Colour Vision Overview 2
What is Colour?
Colour depends on wavelength but is not the same as wavelength. It is not simply defined by wavelength alone as other factors are involved. There is the question of whether colour is entirely subjective.
The Usefulness of Colour Vision
Colour vision is useful for discriminating objects based on colour, especially in scenes with variable lighting (e.g., dappled illumination). Colour vision allows for the grouping of objects based on chromaticity (wavelength) in addition to intensity (luminance).
Consider a scene with blue flowers and green leaves under dappled light. Colour vision helps in:
Grouping green leaves together, despite variations in brightness due to direct sunlight or shadows.
Contrasting the green leaves with the blue petals of the flower.
Is Colour Objectively Real?
There are varying views on whether colour is an objective quality:
Common View: Colour vision is an illusion created by networks in the visual brain used to segment objects.
Alternative View: Colour is real, with objective properties like surface spectral reflectance.
Surface Spectral Reflectance
Any image requires a light source, referred to as the illuminant. The illuminant is spectrally broad, comprising all wavelengths of visible light. Images are formed by this illuminant reflecting off the surface of objects. The reflected light carries information used to form the image.
Surfaces have varying physical and chemical properties, affecting how they absorb or reflect light. Absorption and reflection determine how dark or bright a surface appears.
High absorption leads to less reflection and a darker appearance.
Low absorption leads to more reflection and a brighter appearance.
Surfaces do not absorb all wavelengths of visible light equally, especially if they appear coloured. They reflect certain wavelengths more than others.
Example: A blue petal absorbs more light from the middle and long-wave ends of the spectrum, reflecting more light from the shorter wavelength ends.
Spectra of Reflected Light
Spectra of light reflected from different coloured objects show the relative intensity of reflected light at each wavelength. Each coloured object exhibits peaks at different regions of the spectrum. Even the spectra are broad and reflect all wavelengths but in different amounts.
Wavelength Discrimination
Wavelength discrimination is the ability to distinguish changes in wavelength independently from changes in intensity. It involves the relative amounts of light of different wavelengths forming an image, which is related to the surface spectral reflectance of objects.
The Problem: Disentangling Wavelength and Intensity
Light varies in both intensity and wavelength. A photoreceptor that detects light must differentiate between these two properties.
Experiment to Determine Colour Vision
In experiments to determine if a species has colour vision, the subject might be trained to associate a specific colour with a reward. However, one must control for differences in intensity.
Consider training bees to land on a blue square for a reward:
If the green square appears brighter than the blue, bees might be making the discrimination based on intensity rather than colour.
Carl von Frisch's Experiment
Carl von Frisch trained bees to find sugar water on a blue card, alternating it with many squares of different shades of grey. This was designed to control for chromatic differences in intensity. If bees reliably picked the blue card from various grey shades, it would indicate discrimination on a chromatic basis, i.e., wavelength.
Prerequisite for Colour Vision
Wavelength discrimination is essential for colour vision. The ability to differentiate between wavelengths is required even though colour is not solely defined by wavelength.
Single Spectral Class Photoreceptor
It's important to discuss if this wavelength discrimination can be achieved with a single spectrally tuned photoreceptor. A hypothetical photoreceptor responds best to light at nanometres (green). It responds to a broad range of wavelengths but peaks at this sensitivity. It may initially seem like a "green photoreceptor" but let's investigate.
Limitations
There are limitations to this particular type of photoreceptor. Consider these points:
A low-intensity signal at nm gives a smaller response.
A high-intensity signal at nm (where the response is 20% of that at nm) could produce the same response.
From the perspective of a single photoreceptor, wavelength and intensity cannot be disentangled. Therefore, a single photoreceptor is considered colour-blind which leads to the Principle of Univariance.
Principle of Univariance
The phototransduction signal depends on the rate of photon absorption (quantum catch). The probability of photon absorption depends on wavelength.
Example: A photoreceptor is voltage clamped, showing the photocurrent.
Flashes of light at nm and nm evoke similar responses when the intensity is adjusted.
The intensity at nm is set much higher to compensate for lower sensitivity.
Experimenters set the intensity of the flash to be much greater with the ratio set to at to at . The immediate stimulus was nine times as intense in terms of photon flux as that at .
Implications and Proof
This explains that a single cone is colour-blind:
Based on the signals from a single photoreceptor, it is impossible to determine if the response is due to a low-intensity flash at nm or a high-intensity flash at nm.
Wavelength discrimination requires at least two spectral classes of photoreceptors.
The Addition of a Second Photoreceptor
If you add a second class, as we've done here, then wavelength discrimination becomes possible. Consider first we've got our original one with a peak sensitivity of . Now we're adding a short wave receptor with a peak here at .
Two Spectral Classes of Photoreceptors
Decrease in Intensity: Decreasing the intensity of light at nm will decrease the activity in both photoreceptors.
Change in Wavelength: Decreasing the wavelength from nm (keeping intensity constant) will:
Decrease the response in the original photoreceptor.
Increase the response in the short-wave receptor.
By comparing the responses of the two photoreceptors, changes in intensity can be distinguished from changes in wavelength. Therefore, the number of spectral classes varies across the animal kingdom.
Receptor Classes
Monochromats
Animals with only one spectral class of photoreceptor are monochromats. They are literally colour-blind.
Dichromats
Animals with two spectral classes. Most nonhuman mammals fall into this category. Some humans (typically males) are also dichromats because the genes are X-linked.
Trichromats
Animals with three spectral classes (short wave, medium wave, and long wave). Most humans and old-world primates are trichromats.
Tetrachromats
Animals with four spectral classes of receptors. Most vertebrates (birds, reptiles, fish) are tetrachromatic.
The Importance of Multiple Spectral Classes
Wavelength discrimination depends upon having more than a single spectral class of photoreceptor. More classes enable finer wavelength discrimination.
Conclusion
With at least two spectral classes, wavelength discrimination is possible. This allows for the disentangling of intensity from chromaticity, which is a prerequisite for colour vision.