Readings

• Who proposed the trichromatic theory of colour vision?


  Thomas Young
• In what year was the trichromatic theory proposed?


  1802
• What are the three types of cone photoreceptors called?

  S-,M-, and L-cones
• What do the sensitivities of the cone photoreceptors correspond to?


  The wavelengths of light in the visible range
• How are the three types of cones distributed in the retina?


  They are found in roughly equal numbers and distributed evenly
• What determines the colour sensation evoked by light stimulation?


  The pattern of responses in the three cone types
• Is the trichromacy theory correct?

  No it's incorrect
• What did Ewald Hering propose regarding colour mixtures?


  That blue and yellow, and red and green cannot co-exist in a mixture
• What is the alternative theory to trichromacy theory proposed by Hering?


  Opponent colours theory
• What happens when blue and yellow light are mixed?


  A new sensation of white is seen
• What are the four psychologically primary colours according to Hering?


  Red, green, yellow, and blue
• What is the significance of pure shades of colours?


  They appear to have no trace of any other colour
• How does Hering's opponent colour theory differ from trichromacy theory?


  It proposes four primary colours organized in pairs
• What happens when pure yellow light is mixed with pure blue light?


  The result is white light
• What occurs when red light is mixed with green light?


  A new colour, yellow, is created
• What is the effect of adding blue light to yellow light?


  It progressively weakens the yellow until it disappears
• What is the effect of adding red light to green light?


  It progressively weakens the green until it disappears
• Why do red and green produce yellow instead of white?

  It is a consequence of how the yellow detector is constructed
• What are opponent mechanisms according to Hering?


  They are mechanisms that put colours in opposition
• What are the two opponent mechanisms proposed by Hering?


  Blue-yellow and red-green mechanisms
• How do the colour detectors function in Hering's opponent mechanisms?


  One detector excites the combination unit while the other inhibits it
• What happens when the signal activity from both colour detectors is equal?


  The effects on the combination unit will cancel one another out
• What are the key features of trichromacy theory?


  • Based on three types of cone photoreceptors
  • Each cone has different sensitivities to light
  • The pattern of responses determines colour sensation
• What are the key features of opponent colours theory?


  • Proposes four primary colours: red, green, yellow, blue
  • Colours are organized in pairs (red-green, blue-yellow)
  • Adding opposing colours cancels each other out
• What is the significance of pure shades in Hering's theory?


  • They are experienced as having no trace of other colours
  • They form the basis for the opponent colour pairs
• How do the opponent mechanisms work in the human visual system?


  • Detectors for each colour pair feed into a combination unit
  • One detector excites while the other inhibits the combination unit
• What is the outcome when the signal activity from one colour detector is greater than the other?


  • The colour sensation associated with the greater signal is evoked
• What happens when yellow stimulation is present in the eye??????/

  No blue or yellow is seen.
• What occurs when only blue light is shone into the eye?

  The blue detector is stimulated, increasing activity in the combination unit.
• What does an increase in activity above the resting level signal?


  That blue light is present and blue is seen.
• What happens when only yellow light is shone into the eye?

  Activity in the combination unit decreases, signaling that yellow light is present.
• What is the effect of equal signal activity from excitatory and inhibitory detectors?


  The effects on the combination unit will cancel one another out.
• What does it mean if the signal activity from one detector is greater than that of the other?


  The color sensation associated with the greater signal is evoked.
• What did Hering propose regarding color perception?


  There is a third opponent mechanism that deals with achromatic colors: white, black, and gray.
• Why was Hering's theory largely ignored for several decades?


  Scientists believed the trichromatic theory was correct and thought the two theories were incompatible.
• What led to Hering's theory being taken seriously in the 1950s?


  It was recognized that it could overcome certain difficulties inherent in trichromacy theory.
• How many types of color photoreceptors are there in the human retina?


  Three types of color photoreceptors.
• What does the evidence from the 1950s suggest about Helmholtz and Hering's theories?


  Both theories are partly correct regarding color perception.
• What is the structure of the retina as described in the study material?


  It contains three layers of cell bodies: the ganglion cell layer, the middle layer, and the photoreceptor layer.
• What is the significance of the ganglion cell layer in the retina?


  It contains fewer cells compared to the middle and bottom layers, indicating a convergent pathway.