Color Perception Combined rod + cone response yields both color and brightness perception Cell responses vary with illumination conditions: –low light levels: scotopic visual processing rods predominate, minimal color perception –medium light levels: mesopic processing –high light levels: photopic processing cones predominate

10 4 Color vision is three dimensional; any spectral color can be matched by a mixture of 3 primaries

Wavelength discrimination as a function of wavelength 

Characteristics of human cone types Spectral sensitivity Mosaic Genetics Spectral tuning

Thomas Young

Techniques for measuring photopigment spectra: Derivation from CMF’s Microspectrophotometry (MSP) Electroretinography (ERG) Suction electrode recordings

It is possible to take a set of color matching functions and transform them into the cone spectral sensitivities. MSP: pass a small beam of light through the outer segment and measure the amount of light transmitted as wavelength is varied. ERG measures changes in gross electrical potential as a function of wavelength - in living eyes. By measuring the photocurrent generated in the outer segment in response to a brief flash of light it is possible to estimate the action spectra of single cones.

VariSpec Filter  Shutter Wedge Lamp Condenser lens Achromat lens Beam block Beamsplitter/ combiner Scale ~25 mm Erfle eyepiece Prism 70 o ERG-Flicker Photometry Carroll et al. (2000)

max avg - max avg = nm 0.55

Schnapf & Schneeweis (1999)

Another illustration that individual cones are univariant. If you were monitoring photocurrent, you could not distinguish between a 550 nm flash of 10 3  m -2 and a 659 nm flash of 10 4  m -2.

Characteristics of human cone types Spectral sensitivity Mosaic Genetics Spectral tuning

L/M cone ratio is about 2:1 L/M increases as you move into the periphery S cones make up ~7% No S-cones in central 0.5 deg

50  m Without AOWith AO *Registered sum of 6 images for each condition 1 deg. Temporal Retina, OD 20 yr, female, normal color vision

Selective Bleaching Conditions

Absorptance angle (  ) Number of cones AP (temporal) HS YY HS JC

MD JP JC YY * A. Roorda & D. R. Williams, Nature 1999 * * * HSAP nasal AN RS JW temporal BS JW nasal AP temporal 5 arcmin

100 %L 0 %L Can use ERG to estimate L:M

Consequences of L:M Variation One might expect that if our color perception were constrained by the photoreceptors, large differences in the cone mosaic between individuals would lead to correspondingly large differences in color perception.

Unique Yellow Is thought to represent the point at which the red/green chromatic channel is in equilibrium. Even small differences in L:M ratio would lead to substantial differences in unique yellow.

Predicted Correlation L/M Proportion (%L) Unique Yellow Wavelength (nm) 578 nm Observed

Characteristics of human cone types Spectral sensitivity Mosaic Genetics Spectral tuning

Cone-Opsin Genes

L/M Gene Array 1) L and M genes are highly homologous, 2) Head-to-tail tandem array on the X chromosome, 3) Susceptible to unequal homologous recombination.

One Cell-Type Model Stochastic Pigment-Gene Choice Second Order Neurons ? ? ? random choice mechanism “L vs. M determines gene choice cell type” L M

Characteristics of human cone types Spectral sensitivity Mosaic Genetics Spectral tuning

Wavelength (nm) Absorbance

Mechanisms of Spectral Tuning Changes in the opsin protein Ocular filters (lens, oil droplets) Altering the chromophore Different optical density

Protonated Schiff base of 11-cis-retinal normally absorbs at 440nm in organic solvents. However, most visual pigments have absorption maxima between 360 and 635 nm. It turns out that small changes in the opsin (protein) component of the photopigment are responsible for determining where the pigment will absorb.

Neitz & Neitz (1998)

Rayleigh matching reveals variation in normal pigments... Wavelength (nm) Fraction of incident light absorbed   m

Test R & G Primaries

Variants of the L gene in individuals with normal color vision