Light & The Eye Flashcards
What is light made up of?
photons
How does light behave?
sometimes like a wave other times like a particle
What is wavelength?
distance from peak to peak or crest to crest of waves
What is the effects of different wavelengths on photons?
gives photons different energy levels
What is the wavelengths of visible light?
400-700nm
What is a nanometer?
represented by nm, 1/billionth of a meter or 10^-9
What colour has the highest wavelength? What does this mean for its energy level?
red, lowest
What colour has the lowest wavelength? What does this mean for its energy level?
blue/purple, highest
What wavelengths make up the majority of the solar spectrum?
visible light (400-700)
Explain why the sky is blue?
shorter wavelengths are scattered more when they enter a transparent material like the atmosphere, therefore we see blue since it has the shortest wavelength
Explain why sunsets/sunrise look red?
because of where the sun is positioned light needs to pass through more atmosphere to reach our eyes, therefore blue light is scattered more so only red/orange is left over
Do all animals have the same light sensitivity spectrum?
no
What 3 things can cause photons to not travel in a straight line?
reflection, refraction, absorption
What is reflection? Example?
light bouncing off something and returning back to the medium it came from
mirror
What is refraction? Example?
light travels from one transparent medium to another and it bends/changes direction
distortion from something being udnerwater
What is absorption? Example?
when photon disappears and the energy is transferred to something else
light is absorbed and releases heat
What do photons consist of?
One quantum of energy
Order all categories on electromagnetic spectrum from shortest wavelength to longest?
Gamma rays, x rays, ultraviolet, visible light, infrared, heat, microwaves, radio, television
What is the first thing light passes through when it enters the eye?
Cornea
What is the cornea?
Transparent, at front of eye, let’s light through, has no blood vessels but has transparent sensory nerve ending
Why is it important that the cornea have sensory nerve endings?
Force eyes to close and produce tears of cornea is scratched
What is the purpose of tears?
Tears reduce risk of eye infection, tear film lubes eyes, provides smooth clear surface, helps protect eyes
How does cornea get oxygen and nutrients?
Aqueous humor, also supplies lens
Where is the aqueous humor located? What is its consistency?
Between cornea and lens,
liquid, water like
What is the pupil?
Hole in center of iris, let’s light through, size changes via iris
What is the iris?
Colour part of eye, ring like muscle, controls pupil size
What is the lens also called?
Crystalline lens
What is the purpose of the lens?
Helps focus light on retina, more fine tuning than cornea, changes shape to change focusing power
How many times is light refracted before it hits retina? What does this?
4 times
Cornea, aqueous humor, lens, vitreous humor
What is the longest part through journey of eye by light?
Passing through vitreous chamber that is filled with vitreous humor
Where is the vitreous humor located? What is its consistency?
Between lens and retina
Gel like, viscous, think egg white
What are floaters? What causes them?
When looking at sky might have noticed translucent shapes floating around in eye
Biodebris floating in vitreous humor
How much of the light that arrives at the cornea reach the retina?
50%
Lens power is equal to what?
=1/f
f is focal distance in meters
What are cataracts?
Opacity of the lens, can occur at many ages and take different forms
What are zonules of zinn?
connect ciliary muslce to lens, when muscle relaxes it pulls on lens
What layer of the retina does light reach last?
photoreceptor layer
What is presbyopia?
age related loss of accomadation, makes it hard to focus on near objects
What are bifocals?
correction for your distance vision on the top of the lens, and a correction to help you read on the bottom
What is emmetropia?
no refractive error because length of eyeball is perfectly matched with refractive power of lens
How can myopia be corrected?
w/negative (-) lenses, they diverge the light rays, they are concave shaped
How can hyperopia be corrected?
w/positive (+) lenses, they converge light rays, they are convex
What is the most powerful refractive surface in eye?
cornea
What is astigmatism?
unequal curving of one/more of refractive surface in eye, usually cornea
Each retina contains how many photoreceptors?
