Optics of vision, the retina and the retinal visual field
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Which area of the eye has the greatest visual acuity?
Describe the path of light through the eye
Light passes through the cornea, aqueous humor, lens and onto the fovea.
What is the difference between the visual and optical axis?
Visual axis gives the best colour vision whereas the optical axis gives the most optically clear image
Describe the refraction of light within the eye
Light is first refracted by the cornea then the lens. The cornea causes more refraction than the lens.
How should an image appear on the inside of the eye if light has been refracted properly?
Upside down and reversed
What is focal length?
Focal length is the distance between the focal point and the lens itself.
What has a shorter focal length: a powerful or weak lens?
A powerful (thicker) lens
What is accommodation?
The ability of the lens to change power through ciliary muscles. If the lens is made more powerful we are able to focus on objects closer to us
Horses lack full accommodation abilities - how do they compensate for this?
The shape of the retina means horses can see near and far by moving their head/eyes. Horses see object closer to them by raising their head to increase the focal length and help them to focus on close objects. They lower their head to see distant objects.
What components does the pupillary light reflex test?
Retina, Optic nerve (CN II), Oculomotor nerve (CN III), parts of the brainstem/midbrain and iris
True or false: both pupils should constrict in response to the pupillary light reflex?
True - the pupillary light reflex tests both the ipsilateral and contralateral sides
What happens to the eyes of animals with Horner's syndrome?
They are constricted as the animal loses the ability to dilate their pupil
True or false: you would still expect a response from the pupillary light reflex or dazzle reflex in blind animals?
True - as these reflexes do not involve the visual cortex
What is the dazzle reflex? What structures does it test?
- Aversion reaction in response to sudden intense illumination of the eye
- Oculomotor nerve (CN III), part of midbrain, Facial nerve (CN VII)
What is retinal detachment? Why is this bad?
Retinal detachment is separation of the retina from pigment epithelium. This means there will be loss of blood supply to the retina and rod/cones will start to die off unless reattachment is fairly rapid.
Which receptors are for colour/black and white vision?
- Rods - black and white
- Cones - colour
Which receptor type is more sensitive?
Rods are more sensitive than cones and can function at much lower light levels
How do rods detect light?
Discs stacked up in the outer part of the photoreceptor cells are invaginations of the plasma membrane. Inside these discs are photopigments called rhodopsin. Retinal in the rhodopsin undergoes isomerisation when light hits it (changes from a cis to a trans isomer) which allows photoreceptors to detect light.
What is rhodopsin made up of?
Retinal (a derivate of vitamin A) and opsin
Where in the eye are there no receptors?
A blind spot where the optic nerve leaves the retina
What are the two types of bipolar cell present in the eye?
One responds positively to hyperpolarisation and the other responds negatively
What are the three central visual pathways and what do they do?
- Visual cortex - recognises objects
- Rostral colliculi - localises objects
- Pretectum, pons - proprioception
Describe visual cortex pathway
Information is passed from simple cells to complex cells (which respond to motion) and hypercomplex cells (which respond to particular sizes or lengths of objects). The sum of the inputs to these cells allows us to create a picture of what we are looking at.
What are saccades?
Movements of the eye to look at objects of interest
Describe the rostral colliculi pathway
The prepontine reticular formation is inhibited by pause cells, which are inhibited by the rostral colliculi which is inhibited by the higher visual system. Three negatives make a negative so the higher visual system normally prevents saccades from occurring. Only if the higher visual system thinks something is worth looking at will they allow saccades to take place.
What different things can the eyes adapt to?
- Strong visual signals, they will cut out weak visual signals
- Constant motion
- Light - can see a much wider range of light than we are able to pass on to the visual cortex
What three colour of cones do humans and apes have?
Red, green and blue
What type of protein determines the wavelength of light detected?
How many types of opsin (cones) do most mammals have?
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