Physiology of Colour Vision

Question 1

What is colour vision?

Answer 1

• There is no colour in external world
• ‘Concept’ created by interactions of billions of neurons in brain
• Colour created by neurons using 2 properties of light: energy & frequency of vibration or λ
• Colour only exists/is created by biological systems to code reflected light off objects in different bandwidths

Question 2

What is the purpose of colour vision?

Answer 2

• Need it to give us an internalised template or ‘percept’ of visual scene
• World is really w/o colour
• Colour is used by animal world to discriminate food from poison
• Helps in detection of borders of objects

Question 3

Describe light as an electromagnetic radiation?

Answer 3

• Form of electromagnetic radiation
• Light travels in waves like ripples in pond
• Each light wave has peaks & troughs where electric field highest & lowest respectively
• λ is distance between 2 wave crests or troughs
• No. of wave crests that pass through given point in 1 sec is Frequency – measured in cycles per second (Hz)
• Speed of light wave equals frequency times λ

Question 4

Describe the electromagnetic spectrum?

Answer 4

• Consists of all different λs of electromagnetic radiation, including light, radio waves & X-rays
• Regions of the spectrum are named arbitrarily, but naming helps to describe he energy of radiation e.g. ultraviolet light has shorter λs than radio light
• Gamma Rays  X-ray  UV  Visible  IR  Microwave  Radio Waves
  o Gamma rays about size of atomic nuclei
  o Radio waves about size of buildings
• Only region in entire electromagnetic spectrum that our eyes are sensitive to is visible region
• Equation relating λ & frequency for electromagnetic waves is: λν=c
  o λ is wavelength
  o ν is frequency
  o c is speed of light
• λ & frequency are inversely related
  o Higher the frequency, the shorter the wavelength
• As all light waves move through a vacuum at same speed, the number of wave crests passing by a given point in 1sec depends on λ
  o That number aka frequency, will be larger for a short- λ wave than for a long- λ wave

Question 5

Describe Hue and Wavelength?

Answer 5

• Newton: White light splits into its component colours when passed through a dispersive prism & could be recombined when passed through different prism to go back to white light
• The characteristic colours are, from long to short λs (&, correspondingly, from low to high frequency), red, orange, yellow, green, cyan, blue, and violet
• Hue is caused by a big enough difference between λs to result in a difference in percept coming from it
• The just-noticeable difference in λ varies from about 1 nm in blue-green & yellow wavelengths, to 10 nm & more in longer red & shorter blue wavelengths
• Human eye can distinguish up to a few hundred hues
• However, when pure spectral colours are mixed together or diluted with white light, no. of distinguishable chromaticities can be quite high
• In very low light levels, vision is scotopic: light is detected by rods in retina
  o Rods are maximally sensitive to λs near 500 nm, & play little, if any, role in colour vision
• In bright light, vision is photopic: light is detected by cones which are responsible for colour vision
  o Cones are sensitive to a range of λs, but are most sensitive to λs near 555 nm
• Mesopic vision: both rods and cones provide signals to the retinal GCs
• The shift in colour perception from dim light to daylight gives rise to differences known as the Purkinje effect
• Perception of “white” in animal kingdom is formed by entire spectrum of visible light, or by mixing colours of just a few λs depending on no. & type of colour receptors
• In humans, white light can be perceived by combining λs such as red, green & blue or just a pair complementary colours such as blue & yellow

Question 6

What is chromaticity?

Answer 6

• Chromaticity is colour defined objectively w/ no reference to luminance
• It has 2 independent parameters: hue (h) and colourfulness (s)
• In theories of colour vision hue is usually “degree to which a stimulus can be described as similar to or different from stimuli that are described as red, orange, yellow green blue and violet”
• These attributes are of perceived colour & are related to chromatic intensity
• Interestingly, word ‘hue’ in a painter’s eye is pure pigment – on w/o shade or tint – & includes black & white too
• Colourfulness is “attribute of visual perception according to which perceived colour of an area appears to be more or less chromatic”
• Colourfulness evoked by an object depends not only on its spectral reflectance but also on strength of illumination & ↑with the latter unless brightness is very high (& you lose the colourfulness, just seems like you are in glare)

Question 7

Describe photoreceptors?

