colour perception

• Purpose of colour perception

  Detect ripeness of fruit + veg, poisonous vs non-poisonous berries, fresh vs rotten meat (tho also by smell), social + emotional cues (e.g., blushing, attractiveness, fear), sexual selection (health of potential mate), health (e.g., illness)
• Sclera


  • Whites of your eyes ⇒ easy to tell what direction we're looking (irises stand out) ⇒ see where others are looking (survival, sociability…)
• Light

  Frequency and amplitude (only two light qualities relevant for humans)
• Intensity/amplitude


  How bright light is
• Light waves can have same wavelength while varying in intensity and vice versa
• White light


  Not a colour at all, but the composition of all of the wavelengths in the visible spectrum
• Sun bends fast wavelengths of light (blue) more than slower wavelengths


  Blue sky and blue eyes
• Blue eyes


  Absence of melanin (protects eye from sunlight) in iris
• Green eyes

  Little bit of melanin
• Hue


  Actual colour, determined by wavelengths
• Hot colours


  Larger (slower) wavelengths
• Cold colours


  Shorter (faster) wavelengths
• Wavelengths don't have colour, just speed → colours made up by our perceptual system
• Colour we see

  Everything that the object is not → the refractive property of the object (rejected wavelength that it didn't absorb)

  
• Brightness


  Intensity of light wave (amount of light in a colour) → the higher intensity the light wave (as signaled by its amplitude), the more bright the reflecting object

  
• Black


  Absence of light
• Saturation


  Complexity of the wavelengths of the light entering the eye/amount of white/how pure wavelength is/how colourful colour is with respect to its own brightness

  
• If just one wavelength → completely saturated
• Rarely occurs irl → objects reflect several wavelengths, ⬆ amount of wavelengths that enter the eye ⇒ ⬇dominant wavelengths saturation

  
• White light → no saturation → light saturated w all wavelengths
• Electromagnetic spectrum


  We're sensitive to extremely small proportion of it (visible light) bc atmosphere blocks most other wavelengths of light out (can't come into our environment) ⇒ being sensitive to it wouldn't be useful, no point in evolving to see it

  
• Slower wavelengths (e.g., infrared + microwaves) can get through atmosphere → why are we not sensitive to them?
• We used to be fish → gills migrated and became 3 little bones in our inner ear + jaw, larynx, throat
• The deeper in the water, the less colour you see, but red goes through a little bit (explains why we can see colours of tropical fish)
• Rods and cones


  Contain pigments that change their chemical composition in response to dif frequencies of light
• Rods


  • At night (pigments regenerate every 30 mins), bleached out during day bc light levels too intense, 120 mill rods → in periphery, absent in fovea ⇒ low acuity, high sensitivity
• Cones


  • During day (ˮˮ6 mins), not sensitive enough to convey info in the dark, 67 mill cones → in fovea ⇒ high acuity, responsible for colour vision

  
• Pigment specificity, absorption rates and wavelength sensitivity

  3 dif types of cones → each contains photopigment sensitive to dif wavelengths

  
• Cones absorption rates → no one cone can determine absolute colour because cones have a maximum sensitivity → fires to one wavelength the most, but also a little bit to other wavelengths → the further away it gets from max sensitivity, the less that it fires

  
• A cone will fire more if there's a higher intensity of light → could be same wavelength but at higher intensity ⇒ cone fires more
• Combination of all 3 cones ⇒ figure out what colour it is → why do we need more than one cone? bc they're sensitive to several wavelengths 2 completely dif wavelengths can produce same signal in one photoreceptor ⇒ perceptual system compares differences in signals across the 3 dif photoreceptors to determine wavelength and intensity

  
• 2 intersections ⇒ ambiguous what colour it is (blue or greenish yellow? long wavelength cone firing almost 1 yellow (if it were blue, long wavelength cone would be firing at .25 v low)
• Dif rates of firing of photoreceptors account for all the colour spectrums we see ⇒ produced by combos of RBG

  
• What photoreceptors are actually doing is adding together the amount of RGB in the scene → what TV and computer screens do (tiny pixels of RGB)
• Colourblindness


  Due to individuals missing specific type of cone → dif sensitivity to dif colours
• If missing LW cone → Ø red end of spectrum, if missing SW cone → Ø blue end, LW and MW → only one cone ⇒ can't determine colour w just one cone ⇒ colourblindness

  
• Rods are colourblind (we can't see colours at night)
• The more cones you have, the more you can disambiguate between wavelengths e.g., if w 3 cones i just see blue, a creature w more cones will be able to see dif colours of blue within that blue

  
• Underwater creatures e.g., dolphin, seal, manatee → typically 1 cone (usually blue one bc it's the one that gets through water), dogs, new world monkeys, mice, cats… most mammals → 2, humans, old world monkeys → 3, geckos, birds, fish → 4, butterflies → 6, mantis shrimp → 12
• Dif creatures → dif distributions of what types of light they're sensitive to e.g., bees better w blue spectrum, bird w both sides of spectrum