Lecture 4 - Vision
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Give progress of light entering eye
Cornea -> Pupil (iris) -> cornea+lens -> Vitreous humour -> retina
Define Cornea
clear dome at front of eye, expands and contracts to control amount of light
Define Pupil
circular opening in centre of iris
Define what the cornea + lens do
Bends light - reversing and inverting the image
Define Vitreous gel
Clear gel that light goes through to back of eye
Define retina and what is does
Circular disc at back of eye, changes light into an electrical signal
How does light signals get to brain
Optic nerve -> visual pathway -> occipital cortex (where electrical signals are converted to an image)
What is the function of rods and cones
The photopgiment they contain converts light into a chemical/ neural signal
What are the 2 photoreceptor cells?
Rods and cones
What is the process of light in the retina?
Photoreceptors -> Bipolar cells -> Ganglian cells
Whats the function of bipolar cells?
Intergrate informatino and relay info between photorecptors and brain, via ganglion cells
How many rods and cones are there?
120 million Rods 6 million cones
What do rods code for?
Night vision, sensitive to light intensity, monochromatic information. Poor acuity, more of a broad brush Found mainly in peripheral retina, not fovea
What do cones code for?
Day vision, sensitive to colour, provide info about hue Excellent acuity Found mainly in fovea
At rest, what are photoreceptors like?
Depolarised (voltage gated ion channels open at rest) and releasing glutamate Glutamate inhibits bipolar cells,
How does light reach the brain? Chemical wise
Light -> hyperpolarises photoreceptors ->less glutamate is released onto bipolar cell, so it is no longer inhibited -> bipolar produces more glutamate -> excitates ganglian cell -> AP
When axons leave the eye, what do they form?
Optic tract, optic chiasm and optic nerve
Where do axons from the eye terminate?
Lateral Geniculate nucleus (in thalamus) and superior calliculus
Defing receptive field
The area of visual space which changes the activity (firing rate) of a neuron. Tiny spots in your visual field that corresponds to that photoreceptors precise location on retina. There are millions
What does size of ganglian cell receptive field effect?
Acuity - the smaller the receptive field, the better acuity
Define Fovea
Central part of retina - a small receptive field as less photoreceptors converge on ganglian cell PARVOCELLULAR
Define periphery
surrounding area of retina - Large receptive field as many receptors converge on ganglian cells MAGNOCELLULAR
What is areas with worse acuity used for?
Less important information - e.g. movement
How do on and off areas work?
In opposite - striking centre has complete opposite effect as hitting surround