Answer 7

• Light is detected by outer segment of cones
• Rods are very sensitive to light but their response is slow (& low) in photopic & mesopic light
• Their responses saturate at light levels where cones function optimally
• Cones are less sensitive but are fast (& can sustain their response) & can adapt to brightest lights, being almost impossible to saturate – they only saturate when glare falls on eyes or sun shining straight at you
• Therefore, in modern life cones are ‘the thing’ to have intact & undamaged
• Cones evolved before rods in areas of strong sunlight where vision was a great advantage. Shadows are strong and more important to detect than increments of light in the struggle for survival
• Shadows/the dark depolarise cones leading to a release of glutamate that hyperpolarises/inhibits ‘on-centre’ BCs and depolarises ‘off-centre’ BCs
  o Light switches off glutamate release

Question 8

Describe cones?

Answer 8

• A cone responds only to energy it absorbs
• All λs of light may evoke identical responses from a cone if energy absorbed by cone is same for these λs
• Cones are therefore ‘colour blind’ producing a univariant response reflecting only amount of energy they absorb

Question 9

Describe cones & colour vision?

Answer 9

• Detecting objects by energy reflected from their surfaces, however, can fail when objects reflect similar amount of energy as their background
  o This is where colour vision becomes important.
• λ contrast can detect objects when energy contrast is absent or minimal.
• Object can reflect same energy but seldom reflects same λ composition as its background
• Colour vision combines both energy & λ contrasts to detect objects & this advantage must have evolved early in evolution of vision

Question 10

Describe photo transduction?

Answer 10

1. Light energy (photons) isomerise retinal to its all-trans form, releasing & activating opsin
2. Freed opsin acts enzymatically to catalyse activation of the G protein transducin
3. Transducin catalyses activation of enzyme phosphodiesterase (PDE)
4. Activated PDE detaches cGMP from Na channels by hydrolysing it to GMP
5. Once their ligand (cGMP) is detached, Na channels close, preventing Na+ entry & causing hyperpolarisation  which in turn, prevents neurotransmitter release at synapses w/ BCs
   In light, get hyperpolarisation of rods & cones
   In dark, get sodium entry

Question 11

Describe divariant cones?

Answer 11

• Object detection using cue of differences in spectral reflectance requires 2 or more different types of cones
• Therefore, for divariant colour vision, 2 cone types must exist
• They must be sensitive to different parts of visible spectrum
• They must be as different as possible, preferably
• Visible spectrum range depends on light being able to penetrate eye & be absorbed by photoreceptors

Question 12

Describe the visible spectrum?

Answer 12

• UV light is absorbed by anterior segment of our eyes & seldom reaches photoreceptors
• IR light penetrates our eye readily, but its quantal energy may be too small to activate opsins
• Therefore, colour vision evolved opsins sensitive to middle of visible spectrum 1st
• Near spectral yellow, & a short λ opsin evolved in a 2nd type of cone, near spectral blue
  o 2 types: L (long λ sensitive) & S (short λ sensitive) cones

Question 13

Describe the absorption of L & S cone opsins?

Answer 13

• Normalised absorption spectra of L and S cone opsins that mediate colour vision.
• Strong absorption by S cone opsin on its own induces ‘blue’ percept & strong absorption by L cone opsin results in ‘yellow’.
• Absorption by both give ‘white or grey’ percepts which will depend on achromatic contrast of light & dark.
• If you take the fourth root of the wavelengths each curve (blue or yellow) is identical in shape – they are just shifted along spectrum

Question 14

Describe chromatic aberration?

