Lecture 5 Flashcards
define Light
A narrow band of electromagnetic radiation that can be conceptualized as a wave or a stream of photons.
define Photon:
A quantum of visible light (or other form of electromagnetic radiation) demonstrating both particle and wave properties.
for the purpose of this course what is light considered as
a waveform when traveling through space and a photon when hitting something
what is visible light located on in the spectrul
UV and IR
what colour is associated with shorter wavelengths
blue
what colour is associated with longer wavelengths
purple
how does light interact with things
Light can be absorbed, scattered, reflected, absorbed, transmitted, or refracted.
what happens when light is absorbed
Absorbed: Energy (e.g., light) that is taken up and is not transmitted at all.
what happens when light is Scattered:
Energy that is dispersed in an irregular fashion
Scattered: Energy that is dispersed in an irregular fashion.
When light enters the atmosphere, much of it is absorbed or scattered and never makes where
to the perceiver.
what happens when light is Reflected:
Energy that is redirected when it strikes a surface, usually back to its point of origin
aka when you see a red flower, it is actually a flower absorbing all wavelengths except red because red if being reflected back to you
what heppens when light is Transmitted:
Energy that is passed on through a surface (when it is neither reflected nor absorbed by the surface).
what happens when light is Refracted:
Energy that is altered as it passes into another medium
think pencil in water, it looks broken
The human eye is made up of various parts: what are they
- Cornea:
- Aqueous humor
- Pupil:
- Iris:
- Crystalline lens:
- Vitreous humor:
- Retina:
what is the cornea
The transparent “window” into the eyeball.
allows light to enter the eye
what is the pupil
The dark circular opening at the center of the iris in the eye, where light enters the eye.
because it is a narrow opening, this is what causes the image to flip upside down and left to right before hitting retina
what is the iris
The colored part of the eye, a muscular diaphragm, that regulates light entering the eye by expanding and contracting the pupil.
is a muscle; regulates how much light enters the eye through the pupil
what is the crystalline lens
The lens inside the eye, which focuses light onto the back of the eye.
flexible, causes most refraction in the eye, allows light to be focused at right place in eye
what is the vitreous humour
The transparent fluid that fills the large chamber in the posterior part of the eye.
does nothing to light
what is the retina
A light-sensitive membrane in the back of the eye that contains rods and cones. The lens focuses an image on the retina, which then sends signals to the brain through the optic nerve.
where all the ‘magic’ happens
what is the Aqueous humor: (behind the cornea)
The watery fluid in the anterior chamber.
light starts to refract here
Refraction is necessary for what
to focus light rays onto the retina and this is accomplished by the lens.
what is accomodation
The process in which the lens changes its shape, thus altering its refractive power.
Nearby objects require more or less refraction?
more
because light rays from near objects diverge more.
what is Diopter
The focusing force of a lens can be measured in diopters, i.e. the reciprocal of the focal length.
what is meany by ‘reciprocal of the focal length’
if f = 20 cm,
then D = 1/.2 meters
= 5 diopters
what is Focal length
Distance between the lens and the point at which light rays converge (the focus).
what happens to accommodation with age
maximal accomodation decreases with age (lens cannot contract as much)
what are the problems of refraction (eye problems)
Emmetropia: Myopia: Hyperopia: -> Presbyopia Astigmatism
what is Emmetropia
The happy condition of no refractive error.
what is
Myopia
When light is focused in front of the retina and distant objects cannot be seen sharply; nearsightedness.
light is focused before the retina! (in front of where it should be)
what is Hyperopia
When light is focused behind the retina and near objects cannot be seen sharply; farsightedness.
light is focused behind retina
what is Presbyopia
is a form of hyperopia associated with old age: eventually the lens will loose its elasticity.
