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
What is ‘normal’ vision?
20/20 (feet) or 6/6 (metres)
What would it mean if someone had 20/40 (6/12) vision?
Means that the person can read at 20ft / 6m what a person with normal vision could reach at 40ft / 12m
I.e., their eyesight is half as good
What would it mean if someone had 20/15 vision?
Better than normal vision
The person can see at 20ft what a person with ‘normal’ 20/20 vision can see at 15ft
What is astigmatism?
Blurred vision due to uneven curvature of the cornea or lens
Focal length in the vertical & horizontal planes may be different - causing a refractive error as light is not brought to a focus at one point
Causes blurred vision at all distances
*for an eye without astigmatism, the front surface of the cornea is radially symmetrical and the eye brings light to a point focus
How can you test for astigmatism?
Using an astigmatic fan
Shown if any lines look blacker than the rest, and the blackest lines change as you rotate your head around an anterior-posterior axis
What is the near point?
The nearest point at which you are able to bring an object into focus
What is the far point?
The furthest point at which you are able to bring an object into focus
What is the far point in a normal eye?
At infinity
Parallel rays of light (i.e. from objects at infinity) are brought exactly to focus on the retina
How much refraction occurs at the cornea?
42 dioptres (~2/3 of refraction)
How much refraction occurs at the lens?
23 dioptres (~1/3 of refraction)
What is accommodative power?
A measure of your ability to change the focal length of your lens
What is the difference in the near point between people with normal vision and those with myopia?
People with myopia (nearsighted) have a significantly shorter near point than those with normal vision
When you change your focus from a near object to a far object, what occurs in the eye to allow you to focus on the object?
The ciliary muscles that control the curvature of the lens relax, increasing the tension on the suspensory ligaments (zonule fibres), which stretches the lens to a thinner shape
What is the equation to calculate accommodative power in normal eyes?
Accommodative power (dioptres) = 1/nearpoint (m)
What is the equation to calculate accommodative power in people with myopia?
For someone that is myoptic, then the far point is no longer infinity and needs to be accounted for in the calculation
Accommodative power (dioptres) = (1/near point (m)) - (1/far point (m))
What is presbyopia?
‘Eyesight of old age’
There is a loss of accommodative power, near point recedes beyond a comfortable reading distance
How can contact lenses be used to correct corneal astigmatism?
The space between the lens and the cornea is filled with saline, so any defects of corneal curvature are abolished
What is used to correct presbyopia?
Myoptic subjects may be able to correct by removing their corrective (negative) glasses
Bifocal & trifocal glasses and glasses with a graduated power (higher at the bottom than at the top) help with needing different refractive power for different working distances
What is binocular vision?
Means that you have 2 eyes with overlapping visual fields
Why is binocular vision useful?
Allows for depth perception
Why, when you focus on a distant object and hold your finger up in front of you, why do you see two images of your finger?
Two different images are seen because the images from the object are not foveated and then fall on different parts of the left and right retina (to do with retinofugal projections)
Why is there a blind spot?
The blind spot occurs because there are no rods / cones where the optic disc is situated and therefore no light can be detected at this region
What is the anatomical basis for the blind spot?
The blind spot is where the optic nerve exits the retina - there are no photoreceptors here
How can you test the extent of peripheral vison?
Using a perimeter apparatus
What is the visual field?
The extent of space within which an animal can detect a stimulus, when the eyes are held in a fixed position
What can perimetry test about the visual field?
The extent and integrity of the visual field
Why are perimetry tests carried out on each eye separately?
To detect any defects in binocular regions (central vision) of the visual field that might otherwise be compensated for by the other eye
What does the occurrence of a positive & negative afterimage suggest about the sensitivity of retinal circuits to light?
The sensitivity of retinal photoreceptors decreases gradually while they are being stimulated by light, and increases while they are not
Where are the retinal blood vessels located?
In front of the neural & photosensitive layers
Why can you not see your retinal blood vessels normally?
