Vision

Vision

What are the 4 measured components of light?

wavelength

intensity

• changes in time

• changes in space

What are the units for rate of emission?

Candela

a 60W bulb gives about 100cd

How is light received measured?

Illuminance (lux)

Light received/unit area

full sunlight ~ 100,000 lux

What is the measure of scattered light?

Luminance (cd/m²)

What is albedo?

The light transmitted as a fraction of the light received

Luminance/illuminance

What are the 3 types of vision and their associated light levels and receptor cells?

Phototopic

• well lit

• rods are saturated

• uses cones

mesoscopic

• medium lighting

• uses both rods and cones

scotopic

• dim light

• below cone threshold

• uses rods

What are the 2 ways and timescales that light receptor cells adapt to the dark?

Initial lowering of threshold for both rods and cones (8mins)

Rods shift to become more sensitive to blue light

What is the formula for contrast and why is contrast used by the eye?

C = (L₁-L₂) / (L₁+L₂)

Contrast remains constant for objects and doesn’t change with lighting levels

Where is the light focused in the centre of vision?

Onto the fovea

Which organs focus the light and how refractive are they?

Lens - 19D

Cornea - 57D

How do you calculate the power of a refractive surface?

1/focal length

If measured in a refractive medium then you should multiply it by the refractive index

What is the role of the lens?

Changes shape to alter the focal length by contraction of the cilliary muscle

Define emmetropia

perfect vision

What is myopia and how is it fixed?

Short sightednesss

fixed with a negative lens

What is hypermetropia and how is it fixed?

Long sightedness

positive lens

How does astigmatism cause error?

Curvature of cornea is different in different meridians, so light in a horizontal plane is refracted differently to vertical light.

What 4 optical factors can cause errors of image formation?

Chromatic aberration

spherical aberration

glare

diffraction

How does chromatic aberration impair vision and how does the body react?

Refractive index decreases with wavelength of light

blue light is more refracted than red

monochromatic yellow light gives 25% better acuity

body covers fovea with a yellow pigment

• improves acuity at cost of letting less light in

fewer blue cones are in the centre of the fovea

What is spherical aberration and how is it fixed?

lens is a sphere not an ellipsoid

spheres bend light too much at the edges so light becomes unfocused in periphery

body closes pupil to let less light in around edge

can make lenses out of cyclinders to fix

What is glare?

The scattering of light inside the eye, by the lens or cornea, or reflection off the retina

How does diffraction hinder sight?

light spread out more when passing through a smaller aperture, so gets worse when pupil constricts

What is the point spread function?

Light spreading out before it reaches the retina

What are the 2 pupil muscles, their functions and nervous systems?

constrictor (sphincter pupillae)

• around circumfirence of iris

• allows closing of pupil

• parasynpathetic

dilator

• radially around iris

• increases pupil size

• sympathetic

What is the tradeoff influencing pupil size?

Balancing error due to refractive errors, glare, and other aberrations against diffraction

What are the 3 types of cone cell?

Long, medium and short wave receptors

How do rod cells transduce light?

Rod cell is depolarised when at rest

Light stimulation to rhodopsin causes cGMP Na+ channels to close

causes hyperpolarisation

What are the chemical components to rhodopsin?

opsin

• large peptide

• loops disk membrane 7 times

Retinal

• has 11-cis and all-trans form

• when light hits, goes from 11-cis to all-trans

What is the molecular process behind rod hyperpolarisation?

Stimulated rhodopsin activates transducin

transducin activates a phosphodiesterasae

phosphsdiesterase decreases cGMP concentration

channels close

depolarisation

How do afterimages form?

If they’re flashed with a bright light then receoptors can saturate and remain hyperpolarised for a bit.

What is a receptive feild?

The area on the retina where a cell is responsive to light.

Can be neurones furhter up the chain as well as rods and cones

What are the 2 pahtways from a receptor cell, how do they connect and what are they stimulated by?

On

• sign inverting projection

• excited by the cone being hyperpolarised

Off

• sign preserving connection

• excited by cone being depalarised

• excited by darkness

What is the use of lateral inhibition?

