PTE · Reorder Paragraphs

Mechanisms of Colour Vision

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1

Avian Tetrachromacy

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  1. AConsequently, many avian species rely on these hidden ultraviolet signals to select healthy mates and forage for ripe berries.
  2. BThis sophisticated visual capability relies on four distinct classes of retinal cone cells rather than the three found in humans.
  3. CThe additional cone type contains specialised photopigments that react to ultraviolet wavelengths invisible to ordinary mammalian eyes.
  4. DMost wild bird species possess tetrachromatic vision, which allows them to perceive four distinct channels of colour simultaneously.

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2

Evolution of Primate Trichromacy

  1. AHowever, certain Old World primates later regained a third photopigment through a critical gene duplication event.
  2. BEarly mammalian ancestors lost two of their ancestral cone photopigments, leaving most modern mammals dichromatic.
  3. COver generations, this dietary advantage ensured that trichromatic vision became fixed across the entire lineage.
  4. DSuch chromatic discernment provided an immense evolutionary advantage when searching for tender young leaves and ripe fruit.
  5. EThis genetic mutation allowed them to distinguish red wavelengths from green ones across dense foliage.
3

Mantis Shrimp Photoreception

  1. AThis direct scanning mechanism allows the predatory crustacean to make instantaneous colour judgements in turbid coral reef environments.
  2. BTheir compound eyes contain up to sixteen distinct photoreceptor classes, twelve of which are dedicated exclusively to colour analysis.
  3. CMantis shrimps possess one of the most elaborate and complex visual systems documented anywhere in the animal kingdom.
  4. DInstead of discriminating colours through brain comparison across broad channels, each of these twelve receptor types operates independently.
4

Red-Green Colour Deficiency

  1. AAs a result, the spectral sensitivity curves of the two cone types overlap excessively, blurring chromatic boundaries.
  2. BCongenital colour vision deficiency affects millions of people worldwide, most frequently manifesting as an inability to distinguish red from green.
  3. CBecause biological males inherit only a single copy of this chromosome, they experience the condition far more frequently than females.
  4. DThis specific impairment typically arises from genetic anomalies located on the human X chromosome.
  5. EIn affected individuals, the photopigment within either the red or green cone cells is either missing or functionally altered.
5

The Purkinje Effect

  1. ABecause rod cells are inherently more sensitive to shorter, bluish wavelengths, those tones retain their prominence in dim light.
  2. BAs natural daylight fades into twilight, the human eye undergoes a profound shift in its colour sensitivity.
  3. CIt occurs because visual processing transitions from cone-dominated photopic vision to rod-dominated scotopic vision under low illumination.
  4. DThis phenomenon, known as the Purkinje effect, causes bright red objects to appear darker while blue hues seem to brighten.

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