Vision
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Describe roughly how we see
Light enters our eyes where it acts on photoreceptors in the retina, this triggers processes where neural impulses are generated then travel through pathways and networks of the visual brain. Separate pathways to the midbrain and cerebral cortex mediate different visual functions. In the cortex, neurons in a large number of distinctive visual areas are specialised for making different kinds of visual decisions
How does light enter the eye?
Light enters the eye through the pupil and is focused by the cornea and the lens, on to the retina at the back of the eye
How does pupil become larger or smaller as light levels vary?
Pupil is surrounded by a pigment iris that can expand or contract
How many photoreceptors do humans have?
125 million
How does light become signals to the brain?
Photoreceptors arranged across the surface of retina respond to light that hits them by generating tiny electrical potentials, these signals pass via synapses through a network of cells in the retina, activating retinal ganglion cells whose axons collect together to form the optic nerve, these enter the brain where they transmit action potentials to different visual regions with distinct functions
What is the most numerous photoreceptor?
Rods
What is the least numerous photoreceptor?
Cones
How many times is rods more sensitive than cones?
1000
How do you use the types of photoreceptors?
You see at night with rods but day with cones
How many types of cones are there?
3, sensitive to different wavelengths of light
What are cones vital for?
They are vital for producing colour vision
What happens when one is overexposed to one colour of light?
The pigments in the cones adapt and make a lesser contribution to out perception of colour for a short while
What is phototransduction?
The conversion of light into electrical signals in the rods and cones
What causes the genetic basis of colour blindness?
The absence of certain visual pigments, function of retinal network and presence of two different types of ganglion cells
What are the types of ganglion cells?
90% very small P-type cells, 5% large M-type or mango cellular cells and 5% others
How are optic nerves arranged?
The fibres of each nerves meet at a structure called the optic chiasm, half of them cross to the other side where they joined another half that have stayed uncrossed. Together theses bundles of fibres form the optic tract which projects to the cerebral cortex.
How are optic tracts projected to the cerebral cortex?
Via synaptic relay in a structure called the lateral geniculate nucleus
Where are internal representations of visual space around us created in the brain?
Cerebral cortex
Where is the left-hand side of the visual world in?
In the right hemisphere
What is binocularity?
Neural representation has inputs from each eye and so the cells in the visual areas at the back of the brain can fire in response to an image in either eye
How are cells arranged in the visual cortex?
Columns
What is receptive field?
The region of retina over which the cell will respond to the preferred kind of image
What is V1?
The first stage of cortical processing
What does the neurons respond to best in V1?
Lines and edges in a particular orientation