Vision Flashcards
Week 9
what is the unit of light?
nanometers (nm)
what is the light spectrum?
380-760 nm
what is electromagnetic radiation defined by?
its frequency/ wavelength and ampitude
what are light rays emitted by?
a luminous object can be reflected, absorbed and/or refracted
light source bounces off the object
what is the speed of light?
300 thousand km/hour
where is light processed/ projected onto?
retina
what kind of light is most prominent to bees?
UV light
what does dichromatic mean?
pick up 2 different colours only.
what does light enter the eye from?
through the cornea (clear dome)
where is light processed?
the pupil (centre of the iris)
where in the eye does light bend?
the lens (image reversed and inverted)
what does light continue through the eye?
the vitreous humour (clear gel)
what is the role of the fovea?
picks up detail of an image (high acutany)
what is lens accommodation?
when the optical power of the eyes curvature changes to focus on objects at a range of distances
describe the change in curvature for short focal distances?
when ciliary muscles contract, zonules loosen creating a rounder shaped lens & higher refractive power- see at shorter focal distances
describe the change in curvature for far focal distances?
When ciliary muscles relax, zonules increase in tension, creating a flattening of the lens- increase in further focal distances.
what is presbyopia?
As we get older, objects that are close tend to become harder to focus on
○ As we age lens becomes stiffer- harder for ciliary muscles to change the lens shape
§ Full accommodation range becomes narrowed
§ Gradually starts in 20s but is very subtle so don’t notice until 40-50s.
§ Can be treated through glasses, contact lenses or laser eye surgery.
what are photoreceptors?
convert light energy into neural activity
Have photopigment in cells
what are bipolar cells?
transmit information to ganglion cells
○ Hyperpolarisation of rods and cones, depolarises the bipolar cells
what are ganglion cells?
integrate information and send APs to brain
○ Interpret the message: intensity of firing
what are horizontal and amacrine cells?
ateral neurites influence cells close by
○ Communicate with the bipolar cells- what is firing and at what intensity
* Occipital lobe transduces the message
* Light travels through all the cells
how many rods are there in the body?
92 million
where are rods found?
retinal periphrey (not in the fovea)
are rods sesnsitive to light?
yes- very
what is the function of rods?
identifying light or dark
are rods sensitive to colour?
no- balck and white info only
(monochromatic information)
what type of acuity do rods have?
poor- not sensitive to sahrpness/form of objects
how many cones are there in the eye?
4.6 million
where are cones found?
mainly in the fovea
how sensitive to light are cones?
less sensitive than rods
what type of information do cones provide us with?
info about the hue (colour/wavelength of light)
what type of acuity do cones have?
high- can detectr shape, form, colour
what is an optic disk?
where all axons from the ganglion cells accumulate together to form optic nerve
what is transduction?
- A process that converts an external stimulus to an internal stimulus
- Transduction of light energy into changes in membrane potential
what are receptive fields?
The area of the visual space in which a stimulus must be presented to change the activity of a neuron
what does the size of a neuron’s receptive fiel determine?
its acuity (smaller the better) and sensitivity (larger is better)
what is the fovea?
small as few photoreceptors converge on ganglion cell (parvocellular ganglion cells)
Small receptive field- sharper image
what is the periphery?
large as many receptors converge (magnocellular ganglion cells)
○ Large receptive field- better location of image
○ Lot more ganglion in receptive field- more rods and cones
§ Has boundaries (can be wide or small- according to where ganglions are on the retina)
Provoke action potential or non action potential
where do the rigth and left visual fields cross?
Optic Chiasma
what does ipsilateral mean?
opposite side
what does contralateral mean?
same side
what is binocular vision?
both right and left eye are working together
what happens in the nasal side of the eye?
they cross to the opposite hemisphere of the brain
what happens to the vision in the temproal/lateral side of the eye?
stays on the same side of the hemisphere
what is the role of the LATERAL GENICULARE NUCLEUS (LGN)?
relay information to make it easier to be processed further down
What is the process called through which external sensory stimuli (e.g., light) is transferred into neural activity?
transduction
Where do the axons from each optic nerve meet and cross over (depending on if they are nasal or lateral)?
optic chiasm
what is the LGN part of?
thalamus
what does each LGN recieve?
info from both eyes but only about the contralateral visual field
how many layers of LGN are there?
