4.9 Variation in Color Perception among Humans and Animals

Introduction to the variation in color experience among humans and other animals.

Color blindness is often misnamed; it is more accurately referred to as color deficiency.
Individuals with color deficiency do not experience a complete absence of color vision.
Most people with color deficiency lack the ability to see the same range of colors as the majority of the population.

Dichromatism
- Most common form of color deficiency.
- Normal human vision is trichromatic, utilizing three types of cone cells.
  - These cone types correspond to red, green, and blue.
- In dichromats, one of the cone types (either red or green) is absent or non-functional, leading to limited color differentiation.
Monochromatism
- Very rare condition where individuals have only one functional cone type.
- Individuals cannot distinguish colors and perceive a very limited color spectrum largely in black and white shades.

Common tests for color vision involve images where specific numbers are visible to trichromats but obscured for dichromats.
Example:
- A distinctive number, such as 74, appears to those with normal vision, while those with color deficiencies see merging patterns and struggle to identify the number.

Prevalence of color deficiency varies by gender due to the X-linked genetic trait.
Approximately 8% of males exhibit some form of color deficiency.
- This is because males have one X and one Y chromosome.
Only about 0.05% of females have color deficiency.

Males lack a second X chromosome, which would serve as a backup for a deficient gene for normal color vision.
In females with one mutated gene on an X chromosome, a second X could still provide functional color vision.

Common misconception that certain animals perceive the world solely in black and white.
Example: Sea lions are monochromatic, viewing primarily in shades of gray.

Dogs and horses are not colorblind; they are dichromatic (with two cone types).
- They can see some color but lack the range and diversity of a trichromatic human.

Humans and most primates are trichromatic, possessing three cone types.
Birds exhibit a more advanced color perception:
- Tetrachromatism: Most birds possess a fourth cone type that allows them to see ultraviolet light.
Example with a Bird (Alice):
- Alice, as a tetrachromat, sees colors beyond human capabilities.

The vibrancy and diversity of colors experienced by birds have no human equivalent, including the ability to see ultraviolet light.

Display of a color-rich image as seen by a human versus a dog:
- For humans, reds, greens, and blues are distinctly identified.
- For dogs, colors like red and green are merged due to their limited color perception.

Understanding how animals interact based on color perception can influence human behaviors, particularly in training and play activities.
Example: Throwing a red ball into green grass could confuse a dog due to their inability to differentiate the colors clearly.

Birds, such as crows and ravens, have plumage reflecting ultraviolet light.
To humans, they appear black, but they can discern qualities among birds that remain invisible to us.
Contrast between normal photography and special ultraviolet-sensitive photography highlights these differences in perception that are unnoticeable to the human eye.