First Steps in Vision

Se 3 , Lec 1

What is light made up of?

Photons

What are photons?

Individual packets of electromagnetic energy.

How do photons travel?

Straight line.

How much light is visible to us?

400 - 700 nanometres.

What is a nanometre?

The distance between the peaks and the waves.

One-billionth of a metre.

What happens when light is not reflected, scattered or refracted and instead is absorbed by the eye?

Forms an inverted image on the retina.

What structure does the light first hit before entering the eye?

Cornea

The cornea

The light that was originally travelling in a straight line is then slightly refracted (bent).

The pupil

In dark conditions, the pupil dilates to let more light into the eye and under bright conditions, the pupil constricts to restrict the amount of light entering the eye.

The iris

A muscular structure that gives your eye it’s unique colour.

Is responsible for opening and closing the pupil.

The aqueous humour

A gel-like substance behind the cornea that provides nutrients to the surrounding structures.

The lens

The lens can bulge to help you focus on the image in the back of your retina and flatten out when looking at an object far away in the distance (accommodation).

The vitreous humour

Another gel-like substance that gives the eye it’s shape.

Optical components

Structures that have the capacity to bend the light ray.

What are the 4 optical components?

Cornea, lens, aqueous humour and vitreous humour.

The retina

A sheet of tissue formed of layers of many photosensitive cells (photoreceptors) at the back of the eye where the light can be transformed into something that the brain can interpret into vision.

The fovea

An area in the retina where you have your highest visual acuity (the most detail).

How to get maximum resolution?

Your gaze must be centred towards the exact thing you wish to see for it to reach your fovea.

Why does the periphery not have high resolution?

The light is not centred or hitting the fovea. It is not perfectly focused on the retina.

Refractive error

Image is not focused on the retina.

The optic disc

Where the axons of the ganglion cells are leaving the eye and being transmitted to the brain.

Why is the optic disk also known as the “blind spot”?

There are no photoreceptors in this area.

Any light hitting the optic disc is not seen.

Myopia

“Near-sightedness).

Rays of light converge before they hit the retina. The length of the eyeball is too long.

Image far away will be blurred.

How can myopia be corrected?

Concave lenses can diverge the rays slightly so that they can focus properly on the back of the eye (retina).

Hyperopia

“Long-sightedness).

The length of the eyeball is too short and light ends up focusing behind the retina.

Nearby objects look blurry.

How can hyperopia be corrected?

Convex lenses can be used to make sure the eye can focus properly.

Accommodation

Process by which the eye changes it’s retractive power to focus on objects.

The lens bulges for an object that is nearer and the lens flattens for objects that are further away so that they can form properly on the retina.

What is a dioptre?

Reciprocal of focal length (metres).

How much dioptre accommodation range do we have at birth?

15 dioptre = 1/15 = 0.067 metres = 0.67cm.

Presbyopia

Age-related decline in ability to accommodate.

The ciliary muscle weakens and the lens isn’t able to bulge out as much when you are older.

It becomes harder to see objects closer to us.

Astigmatism

Blur in vision due to irregular curvature of the cornea. A normal cornea is shaped like a rugby ball and an irregular cornea is shaped like a football.

Light rays from different angles don’t focus at the same point.

What cells do light have to pass through before reaching the retina?

Ganglion cells, amacrine cells, bipolar cells and horizontal cells.

Vertical pathway of the retina

Photoreceptors (rods and cones) → Bipolar cells → Ganglion cells.

The direct line sending information to the brain.

Horizontal pathway of the retina

Allowing communication across the vertical pathways.

Horizontal cells → Amacrine cells.

Midget bipolar cells

1 to 1 connection with fovea and cones.

Low convergence but high resolution/visual acuity.

Diffuse bipolar cells

Receiving input from multiple cells, rods and cones.

High convergence but low resolution/visual acuity. This also allows high sensitivity, which means you can detect a very dim spot of light against a dark background.

ON bipolar cells

Depolarise when light increases (membrane potential becomes more positive).

OFF bipolar cells

Hyperpolarise when light decreases (membrane potential becomes negative).

How many ganglion cells are there?

1.25 million

Parvocellular pathway cells (ganglion cells)

Receiving input from midget bipolar cells and are forming a p pathway.

Small dendritic and receptive fields, meaning they are responsible for fine, detail vision.

70% are p cells.

Magnocellular pathway cells (ganglion cells)

Receiving input from diffuse bipolar cells and are forming an m pathway.

Large dendritic and receptive fields, meaning they are responsible for broader vision.

10% are m cells.

Konio cells (ganglion cells)

Connected to S cones so could be related to colour.

20% are konio cells.

Rods

90-120 million.

Active in dim light conditions (scotopic).

Cones

4-5 million.

Active in bright light conditions (photopic), also responsible for colour vision.

Each cone is individually responsible for capturing light in it’s own given region of the retina.

Duplex retina

Organisation of two parts. Some responsible for one function, e.g. rods and cones.

What are the different cone types (and what ratio)?

Short (S), Medium (M), Long (L).

12(M):6(L):1(S)

The fundus

The interior lining of the eye that an optometrist can look at, including the retina, vascular tree, macula (surrounds the fovea), fovea and optic disc.

What photoreceptors are at the fovea?

Mostly cones

What photoreceptors are in the periphery?

Mostly rods

Light transduction

Turning light energy into neural signals.

Photopigment molecules

Known as G-protein coupled receptors.

There are two parts to them: a protein, known as opsin and chromophore which captures light energy.

Photoactivation

Photon energy transferred to chromophore.

Isomerization

Chromophore changes shape from 11 cis-retinal to all-trans-retinal and breaks free from opsin.

Hyperpolarisation

Membrane channels close, Na+ is reduced and the cell becomes negatively charged.

Glutamate

When calcium channels close and calcium decreases, glutamate decreases.

The amount of glutamate at the synapse of the rod or cone is inversely proportional to the number of photons absorbed. The more light the reaches the rods or cones, the less glutamate there will be at the synapse.

What happens when glutamate is reduced?

Reduction in glutamate is a signal to the bipolar cells that a rod or a cone has captured a photon, which then travels through to the ganglion cells and onwards to the brain through the optic nerve.

Receptive field

The region in visual space to which a neuron responds.

Every neuron responds to light in it’s own region of the visual field.

ON-centre receptive fields

Excitation when light is directly in the centre, but inhibited by light in the surround..

OFF-surround receptive fields

Inhibited by light in the centre, excitation when light is in the surround.

Lateral inhibition

Reduction in cell firing rate due to stimulation outside the centre of the receptive field into the surround.

The cell responds the most when spot size is perfectly matched to the centre.

This is why you are able to identify edges in an image.