The brain suppresses the image of the retinal blood vessels
Stationary images fade from vision
What is the Ishihara colour test for?
Red-green colour deficiencies
What is the most common kind of colour blindness?
Red-green deficiencies
Which cones are affected in deutan type colour blindness?
M-type
Which cones are affected in protan type colour blindness?
L-type
Which cones are affected in tritan type colour blindness?
S-type
The optic nerves of each eye are connected to which structure in the brain?
The lateral geniculate nuclei in the forebrain
What % of the male population have various deficits in their ability to appreciate colours?
~ 5%
What is the definition of the ‘receptive field’?
‘The region in SPACE in which the presence of a stimulus will affect the ELECTRICAL ACTIVITY of a neurone’
How are the sizes of visual receptive fields in the retina & the brain measured?
Measured in degrees angle subtended by incident light that stimulates the neurone
What type of receptive field do retinal ganglion cells have?
Centre-surround antagonistic receptive fields (CSARFs)
(Usually have opposite on/off properties)
Why is it useful for retinal ganglion cells to have centre surround receptive fields?
They detect edges - spatial contrast
The way in which the cells respond varies depending on the way that the ‘edge’ / contrast boundary, intersects their receptive fields
How are centre-surround receptive fields of retinal ganglion cells formed?
By lateral inhibition from horizontal cells
Give the names of 2 major retinal ganglion cell populations
- Parvocellular / midget (receptive field)
- Magnocellular / parasol (receptive field)
What are the features of parvocellular / midget receptive fields?
- Can be either ON or OFF centre
- Have sustained responses to light
- Small physical size & small receptive fields
- Occupy mostly the fovea (centre of the visual field)
- Carry information about colour
- Centre & surround sum linearly
What are the features of magnocellular / parasol receptive fields?
- Can be either ON or OFF centre
- Have transient responses to light
- Large physical size & large receptive fields
- Distributed across the whole retina
- Centre & surround do not always sum linearly
- Receives info from many photoreceptors (via BPs)
Parvocellular receptive fields have ‘colour-opponency’, what different types?
M-L opponency (red vs. Green)
S-(L+M) opponency (blue vs. Yellow)
Retinal ganglion cells form mosaics across the surface of the retina. Why is this important?
Important for parallel processing of input
Spatial frequency resolution varies depending on the size of ganglion cell receptive field.
What is the spatial frequency resolution for large receptive fields?
Large RFs = Low spatial frequency resolution
Spatial frequency resolution varies depending on the size of ganglion cell receptive field.
What is the spatial frequency resolution for small receptive fields?
Small RFs = high spatial frequency resolution
How do RGC receptive fields contribute to feature extraction from visual scenes?
Different receptive fields enable detection of a range of levels of detail and contrast - important for parallel processing of visual stimuli
The axons of retinal ganglion cells connect directly to which parts of the brain?
- Superior colliculus - orienting & avoidance responses, eye movements
- Pretectum - pupillary reflex
- Lateral geniculate nucleus (in thalamus) - conveys information to primary visual cortex
The Primary Visual Cortex (V1) is located in which lobe of the brain?
Occipital lobe (at the back of the brain)
What is the ‘WHERE’ visual pathway in the brain?
The dorsal (spatial vision) pathway
Goes to visual processing regions in the parietal lobe
Involved in spatial & motion processing
Associated with the magnocellular pathway
What is the ‘WHAT’ visual pathway in the brain?
The ventral (object recognition) pathway
Goes towards the temporal lobe
Involved in visual object recognition
Associated with the parvocellular pathway
Ascending (visual) information arrives in the cortex from the Lateral Geniculate Nucleus in which layer?
Layer 4
From this layer the information travels to other layers of the cortex via inter-cortical connections (vertical / horizontal)
What is meant by retinotopy (visual field maps)?
Adjacent areas of visual space are mapped to adjacent physical areas on the retina & in the Cortex
The visual cortex has a combined columnar organisation of modular functional maps including…
Ocular dominance
Orientation selectivity
Direction selectivity
