Section 2 Flashcards
Describe Thomas Young’s double slit experiment
Using two slits, young first directed particles at it and noticed it formed a pattern of two lines that aligned with where the slits were. When this was done by shining light at the slits it produced an interference pattern whereby there were alternating light and dark bands on the screen
Describe what happens to waves that are in phase/out of phase
Waves that are in phase will have their amplitudes added to make one large waveform; waves that are out of phase will cancel each other out, resulting in a diminished amplitude
What is a photon?
A quantum of visible light demonstrating both particle and wave properties
Describe the relationship between wavelength and frequency
longer wavelengths have smaller frequencies, and shorter wavelengths have higher frequencies
What range of wavelengths is visible to the human eye?
400nm-700nm
In the range of visible light, which color dominates the lower end of the spectrum, and which color dominates the upper end?
Lower end = blue light
Upper end = red light
How can we differentiate when light acts as a particle or as a wave?
Light will behave like a particle when being emitted by the light source, or when it is interacting with matter (e.g., the retina), but when propagating in space it acts as a wave
Define light absorption
energy that is taken up and is not transmitted at all (object will appear black)
define light reflection
energy that is redirected when it strikes a surface, usually back to its point of origin
How does light reflection relate to the colors we see?
the colors we see are determined by the wavelengths of light that gets reflected
Define light scattering
Energy that is dispersed in an irregular fashion
Why are smaller wavelengths of light more susceptible to scattering?
They are more susceptible to obstacles (e.g., air particles)
Define light transmission
Energy that is passed through a surface (neither reflected nor absorbed)
Define light refraction
Energy that is altered as it passes through another medium (the angle of light changes)
Define specular and diffuse light reflection
Specular: regular and orderly reflection (the smoother the surface it is being reflected on, the more similar the incident and reflective angles will be of the light)
Diffuse: When an object’s surface contains larger irregularities, rays will be reflected in random directions
What is the first tissue in the eye that light will encounter?
The cornea
Why is the minimal light interference when passing through the cornea?
There are no blood vessels in the cornea
What is responsible for the discomfort you feel after wearing contact lenses for too long?
It irritates the transparent nerve endings in your cornea
Where is the aqueous humor located?
between the cornea and lens
What is the role of the aqueous humor?
Provides nourishment to the cornea and lens, and allows the outer layers of the cornea to quickly regenerate if there is damage
What controls the size of the pupil?
The iris
The lens is attached to ciliary muscle via what fibers?
Zonular fibers
What do zonular fibers do?
Stretches the lens when the ciliary muscles are relaxed
What is the main role of the lens?
focuses light on the back of the retina
Which eye structure causes the most refraction of light?
the lens
What is the role of the iris?
Acts as a diaphragm to allow more or less light to get through by expanding/contracting the pupil
What eye structure makes up for most the eyes volume?
Vitreous humor
What happens to light as it passes through the vitreous humor?
light is completely transmitted
Which part of the eye contains photoreceptors?
retina
What are the two types of photoreceptors? What are they in charge of?
Rods: responsible for black and white layout of objects
Cones: responsible for color perception
What two eye structures are considered as part of the CNS?
Retina and optic nerve
What is accomodation?
Process through which the shape of the lens is altered by the ciliary muscles to adjust the focal length
What is the focal length?
Distance between the lens an the point at which light rays converge
What is the relationship between lens shape and refraction?
Flatter lens = less refraction
more compressed lens = more refraction
If an object is near, will the lens refract more or less relative to an object further away? Explain
The closer the object is, the more refraction required because the light rays will be more divergent
What is optical power?
The extent to which the lens bends light
If an object is close and thus the lens is causing more refraction, what happens to the optical power?
Increases
Why does maximal accommodation decrease with age?
The lens becomes more stiff so it is less able to compress when objects are closer to you
What is optical infinity?
The distance at which rays of light are considered to be traveling parallel to each other
Define emmetropia
Light rays are coming from objects at optical infinity so there are no refractive errors
What happens to the angle of light rays as we get closer to objects?
the angle gets larger
What is the maximum accommodation (in diopters) the lens can make at a young age?
