Lec 2/ TB Ch 2

Question 1

• 2 natures of light
• EM spectrum
  
  • shortest wave
  • longest wave
• Visible light
  
  • shortest
  • longest
• Does light have color, why?
• 5 things that can happen to light
  
  • starlight ex
    
    • 2 things that happen @ atmosphere
    • Why is the sky blue
    • Why is the sky red
• Other ex light can be absorbed (2)
• Other ex light is transmitted (1)
• Other ex light is reflected (hint: eye) (2)
• Other ex light is refracted (3)
• Other ex light is diffracted (3)

Answer 1

A Little Light Physics

• 2 natures of light
• Light: conceptualized as a wave OR a stream of photons (i.e. tiny particles that each consist of one quantum of energy)
  
  • We view light as a wave when is passes through a medium
  • We view light as particles when it hits a surface
• EM spectrum
  
  • Gamma rays (short wavelength)
  • Radio waves (long wavelength)
  • Visible light: 400 (violet) to 700nm (red)
• Light waves hv no color
• We see hue b/c our visual system perceives these waves as a specific color
• • Light can be absorbed, diffracted/scattered, reflected, transmitted, or refracted
• Ex. light travelling from a star to our eye
• light reaches atmosphere
  
  • Some photons are absorbed (ex. by water, dust)
  • Some photos are scattered/diffracted (aka Rayleigh scatter)
    
    • This gives the sky color:
    • Blue when the sun is high b/c blue light is scattered more
    • Red when the sun is low (sunset/near horizon) b/c EM radiation has to travel through more atmosphere near Earth’s surface-> more blue light is scattered -> leaving red/yellow
• Light hits surface
  
  • When light hits a “light-colored” surface, most of the light is reflected
    
    • Most of the light bounces off the surface -> we see a “light” surface
  • When EM radiation hits a dark surface, most of the light is absorbed
  • Some light is transmitted through the surface (neither reflected/absorbed)
  • When light travels from air to glass, some light rays are refracted (bent)
• Absorbed: Energy (e.g., light) that is taken up, can be transformed to other forms of energy
  
  • Ex. light energy transduced into neural electrical signals
  • Ex. solar panels absorb light energy, the transform it to electricity
• Transmitted: convey light from one place to another through a transparent medium
  
  • Ex. filters on traditional cameras
    
    • Red filter: absorbs other colors except red light
• Reflected: Energy that is redirected when it strikes a surface
  
  • In the eye, the cornea denser than air -> light is refracted and reflected
  • Application: eye trackers detect these reflections (i.e. Purkinje reflections), and this helps track eye movements
• Refracted: Energy that is altered as it passes into another medium, (e.g., light entering water from the air)
  
  • Depth of a swimming pool
    
    • The floor looks closer than it seems b/c water changes the direction of light
  • Tiny droplets refract light, so we see rainbow
  • Refraction: explains how lenses work, and it provide visual acuity
• Diffracted: Bent, or having waves that spread out, (e.g., waves of sound or light, as they encounter an obstacle, e.g., pass through a narrow aperture)
  
  • When light passes through the pupil, it is diffracted (it spreads out) Unwanted phenomenon the eye
  • Ex. Waves hit opening -> spread out in rings
  • Ex. Sun light spread out

Question 2

• Lec
• 4 reasons why light is a useful source of info
• Acute vision
• Light behavior: lens vs no lens

Answer 2

The human eye

• Why is light a useful source of information?
  
  • When I look at a pile of fruit, it’s ripe -> eatable/ has energy → edible
  • See dangerous animals from a distance, gives us time to run away → danger
  • Get social info from vision (facial expression, gestures) → social info
  • Some receptors in retina sets circadian rhythm; based on presence of light → circadian rhythm

Why visual acuity and how to get it

• If we don’t have visual acuity, we can’t see the letters on the panel when we visit ophthalmologist
• Acute vision: light is projected onto the retina so that light originating from a single point converges back into a point.
• Ex. 2 light sources, each project on the entire surface of the retina -> a mess of lights
  
  • IOW: You can’t see the source; you only see purple + dunno they are 2 diff colors
• Ex. If you hv a lens, the blue light rays hit the lens, the rays get diffracted and bundled -> hits a point on the retina
  
  • Same thing w/ red
    *

Question 3

• Lec
• How did the eye evolve? (from simple “eye” to complex human eye) - 7 steps
  