Answer 14

• Effect produced by refraction of different λs of light through lens at slightly different angles, resulting in failure to focus
• In mammals w/ large eyes L-cones are used to detect both energy & λ contrast but S-cones are used only for λ contrast
  o This is due to chromatic aberration
• Short λ images are out of focus when longer λ images are in focus on photoreceptor mosaic
• Chromatic aberration increases greatly at short λs, which leads to L cone system dominating energy contrast
  o As result, there are many more L than S cones in many mammals in order to gain spatial resolution by achromatic contrast detectable by L cones

Question 15

Describe chromatic aberration in small eyes?

Answer 15

• In small eyed animals like mice and rats, UV light can reach the photoreceptor mosaic
  o Result is that UV sensitive cone opsins have evolved to widen spectral range of vision & if combined with L cones could allow colour vision
• Chromatic aberration is reduced in these small, highly spherical eyes which have outer segments as long as animals with large eyes
• This quality ↑their depth of focus minimising chromatic aberration – advantage of being small
  o But: they compromise on retinal images which are less magnified than those in large eyes

Question 16

What is chromatic contrast?

Answer 16

Comparison of responses between groups of cones of 2 types within same area of visual space

Question 17

Describe Horizontal Cells?

Answer 17

• Cone HCs receive excitatory input from cones & send back inhibitory input to cones
• When cone is hyperpolarised by an increment or depolarised by decrement of light, it receives opposing input from HCs after a brief synaptic delay
• This dampens response & can also reduced effects of scattered light by minimising cone responses outside of focal image on retina
• In colour vision horizontal feedback also narrows action spectrum of cone BCs
  o HCs help you see something that’s blue, something that’s yellow
• In divariant colour vision this can narrow the action spectrum of S cone BCs. This occurs because processes of the H2 HCs that reach L cones are only post-synaptic to S cones
  o Therefore, L cones can send an antagonistic signal to S cones which can reduce effectiveness of λs, absorbed by both L & S cones & this narrows the action spectrum of S cone channel

Question 18

Describe bipolar cells (BCs) and ganglion cells (GCs) in divariant colour vision?

Answer 18

• 1 type of L cone BC depolarises whenever cone or cones they synapse w/ hyperpolarise
  o This set is called on-BC set because they are excited (turned on) by light
• Other set is depolarised whenever cones they synapse w/ detect decrements of light
  o This set is called off-BC set because cells within it are excited by darkness & inhibited (turned off) by light
• GCs serving the two photoreceptor systems are quite different:
  o L cones synapse with unique BCs, called “midget” bipolar (centre of fovea)
   This provides brain with ultimate in spatial resolution, a single cone, & isolates signals of L cones which can be used for colour vision
  o These 2 sets of cone BCs synapse with separate sets of on & off GCs at 2 levels in inner plexiform layer of retina, a more external off lamina & a more internal on-lamina
  o These parallel channels, transmitting lightness & darkness from local retinal areas, are maintained throughout visual pathway to visual cortex
  o This “pushpull” system of neurons is thought to ↑ dynamic range for detecting decrements & increments of light in local areas of retinal image
  o Only L cones seem to be connected to both on & off-BCs while S cones are connected to on-BCs only
   Reason for this may be that latter are only involved in chromatic vision while former are involved in acuity (both chromatic & achromatic vision)
  o Achromatic vision involves detection of lightness & darkness while chromatic vision involves detection of colour
   Not only do S cones lack an off-bipolar system but they have a much different route to the GC output layer
   S cone on bipolar excite internal arbour of a bistratified GC
   Wide-field L cone off bipolar excites external arbour of this same GC
   This S cone system is absent in v centre of fovea where achromatic contrast is mediated by L cone midget system
   Away from fovea L cone midget system begins to contact more than one cone & therefore loses spatial resolution (do not see well here & do not have detailed vision in periphery)
  o There is a 2nd GC system that plays no role in colour vision but is also connected to only L cones. These are parasol GCs (have so many dendritic branches they look like an umbrella/parasol). They are larger cells with faster conduction velocities & they target the magno-cellular layers of LGN
   They play a role in detection of movement & possibly slow tracking movements
   They may receive an input from S cones but detection of such input is difficult to find. Some groups consider that these cells play a role in detection of luminance

Question 19

Describe receptive field organisation of GCs in divariant retina?