100 million
What type of neuron does not spike?
photoreceptors
Do humans have more rods or cones?
rods
Why do humans have duplex retinas?
because we have rods and cones and they operate at different capaciteies
Describe the photoreceptor density grpah?
rods start about mid way and they sloley rise til they get to the fovea and drop to 0, its mirrored on the other side, cones are very low and then quickly increase at fovea, cuts off at optic disc and cotniunues after it
How can you estimate what 1 visual angle looks like?
index finger held up at arms length is 1 visual degree, thumb is 2
If we look at an image that is less thean 1 visual degree where will the image land?
region with only cones
What lighting do rods function best it?
scotopic (dim light)
What lighting do cones function best in?
photopic (bright light)
Why can cones detect colour but not rods?
cones can have three different types of photopigments, rods all have same one
Where do rods and cones get energy?
retinal pigment epithelium which lies below the retina
Why do your eyes adjust after staying in dark for a bit?
the threshold decreases (means sensitivity increases)
What is mesopic vision?
between photopic and scotopic, rods and cones contribute
What are the four ways out visual system adjusts for luminence?
pupil size, photopigment regeneration, dpulex retina, neural circuity
How much does pupil size vary?
2mm to 8mm
How much more quanta can enter the eye when dilated?
16x
How does photopigment regernation affefct out ability to detect luminence?
in dim lighting there is a lot of leftover photopigments but in bright some light is not used b/c no photopigmnet to absorb it, gives up wide range of sensititity
How do our duplex retina allow us a wide sensitivity range?
rods detect dim light, only 1 photon needed as they get saturated cones start to take over, once light dims again and the rods recover we can see in dim lighting again
What are the 5 major classes of neurons in the retina?
photoreceptors, horizontal cells, bipolar cells, amacrine cells, ganglion cells
Where are visual pigments made in photoreceptors? Where are they stored?
inner segment
outer segment
Each visual pigment molecule is made up of what?
protein (opsin) and a chromophore (retinal)
What does chromophores do?
captures light photons
What is the chromophore, retinal, derived from?
vitamin A
Where is rhodopsin found?
only in rods
What is melanopsin?
photopigment that is senstive to ambient light
What photoreceptor cells contain melanopsin?
melanopsin-containing retinal ganglion cells/intrinsically photosensitive retinal ganglion cells
What might influence pupil response?
melanopsin
What is bleaching?
photoactivation, photon is absorbed by photoreceptor and activatews it
What does photoactivation eventually result in?
closing of cell membrane channels that normally allow ions to flow into rods outer segment, makes inside more (-)
What is the amount of glutamate present in the photoreceptor/bipolar cell synapse is proportional to what?
inversly proportional to number of photons being absorbed by photoreceptor
How do photoreceptors respond? (actiona potential or graded)
graded potentials
Are the 3 photopigments in cones distributed equally?
no
What photopigments are least prevalent in cones? Where are they not found?
short wavelength sensitive
fovea
What are the most prevalent cone photopigments?
long wavelength sensitive
What are horizontal cells?
run perpendicular to photoreceptors and make cotnact with photoreceptors and bipolar cells
What plays an important role in lateral inhibition?
horizontal and amacrine cells
What is lateral inhibition?
when an excited cell reduces the activity of neighbouring neurons
What are amacrine cells?
cells in inner nuclear layer that makes synaptic connections with bipolar cells, ganglion cells, and other amacrine cells
What have amacrine cells implicated with?
contrast enhancement and temporal sensitivity (not sure of exact function tho)
What cells form a lateral pathway in retina? What about vertical?
horizontal and amacrine
photoreceptors, bipolar, ganglion
What are bipolar cells?
make synapses with either rods or cones and horizontal cells and then passes signal to ganglion
What is the difference btwn midget and diffuse bipolar cells?
diffuse gets input from many cones and midget gets input from 1
What is ON bipolar cell vs OFF bipolar cell?
the two bipolar cells contacted by a foveal cone, the one that depolarizes is ON cell, the other that hyperpolarizes if OFF cell
Midget bipolar cells send their signal to what ganglion cell?
p ganglion cells, called this because it projects to parvocellular layer of LGN
Diffuse bipolar cells send their signal to what ganglion cells?
m ganglion cells, called this beacuse projects to magnocellular layer of LGN
Are there more P or M ganglion cells?
more P
What do receptive fields of ganglion cells look like?
circularish (center surround)
What are the two types of center surround fields?
ON center and OFF center
How do retinal ganglion cells act as a filter?
will respond best to spots of certain sizes
What are ganglion cells the most sensitive to?
differences in intesnity of light in center/surround (contrast)
Do P gnagliion cells or M ganglion cells have bigger recpetive fields?
P cells have a smaller receptive field
Are M cells or P cells more sensitive?
M cells are more sensitive
Do P cells or M cells have better visual acuity?
