colour vision Flashcards
(21 cards)
Why is it important to assess colour vision in optometry?
Colour vision assessment is crucial for safety (e.g., traffic lights, electrical wiring), vocational advice, detecting ocular disease, and monitoring disease progression, particularly in paediatric cases or patients with poor communication.
What is the Trichromatic Theory of colour vision?
The Trichromatic Theory (Young-Helmholtz theory) states that colour discrimination is derived from three types of cone receptors in the retina:
Long-wavelength (Red)
Medium-wavelength (Green)
Short-wavelength (Blue
What is the Opponent Colour Theory?
The Opponent Colour Theory (Hering) proposes that there are four primary colours arranged in opponent pairs:
Red vs. Green
Yellow vs. Blue
Plus a luminance mechanism involving White vs. Black.
: What is the Zone Theory of colour vision?
The Zone Theory suggests that colour vision is processed in a series of zones along the visual pathway, with trichromatic processing at the receptor level and opponent colour coding at the ganglion cell level.
How is colour vision processed in the retina?
Colour vision is mediated by three classes of cones (red, green, blue). The electrical signals from these cones are processed in the retina and transmitted to ganglion cells, which detect opponent signals and contribute to colour perception.
What is the Stiles-Crawford effect?
The Stiles-Crawford effect describes the phenomenon where cones are more sensitive to light entering the eye axially, and sensitivity decreases as light enters obliquely. This effect can cause changes in hue and saturation perception when monochromatic light enters at an angle.
What is the normal distribution of cones in the retina?
The retina contains a high density of cones in the central 2° of the visual field (fovea), which is essential for colour vision. The distribution of cones decreases towards the periphery.
What is threshold blue sensitivity, and how does it change with age?
Threshold blue sensitivity refers to the ability to perceive blue light, which decreases with age due to the increased absorption of shorter wavelengths by the crystalline lens.
What is the difference between congenital and acquired colour vision deficiencies?
Congenital deficiencies are inherited and result from abnormal or missing cone photopigments (e.g., red-green colour blindness).
Acquired deficiencies arise from ocular or systemic diseases (e.g., glaucoma) or certain medications, and they often fluctuate in severity.
What is monochromacy?
Monochromacy refers to the condition where a person has only one type of functioning cone, making it impossible to distinguish between colours based on wavelength.
What is dichromacy?
Dichromacy occurs when a person has only two types of functioning cones, leading to limited colour discrimination. There are three types of dichromacy:
Protanopia: Lack of long-wavelength (red) cones.
Deuteranopia: Lack of medium-wavelength (green) cones.
Tritanopia: Lack of short-wavelength (blue) cones.
What is anomalous trichromacy?
Anomalous trichromacy is when all three types of cones are present, but one or more cones have abnormal absorption characteristics, resulting in limited colour discrimination. This includes:
Protanomaly: Abnormal long-wavelength pigment.
Deuteranomaly: Abnormal medium-wavelength pigment.
Tritanomaly: Abnormal short-wavelength pigment.
What is the most common form of congenital colour vision deficiency?
The most common form is deuteranomaly, where the medium-wavelength (green) cones are abnormal. It affects 5% of males and is an X-linked recessive trait.
: How is acquired colour vision deficiency related to ocular pathology?
Acquired CV deficiencies are often related to ocular or systemic diseases, such as optic neuropathy, glaucoma, or diabetic retinopathy, and can be used to monitor disease progression.
What are some common causes of acquired colour defects?
Common causes include ocular diseases (e.g., glaucoma), retinal lesions, and systemic conditions (e.g., diabetes, multiple sclerosis). Certain medications (e.g., hydroxychloroquine) can also cause acquired colour vision loss.
What is the Ishihara Test used for?
The Ishihara Test is used for screening colour vision deficiencies, especially red-green defects. It consists of a series of confusion plates where a number is visible to those with normal colour vision and invisible or misidentified by those with a colour vision defect.
How does the City University Test differ from other colour vision tests?
The City University Test is a modified version of the Farnsworth D-15 test, where the patient arranges coloured discs in order. It overcomes issues like soiling of colour caps and is more suitable for patients who struggle with sequencing colours.
What does the Farnsworth 100 Hue Test assess?
he Farnsworth 100 Hue Test assesses the severity and type of colour vision deficiency by asking the patient to arrange a series of coloured caps in order. It is sensitive to red-green and blue-yellow deficiencies.
What is the Hardy-Rand-Rittler (HRR) Test used for?
The HRR Test detects both red-green and blue-yellow colour vision deficiencies and is used to diagnose early signs of glaucoma. It tests each eye separately and looks for asymmetry in the results.
What is the D-15 Test?
The D-15 Test is used to classify colour deficiencies by testing how well the patient can arrange coloured caps. It helps to differentiate between protan, deutan, and tritan defects.
How is protanopia, deuteranopia and tritanopia classified?
Protanopia is the absence of long-wavelength (red) cones, leading to a deficiency in red-green discrimination.
Deuteranopia is the absence of medium-wavelength (green) cones, resulting in a reduced ability to distinguish between red and green colours.
Tritanopia is the absence of short-wavelength (blue) cones, causing difficulty in distinguishing blue-yellow colours.