what is Astigmatism
Unequal curving of one or more of the refractive surfaces of the eye, usually the cornea (cornea will likely be oval or an unevens shape and results in multiple focal points)
-> some of the lines won’t seem to converge on the common focal point, they will seem bent or not converging at centre point
what is Fundus
Using the ophthalmoscope, doctors can view the back surface of patients’ eyes, called the fundus
what are 2 things you would see at the fundus
white spot (optic disk) black/darker spot (Macula)
what is the optic disk
The white circle, know as the optic disc, is the point where the arteries and veins that feed the retina enter the eye, and where the axons of the ganglion cells leave the eye towards the brain. This portion of the retina contains no photoreceptor and is therefore “blind”.
what is the Macula
The dark spot corresponds to the macula. It is situated in the center of eye (i.e. immediately behind your lens). It contains a high densitity of photoreceptors and it is responsible for central vision
what is the Fovea
The central part of the macula is called the fovea. It contains almost no blood vessels, and contains a high density of photoreceptors. It is responsible for sharp central vision
By contrast with other other cranial nerves, the optic nerve and retina is considered part what
the brain.
It is the only part of the central nervous sytem that can be imaged non invasively.
what can the retina be used to diagnise
Can be used to diagnose certain brain disorders like Alzeihmer’s disease
what are Photoreceptors
Photoreceptors are located at the back of the retina, close to the pigment epithelium, which provides vital nutrients to the photoreceptors. The foremost layers of the retina are transparent
why are photoreceptors at the back of the eye when you would expect them to be near the front
because they need lots of nutrients and so they are connected to nerves that feed them from the brain
Photoreceptors transduce light energy into what
neural energy.
what are the 2 types of photoreceptors
Rods:
Cones:
what do Rods do
Photoreceptors specialized for night vision.
- Respond well in low luminance conditions
Do not process color
what do Cones do
Photoreceptors specialized daytime vision, fine visual acuity, and color.
- Respond best in high luminance conditions
are there more cones or rods
Much more rods (~90 millions) than cones (~4-5 millions) in your retina
where are cones concentrated
More cones in your fovea, almost no cones outside of the fovea
why do you have poor colour vision in periphery
More cones in your fovea, almost no cones outside of the fovea.
This means that you have very poor color vision in your periphery. It may seem as if your entire field of view has full-resolution color, but it does not
Note: except for slice 0.0, large cells are cones
what is Visual angle
Vision scientists measure the size of visual stimuli by how large an image appears on the retina, not by how large the object is.
Rule of thumb: If you hold your thumb out at arms length, the width of your thumbnail is about 2 degrees of visual angle.
In summary: The visual angle of an object is a function of both its actual size and distance from the observer.
The foveal has how many rods
none
The foveal “rod-free” area is about what degree of visual angle.
1
The foveal “rod-free” area is about 1 degree of visual angle what does this mean
This means that we can’t see much more than thumb at arm’s length in central (foveal) vision!
how if light converted to a sensory signal
Capturing a photon:
what is capturing a phone/how to
When light hits a photoreceptor, the process of photoactivation begins.
what are the components of Photoreceptors
Contain an outer segment (adjacent to the pigment epithelium), an inner segment, and a synaptic terminal
Visual pigments are manufactured where in the photoreceptor
in the inner segment and then stored in the outer segment
what are Photopigment
Pigments contain a retinal that captures photons and a protein, called an opsin, whose structure determines the wavelength of light to which the photoreceptor responds
what is an opsin
retinal that captures photons and a protein
what kind of opsin do rods have
rhodopsin
what kind of opsin do cones have
Cones have three different opsins, which respond to long, medium or short wavelengths.
what is melanopsin
Some photoreceptors contain melanopsin and can monitor ambient light levels and influence our sleep/wake cycle
Photopigment molecules have two possible shapes, called what
isomers
When a photopigment molecule absorbs a photon of light, it undergoes what
photoisomerization
what is photoisomerization
its shape changes from one isomer (11-cisretinal) to the other (all-transretinal).
what is Photoisomerization
initiates a cascade of biochemical reactions that eventually results in the generation of a neural signal.
what is Photopigment
regeneration, which constitutes the rest of the visual transduction cycle, transforming the all-transisomer back to the 11-cisisomer
give a full summary of transduction fo light cycle
Photopigment molecules have two possible shapes, calledisomers.