15 diopters
Describe presbyopia
Light rays get focused behind the retina because the lens has become less elastic, thus its optical power (accommodation ability) is reduced and the focal length is increased for nearby objects which now look blurry
Describe hyperopia (farsightedness)
Light rays are focusing behind the retina as a result of the eyeball being too short, thus there is not enough refraction
Describe myopia (nearsightedness)
The eyeball is too long and there is too much refraction, resulting in light rays focusing in front of the retina
How do glasses help myopia?
Diffracts the light to balance the extra refraction
Define astigmatism
caused by unequal curving of one or more refractive surfaces of the eye, usually the cornea
The cornea is spherical, however what shape does it adopt in people with an astigmatism?
Cornea becomes more oval
How does the changed shape of the cornea in people with an astigmatism affect their vision?
The changed shape causes some angles of light to diverge, leading to them not landing properly on the retina and resulting in multiple focal points
Where is the fundus located?
the back surface of the eye
Why do your eyes look red after having your photo taken ?
The fundus is completely innervated with capillaries which gives it a red color - this is what causes your eyes to appear red in photos
What is the optic disc and where is it located?
It is the white circle towards the back of the retina, and is the region where blood vessels emerge into the retina
Axons of retinal ganglion cells merge into what structure to enter the brain?
optic nerve
Explain why we have a blind spot in the middle of our eyes, and what structure is responsible for it
The optic disc has so many blood vessels and axons that it has no room for photoreceptors, and since its located in the middle of our eye this creates the blind spot
List the three reasons as to why we do not notice our blind spots
- the blind spot for each eye does not overlap so each eye makes up for the others blindness
- our eyes are constantly moving
- our brain fills in the gaps
Where will you find the macula?
dark spot in the middle of the retina
What is the macula responsible for?
central vision
Fill in the blanks: the macula has a _____ (high/low) density of photoreceptors, and a _____ (high/low) density of blood vessels
High, low
Where is the fovea located?
At the very center of the macula
The fovea is completely deprived of blood vessels; what is it responsible for?
Sharp central vision
The fovea only contains one set of photoreceptors, which type?
Cones
In general, the retina has a higher density of which photoreceptor type?
Rods
We have two main types of vision: central and peripheral. Our ability to discriminate fine details and color is limited to what type?
central vision
What is a visual angle?
Corresponds to the size of the object on the retina
What does the visual angle depend on?
Size of the object and distance from the observer
Tasks that require high visual acuity are limited to what visual angle?
2-3 degrees
What is the difference between how vision scientists measure vision vs optometrists?
vision scientists measure how large the object appears on the retina, whereas optometrists measure the size of the object
What are bipolar cells?
They receive information from photoreceptors and send them to ganglion cells
Photoreceptors are located behind two layers of neurons, all the way at the back of the retina. Why is this?
the photoreceptors are placed directly on top of a pigment epithelium which provides nourishment to the photoreceptors
Where are horizontal cells located?
At the junction between photoreceptors and bipolar cells
List 4 things horizontal cells do
- Receive input from multiple photoreceptors and integrates the signals
- Adjusts signals that will be sent to bipolar cells
- regulates activity of photoreceptors
contributes to enhancing our perception of contrast
What is the function of amacrine cells?
optimizes the signals transmitted from bipolar to ganglion cells
Describe the general architecture of rods and cones
They have an inner segment containing the cell nucleus and synaptic terminals, and an outer segment that contains a stack of discs that are packed with light-sensitive photopigments
Describe the Stiles-Crawford effect
Photons arriving from an unparalleled direction (relative to the photoreceptors) are less likely to be absorbed than those that traverse the inner segment
Why is the Stiles-Crawford effect more prominent for cones than rods?
Cones are less sensitive to unparalleled light, so light coming from the edges of the pupil are less lightly to activate cones compared to rods
What is the name of the process where photoreceptors transform light into electrical signals?
Phototransduction
Each photopigment consists of two things, what are they?
opsin and chromophore
What determines which wavelength a photoreceptor responds to?
the opsin structure
What triggers phototransduction?
the photoisomerization of the chromophore
What name do we give to a chromophore that is isomerized?
bleached
Regeneration of the photopigment is faster in which photoreceptor type?
cones
What is the name of the opsin found in rods?
