Vision (Theme C) Flashcards
Vision is the ability to detect what type of energy?
Electromagnetic
What is the visual range of the electromagnetic spectrum?
~ 380 - 750nm
What are cone cells primarily responsible for?
Colour differentiation
What are rod cells primarily responsible for?
Contrast (light & dark) resolution
What is the region of the eye with the highest visual acuity?
The fovea - a small area in the macular region of the retina
The fovea does not contain which type of cells?
Rod cells
What is the optic disc often referred to as and why?
The ‘blind spot’
As it is where the nerves & retinal blood vessels enter and exit and is devoid of receptors
What is the role (2) of the cornea?
- Refracts & focuses light on the retina - contributes ~65-75% of the total focusing power of the eye
- Acts as a filter - screens out some of the UV wavelengths in sunlight that would damage the lens & retina
The iris controls the size of which eye structure?
Pupil
The size of the lens is controlled by which muscles?
Ciliary muscles
What is the process of ‘accommodation’?
The process by which the eye can vary the shape of the lens for far or near vision (change focus)
The ciliary muscles adjust the curvature of the lens so that light from objects at different distances can be properly focused on the retina
What are the glial cells of the retina called?
Müller cells
Which cells fo the eye are responsible for our night vision?
Rods
Which wavelengths of light are rod cells…
1. Most sensitive to
2. Completely insensitive to
- Most sensitive to: wavelengths in the blue region (~498nm)
- Completely insensitive to: red light (>640nm)
*This explains why at twilight, when rods take over from the cones, blue colours appear more intense than reds
What are the (3) functions of the retinal pigment epithelium?
- Controls the fluid & nutrient environment of the retina - electively transports ions & nutrients between the blood & the retina
- Absorption of light - its pigmentation means that all of the light that reaches the retina is absorbed and so is not reflected back onto the receptors
- Phagocytoses the damaged outer segment of photoreceptor cells (which do not replicate)
How many sets of cones are there in the eye?
3
What are the peak sensitivities of the 3 sets of cones in the eyes?
- 564nm (red)
- 534nm (green)
- 420nm (blue)
What theory underlies most (not all) of the phenomena of colour vision?
The tri-chromatic theory of Young & Helmholtz
Are rods or cones more sensitive to light?
Rods (around 100x more sensitive than cones)
What is the vitreous humor?
Transparent gel that fills the space between the lens & the retina
Many rod cells converge on a single interneurone. What effect does this have on the visual acuity?
Light detection is enhanced but visual acuity is DIMINISHED
As the pooled information is less precise than information that is obtained from a single cone converging on a single interneurone
What causes colourblindness?
People who are colourblind either lack one or more of the sets of cones or have cones that respond to different peak frequencies
What are the different types of colourblindness?
Most common:
Red-green deficiencies (congenital colour vision deficiencies)
Rarer:
Yellow-blue colourblindness (deficiency involving blue cones)
Rarest:
Full colour blindness
Insensitivity to blue light
Congenital colour vision deficiencies affect mostly which types of cones?
Red or green cones
Causing red-green colourblindness
What does protan mean?
Defect in the red cones
What does protanomaly mean?
Altered sensitivity in the red cone function
What does protanopia mean?
Complete red cone deficiency
What does deutan mean?
Defect in the green cones
What does deutanomaly mean?
Altered sensitivity in the green cone function
What does deutanopia mean?
Complete green cone deficiency
What does tritan mean?
Defect in the blue cones
What does tritanopia mean?
Complete blue cone deficiency
What is myopia and how is it treated?
Short-sightedness
Focal point of the image is before the light reaches the retina
Beyond the far point, everything is blurred
Near point is further away than normal
So in correction: a CONCAVE lens is used to bend the light rays outwards before they reach the cornea
What is hyperopia and how is it treated?
Long-sightedness
Light brought into focus behind the retina
Near point is further away than normal
So in correction: a CONVEX lens is used to bend the light rays inwards before they reach the cornea
In normal vision, the image is focused on the retina by the combined contributions of which parts of the eye?
Cornea (about 3/4)
Lens (about 1/4)
What type of lens is used to correct myopia?
Biconcave (diverging)
Used to bend the light rays outwards before they reach the cornea
What type of lens is used to correct hyperopia?
Biconvex lens (converging)
Used to bend the light rays inwards before they reach the cornea
How is visual acuity tested?
Using the Snellen Chart - these display letters of progressively smaller size