6 layers
what are the 6 layers of LGN innervated by?
the contralateral (layers 1, 4 and 6) and ipsilateral eye (layers 2, 3 and 5)
what is the magnocellualr layer of LGN?
layers 1 & 2: relay information about form, movement, depth, light-dark contrast, basics of information
○ Pick up the rods that provide magnocellular layer info
what is the parvocellualr layer of LGN?
layers 3-6: relay information about colour (red & green) and fine detail
○ High acuity- picking up a lot of the cones
* Sublayers are koniocellular – relay information about colour
○ Relay of info about the colour blue (cones)
what happens as we go fruther up the hierarchy of LGN?
more detail recieved by the right and left eyes
what is the straite cortex also known as?
the primarty visual cortex
how many layers is the cortical region of the straite cortex organised into, and what are they?
6 layers
○ V3a motion
○ V3 Form
○ V2 Relay signals
○ V1 catalogues input
○ VP relay signals
○ V4 colour and form
what is the role of the straite cortex?
- Takes on board the theme, form, general layout of the object
- Info passed to the extrastriate cortex: finer detail- colour
what is the dorsal stream?
where object is in space
○ Moving across our vision
○ Helps with perception of how an object is moving across our retinal image
what is the extrastriate cortex?
- Surrounds the Striate cortex / primary visual cortex (V1)
- Combines information for perception
how is the extrasriate cortex arranged hierarchically?
(V2-V8)
* Information moves up the visual association cortices, where it is analysed then passed on to higher centres for further analysis
○ In the parietal and temporal lobes- add info about what we are visualising
* Hierarchy of receptive fields in the visual system: receptive fields become larger and more complex
what are the two cortical processing streams of the extrastriate cortex?
○ Dorsal – “where” pathway
§ Cant identify where the object is in our visual field
§ If object is moving or static
○ Ventral – “what” pathway
§ Cant identify what the object is
what colour are high frequency light waves?
violet
what colour are low frequency light waves?
red
how many light detecting cells are in rods?
one- only see light or no light
how many light detecting cells in cones?
3- can see red, gree, blue (form other colours through a mixture of each of the cones)
what are the two systems of code colour?
- trichromatic coding (cones)
- opponent-processing coding (ganglion cells)
what is trichromatic coding?
- Retina contains three types of cones responsible for colour vision
- Processed by different parts of the visual system
○ Red (long nm)
○ Green (medium nm)
○ Blue (short nm)
what is a comon form of colour blindness?
the confusion between red and green colours.
* Due to a genetic defect (affecting ~10% of males) resulting in “red” cones being filled with “green-cone” photopigment or vice versa.
* 1 in 10 males are red-green colourblind
what is opponent-processing coding?
- Three colour system is converted into opponent-colour system
- 2 types colour-sensitive ganglion cells, respond to colour pairs
○ Yellow - Blue
○ Red - Green - Cant see the two colours at the same time
- 2 types colour-sensitive ganglion cells, respond to colour pairs
what happens when a red light is shown to the eye?
it stmulates the red cone: red-green gangliuon cell= excitatory and signals red
what happens when a green light is shown to the eye?
stimulate sthe green cone: red-green ganglion cell= inhibited, signals green
what happens when the light is shown the colour yellow?
stimulates the red and green cones equally: red-green= exitation and inhibition cancel each other out so no chnage in the signal is detected.
yellow-blue ganglion cells= exitatory so signals yellow
what happens when the colour blue is shown to the eye?
stimulates the blue cone: yellow-blue ganglion cell is inhibited so signals blue
what is negative afterimage caused by?
Due to adaptation in the rate of firing of ganglion cells
what is a rebound effect?
If ganglion cells are excited/inhibited for a prolonged time, they will fire less/more relative to baseline activity
give an example of negative afterimage?
- Looking at the green apple inhibits red-ON green-OFF cells excites green-ON red-OFF cells
○ both interpreted by the brain as green - Looking at a white background (reflecting neutral coloured light containing all colours) makes red-ON green-OFF cells fire more and green-ON red-OFF cells fire less
○ interpreted by the brain as red
