Lecture 19 - Vision I Flashcards
Features of vision
colour
Shapes (object recognition)
Depth
Movement
Stimulus and sensation of light
Sensation is perceived in the brain - form of information sent around in the brain is action potentials but the stimulus for vision is light
Visible light wavelength =
Only a small band of the electromagnetic spectrum comprises visible light (wavelengths of 400-750nm)
Electromagnetic spectrum
Only a small band comprises visible light (wavelengths of 400-750nm)
Have receptors that are responsible for receiving these wavelengths
The longer the wave length the more towards the red end of the spectrum you are
The shorter the wave length, the closer towards the violet end of the spectrum you are
Amplitude is the difference between the peak and the trough - larger amplitude means a greater intensity of colour/brighter
The visual system detects this and forms our visual perceptions of the world
The longer the wavelength
The longer the wave length the more towards the red end of the spectrum you are
The shorter the wavelength
The shorter the wave length, the closer towards the violet end of the spectrum you are
Amplitude in terms of light and vision
Amplitude is the difference between the peak and the trough - larger amplitude means a greater intensity of colour/brighter
Eye purpose
the eye is the structure that receives the light stimulus
The eye detects light and processes visual information
Creates visual perceptions and guides behaviour
Components of the eye
The eye consists of two basic components
- Optical component – collects and focuses light onto the plane of the retina
- Neural component – converts light energy into patterned changes of membrane potential that the brain can decode to create visual perceptions
Optical component
- Optical component – collects and focuses light onto the plane of the retina
Neural component
- Neural component – converts light energy into patterned changes of membrane potential that the brain can decode to create visual perceptions
Sclera
Fibrous capsule, white in our eyes which covers most of our spherical eyeball
Cornea
Sclera modified at the front of the eye, cells are no longer white and fibrous and they are now transparent
Cornea is avascular and has no direct blood supply, nutrients are received through diffusion. Adaptation that allows light to come through unhindered by pigmented cells
Conjunctiva
In front of the cornea another thin layer of cells called the conjunctiva and in front of this there is another layer (not cells or tissue) is tears
Anterior chamber
Can be used interchangeably with the aqueous humour
Fluid filled chamber responsible for inflating the eye and maintaining intraocular pressure
Increase in pressure in the anterior chamber = glaucoma
Iris
Coloured part of the eye
Sits under the cornea
Pupil
Black hole in the middle of the eye and what we are actually seeing is the back of the eye and more specifically the choroid pigment epithelium
Lens
Fibrous capsule which contains crystallised lens cells and is held in place by zonule fibres
Avascular so no direct blood supply to again help with light passing through
Zonule fibres
The zonules are the tiny thread-like fibers that hold the eye’s lens firmly in place.
Help the lens accommodate
Vitreous humour
Posterior chamber of the eye
Jelly like
Choroid epithelium
Densely pigmented cells with black pigment as black absorbs all of the light
Disorder where these are not pigmented is called albinism (albino) so light reflects all around the back of the eye so present with lots of visual problems
the actual choroid supplies vasculature
Extraocular muscles
Muscles that come out of the sides of the eye are called the extra ocular muscles which are responsible for moving the eyes around in the skull
Retina
Retinal ganglion cells line the whole of the back of the eye and these leave the eye at a place called the optic disc which is actually our blindspot and all of these retinal ganglion cells together form the optic nerve which heads off to different parts of the brain
Retina is well supplied with blood as it is part of the brain
Fovea
Depression in the retina, where we have our highest visual activity/ strongest vision
Optic disk
The optic disc (optic nerve head) is the location where ganglion cell axons exit the eye to form the optic nerve.
Blood vessels come up through the optic disk and become ramified around the eye ball **
Blind spot
What components of the eye are technically part of the brain?
Note - choroid, retina, fovea, optic disc and optic nerve are technically part of the brain
Refraction
Refraction is the “bending” of light as it passes from one substance to another e.g., air to water
Refraction is related to the difference between the refractive indices of the two media, and to the curvature of the refractive surface
What part of the eye refracts?
Cornea and the lens are able to refract light on to the plane of the retina
What part of the eye has the highest refractive power?
CORNEA
Increase in refractive index from 1 to 1.3 from the air to the corneas therefore this increase causes the bending of light. The others are relatively the same therefore cornea is responsible for the majority of the eyes ability to bend light
Optics - refractive power
The ability of a substance to “bend” light (refractive power) is measured in diopters. Reciprocal of focal length in metres.
A lens that converges light to a focal point 1 metre beyond the lens has a power of 1 diopter. A 2 diopter lens would convert rays 0.5m beyond the lens, 10 diopter at 0.1m etc.
Double the power, you reduce the metres by half
The eye has a refractive power of about 60 diopters – this equates to a focal point of about 17mm beyond the lens.
This is not always the case
The eyes refractive power…
The eye has a refractive power of about 60 diopters – this equates to a focal point of about 17mm beyond the lens.
This is not always the case
The things involved in the near response
As you shift your gaze from distant to near objects, three processes occur …
1 - Accommodation
2 - Constriction of pupil
3- Convergence of eyes
Near response - accommodation
Accommodation - Contraction/relaxation of ciliary muscle to alter lens shape and change refractive power
Near response - constriction of pupil
Constriction of pupil - Improved depth of focus, fewer optical aberrations by excluding edges of lens (periphery reduces a little bit)
Near response - convergence of eyees
Convergence of eyes - Objects remain in register on corresponding parts of the two retinae (cross eyed)
Describe accomodation in depth
When looking at distant objects the ciliary muscle is relaxed due to low parasympathetic activity, so the zonular fibres are taut, and the lens is flattened.
When gaze shifts to close objects parasympathetic activation of ciliary muscle increases, ciliary muscle contracts, tension removed from zonular fibres, lens becomes more spherical due to natural elasticity (eye’s total refractive power increases from about 60 up to about 75 diopters).
Slackens annular fibres producing a rounded lens - increasing curvature so increases eyes refractive power from about 60 to 75 diopters - this is under the optimal conditions
This is known as emmetropia = normal sight, far away object is clear
Myopia
Cant see things from far, can see close
Near sighted
Eye ball is slightly mishappened (elongated) and light is focused in front of the plane of the retina so things from far away are left out of focus
Concave lens correction
Incidence of myopia is increasing - in the 1950s in south east Asia incidence was approximately 20% and now 2010 is 80% incidence this is technology driven and the fact that there is a highly competitive school system ( need sun for eyes to grow properly - idea of the Singapore national myopia programme)
Hypermetropia (hyperopia)
Things up close are out of focus
Farsighted
Eyeball is too short
Usually by genetic factors but unsure about what these are
Convex lens correction - increases focal power by a couple of diopters and helps to bring forward the focal point
Astigmatism
Curvature of cornea and/or lens is aspherical
Different amount of refraction in different planes
Corrected with a cylindrical lens
Presbyopia
Age-related loss of accommodation
Lens loses elasticity (ie becomes stiff), accommodation falls from about 15 diopters in children, to 2 diopters at around 45-50 years, to essentially 0 at 70 years
Lens needs elasticity for accommodation
In consequence, near point recedes from 10cm at 20 years to 80cm at 60 years
Corrective lens needed (convex) to restore near vision (ie “reading glasses”)
Cataracts
Cells in the lens die as you are older, are no longer transparent and become opaque
Lens becomes opaque, especially with age
Opaqueness causes light to scatter rather than converge
Lens can be surgically removed and a plastic lens installed
Ability to accommodate is lost post-surgery
Still end up with myopia and so glasses are still needed