- When a photopigment molecule absorbs a photon of light, it undergoesphotoisomerization—its shape changes from one isomer (11-cisretinal) to the other (all-transretinal).
- Photoisomerization initiates a cascade of biochemical reactions that eventually results in the generation of a neural signal.
- Photopigment regeneration, which constitutes the rest of the visual transduction cycle, transforming the all-transisomer back to the 11-cisisomer.
what is The duplex retina
rods and cones have different functions!
what is another term for night vision
scotopic
what is another term for daytime seeing
photopic
the cycle of regeneration is faster in cones or rods?
cones
Highest level of light we can see is how high compared to the lowest level of light we can see
107 vs 1013 photons per second hit the eye
Highest level of light we can see is a million time more intense than the the lowest level of light we can see
what is Dark adaptation
When entering a dark room (cinema) after being outside, your visual system slowly adjusts itself.
what are the Four mechanisms for dark adaptation:
- Pupil changes its size; fast, but effects are relatively limited
- Rods and cones will gradually become more sensitive to light
- cones will first gain a lot of sensitivity, but will subsequently level off after 5-8 minutes.
- rods will steadily increase their sensitivy up to ~25 minutes (This corresponds to the time it takes for photopigments to regenerate in rods and cones) - The duplex retina: rods will take over cones (closely related to 2, just 2 has to do with regeneration so regenerated pigment will star building up in receptors)
- Neural circuits enhance contrast, making vision possible regardless of global luminance levels.
see messages for graph
study and understand it
How can we measure the sensitivity of cones separately from rod?
Use colored stimuli
Compare stimuli presented in the fovea vs periphery
do photoreceptors fire action potentions
NO
explain action potentials
- Resting membrane potential of a neuron is normally around – 70 mV. The neuronal is said to be polarized.
- - There is more positive sodium ions (Na+) outside than positive potassium ions (K+) inside of the cell. (outside has more Na+ than inside) - Activating certain receptors at the surface of the neuron will cause its membrane to depolarize.
- - positively charged sodium ions enter the cell and bring the membrane potential up to +40mV. (Na+ is let in and it depolarizes) - After the neuron has discharged, positively charged potassium ions will leave the neuron and repolarize.
- The sodium-potassium pump swaps potassium for sodium. (reestablish balance at beginning)
Once photoactivation starts what happens to photoreceptors
photoreceptors become hyperpolarized (negatively charged).
what is hyperpolarization
hyperpolarization is the opposite of a neuron discharging (or depolarizing). Normally it is associated with inhibition of neuronal activity. The fact that it is associated with the transduction of photon is an exception here.
Changes in photoreceptor activation are communicated to where
the bipolar cells
Changes in photoreceptor activation are communicated to the bipolar cells in the form of what
graded potentials
Graded potentials vary continuously in what
their amplitudes
Bipolar cells synapse with what
retinal ganglion cells
Bipolar cells synapse with retinal ganglion cells, which fire in what form
an all-or-none fashion rather than in graded potentials
summarize the Transduction of light by photoreceptors
- Once photoactivation starts, photoreceptors become hyperpolarized (negatively charged).
- > Note: hyperpolarization is the opposite of a neuron discharging (or depolarizing). Normally it is associated with inhibition of neuronal activity. The fact that it is associated with the transduction of photon is an exception here.
2) Changes in photoreceptor activation are communicated to the bipolar cells in the form of graded potentials.
Graded potentials vary continuously in their amplitudes.
3) Bipolar cells synapse with retinal ganglion cells, which fire in an all-or-none fashion rather than in graded potentials.
what is the Receptive field
The region on the retina in which stimuli influence a neuron’s firing rate.
Receptive fields have what kind of organization
center-surround organization
Receptive fields have a center-surround organization what is this in simple terms
ON/OFF or OFF/ON
Receptive fields have a center-surround organization (ON/OFF or OFF/ON)
what does this allow for
This allows to increase contrast
Receptive fields have a center-surround organization (ON/OFF or OFF/ON)
what does the organization depend on
This organisation depends on horizontal cells