Increases sensitivity to contrast

What is the nature of the receptive field of a bipolar cell?

Centre-surround receptive field

How is the centre-surround receptive field of a bipolar cell achieved?

The bipolar cell connects directly to its centre

Connects via horizontal cell to surround

How does information from rods integrate with information from the cones?

Amacrine cells (above the rod bipolar cells) project to the cone bipolar cells

How does the eye react to light intensity change?

Pupil can change size from 2mm to 8mm diameter

Photoreceptors can change sensitivity

How does a photoreceptor change its sensitivity?

Some way as it depolarises

Increased light exposure increases rhodopsin activity

more transducin is activated

Phosphodiesterase becomes more active

Less cGMP

Fewer channels open so less Ca²⁺ influx

less Ca²⁺ means less active rhodopsin

What are the types of ganglion cells, their features and ratios?

Magno

• 5% of ganglion cells

• llarger Rf

• faster and more transient, lower threshold

Parvo

• 90% of ganglion cells

• smaller RF

• slower, longer signals, but higher threshold

• can be sensitive to differences in wavelength, so colours can cancel each other out

  • Red-green

  • blue-yellow

non-P, non-M

• 5%

• do other stuff

What 2 theories have informed our understanding of colour perception?

Trichomic theory

• any natural light can be made by combining Red, Blue, and Green

Opponent process theory

• There are some colour combinations that we’ll never see like reddish greeen or blueish yellow

What are the causes and prevelances for colour-blindness?

Caused by only have 2 out of 3 types of cones

1% of men don’t have L and 1% don’t have M

0.1% of all people don’t have S

What are the direction terms for the sides of the retina?

Nasal and temoral

How where does the nasal side of each retina transfer to?

The contralateral side

What is the pathway visual signals take to get to the brain?

Optic nerve

Optic chiasm (meeting point of optic nerves)

Optic tract

What are the targets of the optic tract and what do they do?

Lateral geniculate nucleus

• Main gateway to primary visual cortex

• Target of 90% of retinal ganglion cells

• underlies concious peception

Superior colliculus

• target of 10% of retinal ganglion cells

• Good response to moving objects

• fight or flight response

• Motor command signals to help plan and generate eye movement

Non-imageforming pathways

• Hypothalamus

  • Synchronising biological rhythms

• Prectectum

  • Controls pupil size

Where does the optic tract project to in non-primates?

Superior colliculus is the main target for non-primates

• 90% of retinal ganglion cells in mice with only 35% going to the LGN

Superior colliculus is called the Optic tectum in non-mammalian vertebrates and is a major target

How does the signal get from the Lateral Geniculate Nucleus to the Visual Cortex?

Via the Optic radiation

What is cortical blindness and it’s cause?

No concious perception of light

Still reacts to stimuli

Will catch a ball, but can tell you anything’s coming towards them

Caused by a lesion somewhere in the pathway from eye to end of optic radiation

What is the structure of the lateral Geniculate nucleus?

6 layers bent like a knee joint

number 1 is most ventral

Where do the areas of the retina synapse in the Lateral Geniculate Nucleus?

Temporal retinal signals synapse on 2,3 and 5 on their ipsilateral side

Nasal retinal signals go to 1,4 and 6 on their contralateral side

What is the input for each layer of the Lateral Geniculate Nucleus and how big are the neurones?

Layers 1 and 2

• Magnocellular (input from M cells)

• Larger neurones

Layers 3, 4, 5 and 6

• parvocellular (input from P cells)

• smaller neurones

Where do non-P non-M cells project to in the LGN?

K1-K6

Gaps ventral to each principal layer

What are the features of the different types of LGN cells?

Magnocellular LGN cells

• Large centre-surround receptive feild

• Transient response

• Insensitive to wavelength differences

Parvocellular cells

• Small centre-surround RF

• sustained response

• Can have colour opponency

Koniocellular cells

• centre surround

• either light/dark or colour opponency

What are the layers of the Primary visual cortex (V1)?

I

II

III

IV

• A

• B

• C

  • Alpha

  • Beta

V

VI