Recall that our first case of thinking about pigments in the eye was to consider rhodopsin alone (one pigment) and the implications of its absorption spectrum. We had two lights, each with one wavelength. The left hand light was fixed and the right hand light could be varied in intensity. Could there be any intensity where the two lights look identical even though their wavelengths are different? Now add a pigment to the receptor system. Compare the response of two pigments to the two lights, each with a different wavelength from the other (one wavelength).

Case 1 -- Equal number of photons at each of the two different wavelengths

Two different wavelength lights indicated by Greek lambdas Incident Photons Pigment 1 Photons Absorbed Pigment 2 Photons Absorbed Incident Pigment 1 Pigment 2 1000 460 150 1000 260 410

Case 2 -- Turn up the intensity of the adjustable light on the right Case 2 -- Turn up the intensity of the adjustable light on the right. What happens to the number of photons absorbed by pigment 1 and pigment 2? Does either pigment match the corresponding number for the left hand light? Remember: Once the wavelengths have been selected and the observer with a certain number of visual pigments has been selected, only the intensity (amount, number of photons) of the lights can be changed in the experiment.

Incident Photons Pigment 1 Photons Absorbed Pigment 2 Photons Absorbed Incident Pigment 1 Pigment 2 Photons Absorbed 1000 460 150 1770 460 725

Case 3 -- Turn down the intensity of the adjustable light on the right Case 3 -- Turn down the intensity of the adjustable light on the right. What happens to the number of photons absorbed by pigment 1 and pigment 2? Does either pigment match the corresponding number for the left hand light?

Incident Photons Pigment 1 Photons Absorbed Pigment 2 Photons Absorbed Incident Pigment 1 Pigment 2 1000 460 150 366 95 150

The two lights NEVER match

Summary Table What have we learned? Y Number of underlying pigments As the wavelength intensity is adjusted, is there any place where the two lights exactly match? Yes, if we have one underlying pigment. No if we have 2 underlying pigments Number of wavelengths in the adjustable light 1 2 3 4 5 6 Y 2 N Number of underlying pigments

Now add a second adjustable wavelength to the light on the right Now add a second adjustable wavelength to the light on the right. Instead of one adjustable wavelength, we now have two adjustable wavelengths in an additive mix in the light on the right.

Spectral absorption curves for the two hypothetical pigments and two adjustable wavelengths in the light on the right (2 and 3 ) compared to the fixed light on the left (1)

Pigment 1 Pigment 2 Wavelengths in Lights

Person whose eye has two cone types (pigments) Matching task: Can we adjust the amount of light in each light so that the lights look identical? If “yes,” then wavelength and intensity (amount of light) cannot be distinguished. If “no,” then wavelength and intensity can be distinguished. l1 l2 + l3 Set-up: Light with one wavelength compared to light with two wavelengths. Eye Person whose eye has two cone types (pigments) Blow-up diagram of small portion of retina – suppose there are two cone types, meaning 2 pigments

l1 l2 + l3 Incident Photons Pigment 1 Photons Absorbed Pigment 2 1 1000 530 260 2 0 0 0 3 0 0 0 -------- ----------- Total Effect 530 260 0 0 0 1000 360 110 1000 270 410 ----------- ----------- 630 520

Incident Photons Pigment 1 Photons Absorbed Pigment 2 Photons Absorbed Incident Pigment 1 Pigment 2 1 1000 530 260 2 0 0 0 3 0 0 0 -------- ----------- Total Effect 530 260 0 0 0 1250 450 137 300 80 123 ----------- ----------- 530 260

Now you got the two lights to match perfectly again Now you got the two lights to match perfectly again. To do this you had to have two adjustable wavelengths being shined on two pigments

Summary Table 1 2 3 4 5 6 Y Number of underlying pigments N Y As the wavelength intensity is adjusted, is there any place where the two lights exactly match? Number of wavelengths in the adjustable light 1 2 3 4 5 6 1 2 3 4 5 6 Y Number of underlying pigments N Y

Now add a 3rd underlying pigment What will happen when we try to match the light on the right, with two adjustable pigments, to the light on the left when there are 3 underlying responding pigments?

Fig. 14 shows the normalized absorption spectra of the three cone mechanisms that mediate primate trivariant color vision. The original L cone system of divariant color vision split into two slightly different long wavelength sensitive cones straddling the yellow region of the visible spectrum -- Color vision chapter of WebVision book linked to syllabus

Person whose eye has three cone types (pigments) Matching task: Can we adjust the amount of light in each light so that the lights look identical? If “yes,” then wavelength and intensity (amount of light) cannot be distinguished. If “no,” then wavelength and intensity can be distinguished. l1 l2 + l3 Set-up: Light with one wavelength compared to light with two wavelengths. Eye Person whose eye has three cone types (pigments)

No Match

Summary Table 1 2 3 4 5 6 Y Number of underlying pigments N Y N N As the wavelength intensity is adjusted, is there any place where the two lights exactly match? Number of wavelengths in the adjustable light 1 2 3 4 5 6 1 2 3 4 5 6 Y Number of underlying pigments N Y N N

Now add a 3rd adjustable wavelength to the light on the right

Person whose eye has three cone types (pigments) Matching task: Can we adjust the amount of light in each light so that the lights look identical? If “yes,” then wavelength and intensity (amount of light) cannot be distinguished. If “no,” then wavelength and intensity can be distinguished. l1 l2 + l3 + l4 Set-up: Light with one wavelength compared to light with three wavelengths. Eye Person whose eye has three cone types (pigments) Blow-up diagram of small portion of retina – suppose there are three cone types, meaning 3 pigments

Summary Table 1 2 3 4 5 6 Y Number of underlying pigments N Y N Y As the wavelength intensity is adjusted, is there any place where the two lights exactly match? Number of wavelengths in the adjustable light 1 2 3 4 5 6 1 2 3 4 5 6 Y Number of underlying pigments N Y N Y

Now add a 4th underlying pigment Now add a 4th underlying pigment. Will you ever get a match between the lights if there are 3 intensity adjustable wavelengths added together in the light on the right?

What would the table look like if we kept filling it in?