The Eye and Vision Flashcards
Describe how vision is created
When parallel rays of light hit the eye lens, the rays are diverging. This means that the rays need to quickly converge in order to hit the back of the eye at the retina and be seen clearly.
When an object we are trying to see is closer to us, the lens of our eye has the ability to adapt and change shape to be more convex, increasing its power to focus on the object (accomodation).
Describe refractive error
Presbyopia: the gradual loss in our eye’s ability to change the shape of its lens to adapt to look at nearby objects. Consequence of ageing.
Myopia (short-sightedness, ‘-‘ concave lens): due to the eyeball being to big this can result in the converging of the rays entering the eye anterior to the retina. This means when the light hits the back of the eye, the rays are diverging creating a blurry image.
Hyperopia (long-sightedness ‘+’ convex lens): due to eyeball being to small this can result in the the converging of light rays behind the eye meaning that by the time the rays have hit the back of the eye they have not yet met at a single point and the image is blurry.
Describe the cells in the eye that make up vision
Rod cells (120 million - found in the periphery of the retina): respond to 1 wavelength of light and therefore is not able to generate colour vision. Works best in dim light and is very sensitive to packets of light. Poor resolution.
Cone cells (6 million - found centrally in the macula and peaking at the fovea): respond to 3 wavelengths of light (red, green, blue) and is what allows the eye to see colour. Works best in bright light and does not require a lot of light to project to the back of the eye.
There are 3 types of photoreceptors in the eye all with varying sensitivities;
- 2 photoreceptors peak close together in the green-red area of the light spectrum meaning that our eyes can differentiate between green and red colours well
- the other photoreceptor peaks further away in the blue area of the light spectrum meaning our eyes cannot differentiate between green-blue colours very well
Explain the path that light takes through the eye
Light hits the eye and the photons excite photoreceptors. This stimulates bipolar cells which carry information to the ganglion cells. The axons of these cells carry information out of the back of the eye forming the optic nerve.
Describe glaucoma
Glaucoma is an eye condition caused by damage to the optic nerve.
It is characterised by enlarging of the retinal cup in proportion to the optic disc (due to increased pressure and/or reduced blood flow to the nerve causing fibres to die). It also causes thinning of the neuroretinal ring
Describe the pathway of the visual system
The optic nerve carries information out of the back of each eye. They meet and cross over quickly in the optic chiasm. The information moves posteriorly in the optic tract towards the occipital lobe where it synapses on the LGN. Here the information splits into pathways where half of the information moves superiorly to the parietal lobes and the other half moves inferiorly to the temporal lobes. All the information meets at the occipital lobe which processes it and distributes the information back to the parietal and temporal lobes
Describe the visual development at the beginning of life
birth to 3 months: cannot perceive colour, poor contrast sensitivity. Can fixate on faces but only close with poor switching between faces and objects.
3-6 months: can fixate on faces further away, can start to recognise familiar vs unfamiliar faces, development of stereo vision
6-12 months: can fixate on faces over 2m away, more interest in abstract TV and books, increased integration of visual and motor systems
12-18 months: 6/18 visual acuity, can point to pictures in books and find hidden toys
Describe the visual development at the beginning of life
birth to 3 months: cannot perceive colour, poor contrast sensitivity. Can fixate on faces but only close with poor switching between faces and objects.
3-6 months: can fixate on faces further away, can start to recognise familiar vs unfamiliar faces, development of stereo vision
6-12 months: can fixate on faces over 2m away, more interest in abstract TV and books, increased integration of visual and motor systems
12-18 months: 6/18 visual acuity, can point to pictures in books and find hidden toys
