Lecture 19 - Vision I

Question 1

Features of vision

Answer 1

colour
Shapes (object recognition)
Depth
Movement

Question 2

Stimulus and sensation of light

Answer 2

Sensation is perceived in the brain - form of information sent around in the brain is action potentials but the stimulus for vision is light

Question 3

Visible light wavelength =

Answer 3

Only a small band of the electromagnetic spectrum comprises visible light (wavelengths of 400-750nm)

Question 4

Electromagnetic spectrum

Answer 4

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

Question 5

The longer the wavelength

Answer 5

The longer the wave length the more towards the red end of the spectrum you are

Question 6

The shorter the wavelength

Answer 6

The shorter the wave length, the closer towards the violet end of the spectrum you are

Question 7

Amplitude in terms of light and vision

Answer 7

Amplitude is the difference between the peak and the trough - larger amplitude means a greater intensity of colour/brighter

Question 8

Eye purpose

Answer 8

the eye is the structure that receives the light stimulus

The eye detects light and processes visual information
Creates visual perceptions and guides behaviour

Question 9

Components of the eye

Answer 9

The eye consists of two basic components

1. Optical component – collects and focuses light onto the plane of the retina
2. Neural component – converts light energy into patterned changes of membrane potential that the brain can decode to create visual perceptions

Question 10

Optical component

Answer 10

1. Optical component – collects and focuses light onto the plane of the retina

Question 11

Neural component

Answer 11

2. Neural component – converts light energy into patterned changes of membrane potential that the brain can decode to create visual perceptions

Question 12

Sclera

Answer 12

Fibrous capsule, white in our eyes which covers most of our spherical eyeball

Question 13

Cornea

Answer 13

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

Question 14

Conjunctiva

Answer 14

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

Question 15

Anterior chamber

Answer 15

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

Question 16

Iris

Answer 16

Coloured part of the eye

Sits under the cornea

Question 17

Pupil

Answer 17

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

Question 18

Lens

Answer 18

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

Question 19

Zonule fibres

Answer 19

The zonules are the tiny thread-like fibers that hold the eye’s lens firmly in place.
Help the lens accommodate

Question 20

Vitreous humour

Answer 20

Posterior chamber of the eye

Jelly like

Question 21

Choroid epithelium

Answer 21

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

Question 22

Extraocular muscles

Answer 22

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

Question 23

Retina

Answer 23

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

Question 24

Fovea

Answer 24

Depression in the retina, where we have our highest visual activity/ strongest vision

Question 25

Optic disk

Answer 25

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

Question 26

What components of the eye are technically part of the brain?

Answer 26

Note - choroid, retina, fovea, optic disc and optic nerve are technically part of the brain

Question 27

Refraction

Answer 27

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

Question 28

What part of the eye refracts?

Answer 28

Cornea and the lens are able to refract light on to the plane of the retina

Question 29

What part of the eye has the highest refractive power?

Answer 29

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

Question 30

Optics - refractive power

Answer 30

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

Question 31

The eyes refractive power…

Answer 31

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

Question 32

The things involved in the near response

Answer 32

As you shift your gaze from distant to near objects, three processes occur …
1 - Accommodation
2 - Constriction of pupil
3- Convergence of eyes

Question 33

Near response - accommodation

Answer 33

Accommodation - Contraction/relaxation of ciliary muscle to alter lens shape and change refractive power

Question 34

Near response - constriction of pupil

Answer 34

Constriction of pupil - Improved depth of focus, fewer optical aberrations by excluding edges of lens (periphery reduces a little bit)

Question 35

Near response - convergence of eyees

Answer 35

Convergence of eyes - Objects remain in register on corresponding parts of the two retinae (cross eyed)

Question 36

Describe accomodation in depth

Answer 36

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

Question 37

Myopia

Answer 37

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)

Question 38

Hypermetropia (hyperopia)

Answer 38

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

Question 39

Astigmatism

Answer 39

Curvature of cornea and/or lens is aspherical
Different amount of refraction in different planes
Corrected with a cylindrical lens

Question 40

Presbyopia

Answer 40

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”)

Question 41

Cataracts

Answer 41

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