Answer 19

• If have an S cone ON then have an L cone OFF
• Core extensive antagonism
• When have L cone ON & S cone OFF  may have chromatic contrast BUT may not as system may be concentrating on achromatic or detailed contrast
• Centre/surround antagonism
• If have L wavelength OFF bipolar GC, shining a light just gives achromatic contrast

Question 20

What can a divariant colour vision system detect?

Answer 20

A divariant colour vision system can detect spectral contrasts that reflect more at one end than at the other end of the spectrum; these reflectances tend to tilt the solar spectrum

Question 21

Describe blue/yellow colour vision?

Answer 21

• As long as different bipolar inputs to the bistratified GC are excited by white light, they do not oppose each other
• Antagonistic input comes from H1 & H2 cells & provides spectral antagonism to bistratified GC because light activation of L cones will produce a depolarising signal in S cones that counteracts hyperpolarisation produced by short λ light
• The strong response to white light, however, implies that this H2 mediated antagonism is relatively weak
• Channel transmitting L cone signals for spectral contrast in visual cortex has been considered to be midget system
  o These midget GCs are considered to receive no input from S cones, either synergistic (in concert w/ activity of L wavelength system) or antagonistic (against the activity), although there are connections to S cones through H2 HCs
• In trivariant (3 colours) monkey retina there is no evidence of S cone input to either the midget or the parasol GC systems which implies that H2 horizontal cells are only post- & not pre- synaptic to L cones
• H1 HC only contacts L cones & therefore provides only spatial antagonism to neighbouring L cones & does not produce spectral antagonism
  o H1 HC does not care much about colour, more interested in achromatic/detailed vision

Question 22

Describe colour opponency?

Answer 22

• Evidence exists that L cone ON/S cone OFF GC exists to provide an opponent L cone on input to visual cortex
• Difficult to find such a GC in primate retina, but they have been reported in koniocellular layers of primate LGN
• These geniculate cells are traced back to their retinal GC inputs, which have their dendritic arbours (branches) in ON-lamina of inner plexiform layer
  o Quite different than the bistratified arbours of S ON/ L OFF GCs
• This produces a curious difference in blue/yellow channels of colour vision in primates.
  o In small animals, ground squirrels, guinea pigs & mice, there appears to be a more symmetrical system of S cone ON/L cone OFF & M cone ON/S cone OFF opponent GCs, both of which send their dendrites to ON-lamina of inner plexiform layer.
• This implies that HCs &/or intervening amacrine cells are involved in their unique opponent organisation, perhaps more similar to rod system

Question 23

What is chromatic & achromatic contrast?

Answer 23

• Parallel pathways extracting both chromatic & achromatic contrast from same cone mosaic
• Achromatic space is smaller than chromatic space
  o What get in small space, is an impression of some yellow, some brown, some blue but want to see white, grey or black (as that is achromatic)

Question 24

Describe land, colour constancy & double opponency?

Answer 24

• Experiment by Edwin Land: 2 projectors cast a mixture of white & yellow light on a blue screen. An object (black arrow) is placed in yellow beam leading to appearance (percept) of a ‘blue’ shadow of arrow on screen. This paradoxical appearance of shadow can be explained if assume that effects of separate cone mechanisms viewing screen have their responses normalised across it before being compared w/ each other to produce percept of colour (i.e. blue)
• Double opponent cell construction:
  o Central 2 ‘ON’ cells, one S & other L, inhibit each other
  o Surround L ‘ON’ cells excite central S ‘ON’ cell & inhibit central L ‘ON’ cell
  o Surround S ‘ON’ cells excite central L ‘ON’ cell & inhibit central S ‘ON’ cell
• Despite colour of light which lights a room, have a v strong percept in our brains of what a colour is & this is where colour constancy comes in

Question 25

Describe trivariant colour vision?

