

Answer 10

sensitivity = how sensitive the cone is to that certain wavelength

light intensity = how intense/how much light there is

• can get any response by adjusting the intensity of the light
• see week 2 written notes for how to read wavelength graph and example question for this equation

Question 11

principle of univariance

Answer 11

• we can get any response using any wavelength of light (or mixture) through the adjustment of light intensity
• stimulating a receptor alters its response (change in membrane potential)
• response of a photoreceptor cannot tell you the wavelength or intensity of a stimulus - cannot say what contributed to a response
• if only one photoreceptor used = different stimuli (different colours) would only appear as different shades of a single colour

Question 12

what trichromacy cannot answer

Answer 12

• why when you mix yellow and blue you don’t get yellowish blue
• and why you don’t get reddish green when you mix red and green
• why it’s not a mix and instead it’s a different colour