  • 1 simple animals
    
    • what do they have
    • fx
    • Is it an eye?
  • 2 animals w/ retinas
    
    • what does it hv?
    • fx
    • What info does this animal get?
  • 3 shape change
    
    • what happened to the shape?
    • What 2 types of info can you obtain now?
  • 4 shape change again
    
    • what happens to the shape?
    • What 2 abilities does this shape provide?
  • 5. smth is added in front of the pinhole
       * What is added?
       * fx
  • 6 smth develops
    
    • What is developed
      
      • 2 fx
  • 7 shape change
    
    • How does the shape change?
    • What is it’s fx
    • Cornea vs lens

Answer 3

• How did the eye evolve? (from simple “eye” to complex human eye)
  
  • 1. Simple animals have a few photosensitive cells that have light sensitive protein (ex. opsin)
       * light sensitive protein: opsin
       * Not an eye
       * It helps them go towards or away the light
  • 2. Then you have some animals w/ retinas
       * The retina has a few photoreceptors
       * Photoreceptors: has a bunch of opsins
       * These photoreceptors can convert light into neural signals, which is send via nerve fibers
       * This animal does not have visual acuity, blurry vision
       *
       * IOW: you can only tell when it is day vs night time
  • 3. The photoreceptors are depressed/folded in
       * The yellow photoreceptors on the top receive light from the bottom
       * Those on the bottom receive light from abv
       *
       * IOW: you can tell day/night time & the direction of light source
  • eye spots
  • 4. The fold becomes a pinhole
       * There’s direction sensitivity and spatial acuity (see fine details)
       * Specifically, the light rays (red lines) only hit a single point at the retina
       * IOW, the “pinhole” shape provides visual acuity (NOT the lens)
       *
  • 5. Cover the pinhole w/ a transparent membrane
       * Separates the inside from outside
       * If the animal swims, this prevents parasites travelling into the pinhole
  • 6. The lens develops
       * Has refractory power, provides more acuity
  • 7. Develop a bulging surface (cornea)
       * Since the cornea bulges, it fx like a lens
       * Cornea has a greater refractory power than the lens
       * Cornea vs lens
       
       • Lens can change shape

Question 4

• LABEL DIAGRAM
• Cornea
  
  • what is it
  • fx
  • Does it hv blood vessels?
  • What happens if it is scratched? - 3 steps
  • Purpose of tear film (4)
• Acqeous humor
  
  • What is it derived from?
  • 3 fx
• Sclera
• Lens
  
  • Is there blood supply?
  • fx
  • controlled by?
• pupil
• Iris
  
  • 4 fx
  • 2 settings for pupillary eye reflex
• Vitreous humor
  
  • 2 fx
• ciliary muscle
  
  • fx
  • 2 scenarios
• Zonules of Zinn
• Retina
• Fovea
  
  • aka in latin
  • Why is vision acute here?
• Optic nerve
• Optic disk
• Choroid
  
  • fx
  • Noctural animals
  • Autoimmune disease and Choroid

Answer 4

• Cornea: transparent membrane, helps focus light (constant refractory power)
  
  • Made of fibres, no blood or blood vessels
  • Scratched
    
    • 1 If cornea is scratched, the sensory nerves force eyes shut and produce tears
    • 2 Tears keep cornea transparent
    • 3 External layer of cornea regenerate fast, in 24 hr
  • Tear film:
    
    • protect and lubricate eyes
    • wash away dust
    • contact lenses sit there
    • Tears reduce risk of eye infection
• Aqueous humor:
  
  • derived from blood
  • Provide O2 and nutrients, remove waste from cornea and lens
• Sclera: cornea melts into sclera (white in humans) and wraps the eye
• Lens
  
  • No blood supply, transparent
  • helps focus light, and change shape which changes the refractory power of the eye
  • Controlled by ciliary muscle
• Pupil: hole/opening in the iris
• Iris: gives color,
  
  • has ring muscle controls size of the pupil
  • IOW: controls the amount of light reaching the retina
  • Helps w/ adaptation
  • Pupillary light reflex
    
    • Dark: makes pupil a big black spot
    • Day: makes pupil a tiny hole
• Vitreous humor = thick liquid
  
  • refract light
  • keep eye shape round
• Cillary muscle: another muscle ring, changes shape of lens
  
  • When it contracts, the lens contracts
  • When it relaxes, the lens relax and gets wider
• Zonules of Zinn: connects lens w/ ciliary muscles
• • Retina: transduction
• Fovea: (pit in latin) pit on the retina
  