Answer 25

• In primates, high resolution vision & trivariant colour vision evolved to enhance survival
• Fovea formed to facilitate high resolution achromatic vision & a 3rd opsin evolved from original mammalian L cone opsin to create a new dimension of colour in higher primates
• Gene for L cone opsin duplicated itself & one of paired genes developed polymorphisms to absorb further into long λ region of the spectrum  trivariant colour vision system is formed
• Original long wave sensitive L cone now became an M cone being the partner of an even longer wavelength sensitive L cone
  o M being green and L being red (due to λ)
• A trivariant system can detect a larger variety of spectral reflectances

Question 26

What happens when a surface reflects less from both sides of visible spectrum?

Answer 26

• If a surface reflects less from both sides of visible spectrum, i.e. bending solar spectrum, it might be invisible to a divariant system because both cone types could be absorbing same amount of light from object & its background
• A trivariant system detects this object because it is impossible for all 3 cones to be absorbing same amount of light from object & its background
• More complex reflecting surfaces might confuse even a trivariant system but they are probably very rare in the natural world

Question 27

Describe trivariant system?

Answer 27

-Trivariant system more sensitive to detecting colour
-Yellow region of original spectrum was split & created 2 new chromatic percepts, red & green. A rise in beta-band absorption of this new L cone pigment also provides a long wave influence at short λ region of the spectrum
-When in dark and turn red light on, red light helps us see like night-vision goggles –> will not see green light very clearly in scotopic region
-A trivariant system facilitates distinguishing of red & green objects which would remain indistinguishable for a divariant observer
-Midget cell system assumed to play role as “double duty” detector contributing to both high spatial resolution achromatic vision & lower spatial resolution chromatic vision
-Blue-yellow & red-green are the 2 systems

Question 28

Describe the L & S Cone System Electrophysiological Responses?

Answer 28

• Responses of L cone on midget GC (large amplitude) & a M cone on midget-like GC (small amplitude) responding to small red spot in presence of same adapting fields
• Red adapting field continuously excites L cone cell & inhibits M cone cell; under these conditions small red spot inhibits L cone cell, an inhibition mediated by M cones
• Cell is excited by blue spot on yellow background & is profoundly excited by turning off of yellow adapting light but is inhibited by red spot in absence of adapting light

Question 29

Describe the Achromatic & Chromatic L & M Cone System?

Answer 29

• Evolution of trivariant colour vision in higher primates increased repertoire of colours perceived & power of spectral contrast to detect objects
• Original blue/yellow form of colour vision was now accompanied by a parallel system of red/green colours occurring in yellow region of spectrum where brightness is maximal (central vision)
• Where dynamic range of achromatic contrasts are maximal is where chromatic contrast can contribute most to detecting edges/borders when achromatic contrast is minimal

Question 30

Describe trivariant colour vision?

Answer 30

• Activity of trivariant cone system contributes to variety of major colours seen
• Borders of effective energy contrast for each of these colours are minimal, which is where colour contrast is especially important

Question 31

Describe trivariant cone mosaic?

Answer 31

• Division of a single L cone type into 2 spectrally different L & M cones produces a mosaic of L, M and S cones from which each GC picks out input of one or other of these cone mechanisms to transmit to brain
• Adaptive optics has revealed significant differences in number of L vs M cones in normal subjects

Question 32

Describe the parallel system of achromatic GCs

Answer 32

• In addition to midget & midget-like GCs described there is another parallel system of larger GCs w/ less representation in fovea than midget cell system
• These GCs also have: ON- & OFF-varieties mediated by a separate set of cone BCs & are known to be “parasol” GCs found by the Golgi method of silver impregnation
  o Movement perception
  o These parasol GCs seem to receive their inputs from only L & M cones through a different set of BCs than those serving midget cell system
• The retinal latencies of these 2 types of cells show significant difference between tonic (continuously firing) & phasic (firing in bursts) cells indicating a faster transmission of latter through inner nuclear layer by phasic system
• In addition, larger size of axons of phasic system allows this system to transmit its signals to visual cortex much quicker than tonic system
• Role of phasic “parasol” GC system in vision is not entirely clear
• It seems to play a role in detection of motion & to target different areas of visual cortex