Rhodopsin
How many opsins are found in cones? Name them
3: S-, M-, and L- opsins
Fill in the blanks:
S-cones are responsible for ____ (long/medium/short) wavelengths and responsible for perceiving the color ___
M-cones are responsible for ____ (long/medium/short) wavelengths and responsible for perceiving the color ___
L-cones are responsible for ____ (long/medium/short) wavelengths and responsible for perceiving the color ___
short/blue, medium/green, long/red
What is the role of melanopsin?
Monitors ambient light levels to influence our circadian rhythm
Which photoreceptor is more sensitive to light?
rods
Why are rods useless in bright environments?
Their high sensitivity means they will be constantly bleached
match each photoreceptor with the correct type of vision:
rods, cones, photopic, scotopic
rods = scotopic vision cones = photopic vision
Scotopic vision is associated with ____ (high/low) acuity and ____ (high/low) sensitivity
low/high
Photopic vision is associated with ____ (high/low) acuity and ____ (high/low) sensitivity
high/low
What is dark adaptation?
The process by which you go from a brightly lit to low lit room, and your photoreceptors need time to adjust before you can clearly see in the dark
list the 3 mechanisms that contribute to dark adaptation
- Pupil dilation
- Photoreceptor’s gain in light sensitivity
- contrast enhancement
How does pupil dilation aid in dark adaptation?
Dilating your pupils allows more light to hit your retina
Describe the gain in sensitivity pathway for rods and cones
A dark environment will promote photopigment regeneration rather than bleaching, allowing the photoreceptors to become more receptive to light. Because cones regenerate faster, they will hit their maximum sensitivity faster. Rods, however, take more time to reach maximal sensitivity so once you hit the rod-cone break (the point where the sensitivity of cones=rods) the continued improvement of your vision in the dark room is solely due to the increasing sensitivity of your rods
Why is the maximal sensitivity of rods greater than that of cones?
Rods have a greater reservoir of photopigments
Briefly describe light adaptation
This occurs when going from a dark to bright room. Initially all your photoreceptors will get bleached which is why when you’re first exposed you tend to cover your eyes, but then the cones will quickly regenerate (faster than rods) which will allow you to see clearly
What is a receptive field?
The region on the retina in which stimuli influence a neuron’s firing rate
Describe the organization of the receptive field of an ON-center/OFF-surround ganglion cell
the cell is excited when light hits the center of its receptive field, but inhibited when it hits the surrounding edges
Describe the organization of the receptive field of an OFF-center/ON-surround ganglion cell
The cell is inhibited when light hits the center of its receptive field, but excited when it hits the surrounding edges
What happens to cones when light hits it?
It becomes hyperpolarized, and releases little glutamate
How is communication done between photoreceptors and bipolar cells?
through graded potentials
What happens to the signal of a hyperpolarized cone when it reaches an ON-center and OFF-center bipolar cell?
The ON-center bipolar cell will reverse the signal, leading to its own depolarization
The OFF-center bipolar cell keeps the same signal, leading to its own hyperpolarization
What cell type (besides bipolar and ganglion) is involved in lateral inhibition?
horizontal cells
Explain how horizontal cells trigger lateral inhibition
When a center photoreceptor is presented with light but the surrounding photoreceptors are in the dark, those surrounding photoreceptors will release a lot of glutamate and activate horizontal cells. The horizontal cells are inhibitory, and will send inhibitory feedback to every photoreceptor it is connected to (includes the center photoreceptor). This will increase the inhibition on the center photoreceptor, leading to greater activation of the ON-center bipolar cell, making it seem as if there is more light than there actually is and thus increasing contrast
Why is there very little contrast when light is shined on all the photoreceptors?
The light on the surrounding photoreceptors will not trigger the inhibitory feedback from horizontal cells so the center photoreceptor will not receive any extra inhibition
What is the most important function of lateral inhibition?
Enhance the perception of edges
Which type of vision has a smaller receptive field, foveal or peripheral?