  • other cells pushed aside, and only has photoreceptors
  • Most acute vision due to many photoreceptors there (nothing to due w/ cornea and lens)
• Optic nerve: receive signals from retina and send signals
• Optic disk: blind spot, no photoreceptors, where signals leave via optic nerve
• Choroid (brown/black layer): light that is not absorbed by the photoreceptors on the retina is absorbed here
  
  • So, the excess light rays won’t bounce back to blur our vision
  • Nocturnal animals: Choroid is reflective, so their vision is more sensitive and has a bit of blur -> see in darkness
  • Autoimmune disease: can attack choroid, inflame it, and damage retina

Question 5

• Process to focus light
• four optical components that focus light
  
  • Which one has the highest refraction?
  • Which components’ refractive powers are fixed
• How can we focus light on the retina?
• Accommodation process: 3 steps
  
  • accommodated state vs unaccomodataed state
    
    • ciliary muscles state
    • Zonules state
    • Lens state
    • Focus on close vs distant object
    • Describe light rays in image
• Power formula
• diopters
  
  • Diopter and focal length relationship
• Age and accommodation relationship
• Presbyopia
  
  • cause
  • what happens
  • 2 issues
  • Solution
• Crystallins
• How do lens become opaque?
• What is the condition called?

Answer 5

Focusing Light onto the Retina

• Refraction is necessary to focus light rays.
• This is done by the four optical components.
  
  • Lens, cornea, vitreous humor, aqueous humor
• Cornea: is curved, so it has a high refractive index (1.4 vs 1 in air)
  
  • it is the most refractive surface in the eye
• The aqueous and vitreous humours can refract light, but their refractive power is fixed (incl cornea)
  
  • IOW: can’t bring close objects into focus
• Lens: brings close objects into focus
• To focus an image on the retina, the refractive power of the 4 optical components has to match the length of the eyeball
• Accommodation (change in focus): lens change shape -> change refractive power -> bring close objects into focus
  
  • Ciliary muscle is attached to lens via tiny fibers (Zonules of Zinn)
  • Unaccommodated state
    
    • Ciliary muscle is relaxed
    • Zonules are stretched
    • Lens is flat
    • Eye is focused on distant objects (ex. stars)
    • → Since the dot is so far away, // light enters. The bend/refraction is not so extreme.
      
      • Ciliary muscles relax, so the lens is stretched
  • Accommodated state
    
    • Ciliary muscle contract
    • Reduced tension on Zonules
    • Lens bulge
    • Fatter lens -> Eye is focused on close objects (ex. iPhone)
    • → If the object is very close to you, you need more refractory power so that the light rays can hit the retina
      
      • Here, the lens is fat/relaxed; ciliary muscles contract/stretched -> more refractory power
• Accommodation can change the power of the lens
• Lens power (P) = 1/f
  
  • f = focal distance (m)
    
    • d b/w lens (or mirror) and object
• Diopters (D): a measure of optical power of the lens
  
  • D = 1/f
  • Ex. 15 diopters of accommodation: you can read your watch at 6.7 cm
• Ability to accommodate declines w/ age
• Presbyopia: aka old sight
  
  • Age-related loss of accommodation,
  • lens hardens,
    
    • → lens can’t get fat to focus on nearby shit; lens can’t stretch to see far stuff
  • We have bifocals
    
    • Lenses that have power x at the top so we can see distant objects, and power y at the bottom so we can see objects at reading distance
• Our lens is transparent
  
  • Crystallins: proteins that make up the lens, lined up uniformly
  • If it is not lined up uniformly, the lens becomes opaque
• Cataracts: lens become opaque
  
  • Happen at diff ages
  • Congenital cataracts: present at birth, rare
  • Other cataracts: discovered after 50 yo; older = higher chance of cataracts
  • It affects vision as they absorb and scatter more light
  • Treatment of cataract: remove opaque lens, replace w/ silicone implant
    
    • 30 min surgery

Question 6

• Emmetropia
  
  • definition
  • How are diff length eyeballs emmetropic?
  • Why are most bb hypertrophic
• Refractive errors
  
  • 4 conditions
• Myopia
  
  • issue
  • 2 reasons
  • correction
• Hyperopia
  
  • issue
  • 3 reasons
  • correction
• Presbyopia
  
  • aka
• Astigmatism
  
  • cause
  • What do you see?
  • 2 corrections

Answer 6

• Emmetropia: refractive power of the 4 optical components of the eye (cornea, aqueous humor, lens, and vitreous humor) match the length of the eyeball
  