P cells
How do P cells react to sustained light on center?
sustained firing for entire duration
How do M cells react to sustained light on center?
will fire and then return back to normal even if light remains same
What info do P cells mainly provide?
contrast in retinal image
What info do M cells mainly provide?
how image changes over time
Whaat are the first photoreceptors to mature in retina?
intrinsically photosensitive gnaglion cells (babies can detect light before they can see images)
What is a cycle when talking about grating?
1 black and 1 white stripe
What determines the resolution acuity limit of spatial vision?
Spacing of photoreceptors in the retina
How many cones per cycle do we need to properly perceive sine wave gratings?
2 cones per cycle
What is horizontal and vertical asymmetry mean?
Vision falls of faster along vertical midline of visual field when compared to horizontal midline
What does vertical meridian asymmetry mean?
Better acuity a fixed distance below the midline of visual field than above
Is central or peripheral vision faster? Why?
Peripheral is faster
Foveal cones have longer axons because they are tightly packed
High contrast sine gratings can be distinguished from a grey field is the stripes are separated by what?
At least 1 arc minute of visual angle
What is spatial frequencies?
Number of times a pattern repeats in a given unit of space (visual angle)
What does contrast sensitive function describe?
How the sensitivity to contrast depends on spatial frequency of stimulus
What is the left y axis of CSF? How do we obtain the units?
Contrast sensitivity (starts at 1 goes up to 1000)
Taking reciprocal of right y axis
What is the right y axis of CSF?
Contrast (%)
How is contrast calculated according to michealson?
C= Lmax-Lmin/Lmax + Lmin
What are some factors that influence the form of CSF?
Adaption level of eye, temporal modulation (how change over time), and age and refractive state of individual
How does the visual system appear to break down images?
Components that are sine wave gratings with particular spatial frequencies
What is Fournier analysis?
Procedure where any signal can be separated into component sine waves at different frequencies
What is the phase of a sine wave?
It’s position relative to a fixed marker
Where do the axons of the retinal ganglion cells end?
The lateral geniculate nuclei (on both sides)
How many layers are in the LGN of primates?
6 layers
What are the magnocellular layers of lgn? What about parvocellular?
1, 2 (inner most)
3, 4, 5, 6 (outer)
What do magnocellular layer respond to vs parvocellular?
Large, fast moving objects VS details of stationary objects
What are between the layers of LGN?
Koniocellular layers
What projections reach the left LGN?
Left side of retina in both eyes
What projections reach the right LGN?
Right side of retina in both eyes
What layers receive info from the same eye?
2, 3, 5
What layers of LGN receive info from opposite eye?
1, 4, 6
How is the LGN mapped?
Topographically
What larger brain structure is the LGN a part of?
Thalamus
What are the other names of the primary visual cortex?
V1, area 17, striate cortex
How many layers does V1 have?
6
Where do the fibres from the LGN mainly project?
layer 4C, magnocellular ones go to upper layer (4Calpha), and parvo go to lower layer (4Cbeta)
What is cortical magnification?
how different amounts of cortical area are dedicated to certain regions of visual field
What did the earliest studies on cortical magnification/topography in humans use?
lesions on the cortex
What is eccentricity?
disntace between retinal image and fovea, angular distance from fovea
What do BOLD signals mean?
blood oxygen level dependent (fMRI)
What is the extrastriate cortex?
V2-V4, MT(or V5)
What is a consequence of cortical magnification?
visual acuity declines in an orderly fashion with eccrentricity
What is the biggest obstacle to object recognition in periphery?
visual crowding, deleterious effect of clutter on peripheral object recognition (obejcts that are easily identified on own seem indistinct when surrounded by other objects)
Why might visual crowding be essential?
simplifies appearence of objects in periphery by promoting consistent appearence
How does the receptive fields differ between neurons in retina/LGN vs cortex?
not circular in cortex, more elongated (respond better to edges, lines, bars, gratings, rather than spots)
What is orientation tuning? How much do they generalize?
cell is tuned to detect/respond to lines in a certain orientation
up to 30 degrees different they will still respond
Which lines are cells least responsive to? Hozionrtal, vertical, or oblique?
oblqiue
How do simple cells respond?
detect sine waves, are phase sensitive, detect more contrast
How do complex cells respond?
detect sine waves, not phase selective
What do we find for the first time in striate cortex?
orientation selective cells, motion selective cells, binocular cells that has varying degrees of dominance
What is a diopter?
unit of measurement used for an eye prescription, expresses optical power of lens