Foveal
Define acuity
the smallest spatial detail that can be resolved
How would an eye doctor define acuity?
The smallest letter one can read at a distance of 20 ft compared to what the average person can read
What is the smallest angle of a visible cycle you can identify?
one minute of an arc (or 1/60 of one degree of visual angle)
Why would our eyes fail to discriminate visual gratings smaller than 1 arc minute?
Because the cones in the fovea have an average center-to-center distance of 0.5 arc minute (two on cones or off cones will never be separated by less than 1 arc minute
How do visual scientists assess visual acuity?
Garbor patches
What are Garbor patches?
Standardized stimuli that allows you to adjust both spatial frequency and contrast
What is the maximum spatial frequency where increasing contrast helps you discriminate gratings?
60 cycles/degree
In low contrast environments, what does visual acuity depend on?
Spatial frequency
At what spatial frequency is visual acuity optimal?
7 cycles/degree
What is the simplest way to express spatial frequency?
As a sine wave
How do frequencies affect what we can see in an image?
Low frequencies allow us to see the general picture, however high frequencies allow for the perception of details
How do phase shifts impact photoreceptor activity?
A 90 or 270 degree shift will misalign the grating with the ganglion cell, resulting in no response
If you impose a phase shift of 180 degrees, what happens to an ON-center ganglion cell?
It will have a negative response (fire less than baseline)
What three things does degree of convergence depend on?
- size of receptive field
- acuity
- light sensitivity
Why do rods have a high degree of convergence?
Multiple rods converge onto a single bipolar cell
what is the name of a bipolar cell that receives input from multiple axons?
diffuse bipolar cell
What is the name of ganglion cells that receive input from multiple bipolar cells?
parasol ganglion cells
Parasol ganglion cells have ____ (small/large) receptive fields, and _____ (good/poor) spatial resolution
large/poor
Why does central vision have a low degree of convergence?
A single cone projects to a single bipolar cell, which projects to a single ganglion cell
what is the name of bipolar cells that receive input from single photoreceptors?
Midget bipolar cells
what is the name of ganglion cells that receive input from single bipolar cells?
midget ganglion cells
What is the trade-off between sensitivity and acuity?
vision with higher sensitivity will have lower acuity, and vice versa
What is the optic chiasm?
It is a structure located at the front of the hypothalamus where half of the optic nerves from the right eye cross into the left hemisphere and half the optic nerves coming from the left eye cross into the right hemisphere
The visual cortex processes information coming from the ______ (ipsilateral/contralateral) visual field
contralateral
What do you call retinal projections past the optic chiasm?
Optic tract
where is the lateral geniculate nucleus (LGN) located?
on both sides of the thalamus
Where does the LGN send the information it gets from the retina?
to the primary visual cortex
Why is the LGN connected to the brainstem?
It is involved in gating sensory information during sleep
Provide the names of the different layers of the LGN
layer 1/2: magnocellular layers
layer 3/4/5/6: parvocellular layers
Which types of ganglion cell do magnocellular layers receive input from?
parasol ganglion cells
What type of vision does the magnocellular layer of the LGN process?
Peripheral vision
What type of ganglion cell do parvocellular layers of the LGN receive input from?
midget ganglion cells
What type of vision does the parvocellular layer of the LGN process?
Central vision
What is thought to be the role of koniocellular layers in the LGN?
color perception
each LGN processes information from the ______ (ipsilateral/contralateral) visual field
contralateral
Each LGN is connected to the _____ (temporal/nasal) part of the ipsilateral retina and the ______ (temporal/nasal) part of the contralateral retina
temporal/nasal
What does it mean for each LGN to be strictly monocular?
Layers 2,3, and 5 receive info only from the ipsilateral eye, whereas layers 1,4, and 6 receive info only from the contralateral eye
What does it mean for each LGN to maintain a retinotopic organization?
The spatial layout of information on the retina is the same in the LGN
What lobe houses the primary visual cortex?
occipital lobe
TRUE/FALSE: retinopathic mapping is conserved in area V1
true
What is cortical magnitifcation?
Scaling of information in the cortex from different parts of the visual field - more cortex is devoted to processing foveal information than peripheral