  • acute vision w/o glasses
  • Human eye: 24 mm, diameter of quarter
  • Some eyeballs are longer/shorter but are still emmetropic
  • This is b/c eye grow to match the power of optical components we’re born with
  • • NOTE: most bb are hyperopic
  • The optical components are more developed than the length of their eyeballs
• Refractive errors: image of the world cannot focus on the retina
  
  • Happens when the eyeball is too long or short relative to the power of the 4 optical components
  • Ex. myopia, hyperopia, astigmatism, presbyopia
• Myopia/near sight:
  
  • refractory power is too strong, and it converges b4 the retina
    
    • Reasons
    • 1. longe eyeball
    • 2. Cornea is more curved -> more refractory power
  • Correction for myopia
    
    • Need to remove refractory power (less bulge)
    • Put concave lens, to cave things in
    • So the light converges at the retina
• Hyperopia/farsighted: image (of the star) is focused behind retina
  
  • 3 reasons
  • 1. shorter eyeball -> weaker refractory power -> light converges behind the retina
  • 2. Refractory power is low (less curve cornea)
  • 3. When ppl get older, the lens gets stiff -> less curved -> less refractory power
  • If it is no too severe, the person can compensate by accommodating (increase power of the eye) → squint
  • Correction:
    
    • Use convex lens (create more bulge) -> add refractory power
  • • Presbyopia = How do you hv myopia and hyperopia?
  • You can have myopia for things that are far away
    
    • When objects are farm, the lens can’t be stretched (inelastic)
  • And have hyperopia for things that are close by
    
    • When objects are close, lens is not round (inelastic)
• Astigmatism
  
  • Normal Cornea: round like a basketball
  • Ppl w/ astigmatism has cornea that are longer like American football
    
    • Things are blurry due to cornea
  • IOW: some lines seem out of focus, some are sharp
  • Correction
    
    • Lenses w/ 2 focal points (provide diff amounts of focusing power in the horizontal and vertical planes): correct astigmatism
    • LASIK or refractive surgery: change cornea’s refractive power

Question 7

• Ophthalmoscope
• yellow dot
  
  • what is it?
  • Characteristics
• brown circle
  
  • what is it?
  • Where is the fovea?
    
    • fx
  • Where is the vascular tree?
• Photomicrograph
  
  • 2 layers of retina
  • color

Answer 7

What the Doctor Saw

• Ophthalmoscope: Dr shine bright light into your the back of eye (fundus)
  
  • Yellow dot = optic disk, no photoreceptors, blind spot
  • Brown circle = macula (spot)
    
    • fovea (v tiny) is there; for acute vision
    • Vascular tree spreads across retina but stops at fovea
• • Photomicrograph: provides a detailed view on retina’s structure (Ophthalmoscope can’t)
    
    • Retina: transduction happens here, 2 main layers
      
      • Layer 1: has layers of clear neurons,
        
        • Half the thickness of credit card
      • Layer 2: A layer of darker cell

Question 8

• 2 steps in transduction
• Why travel all to the back?
  
  • 2 main reasons

Answer 8

• Process of transduction
  
  • 1. Light will travel through all the layers to reach the photoceptors at the retina; reaches outer segment first, then inner segment; transduction happens
  • 2. Neural signals than travel from photoreceptors -> bipolar cells & horizontal cells & amacrine cells -> ganglion cells
  • • Why are photoreceptors at the back/last layer?
      * Although light needs to pass through ganglion, horizontal, amacrine cell, these cells are transparent
      * Photoreceptors rely on specific cells to provide nutrients and housekeeping, and those cells are opaque
      * Those cells are also located in a pigmented epithelium

Question 9

• 5 cell types
• photoreceptors role
• duplex retina
  
  • fx
  • #
  • location
  • size
  • implication
• Optic disk: # cones, # rods
• only rods animals
• only cones animals
• 3rd photoreceptor type
• Eccentricity
• Why do have rods when we mainly use cones to see these days?

Answer 9

Retinal Geography and Function

• 5 cell type: photoreceptors, amacrine, horizontal, bipolar, ganglion
• Photoreceptors: transduction
• Duplex retinas: rods & cones
  
  • Rods: Photoreceptors that are specialized for night = scotopic vision (90 million)
    
    • Location: periphery mainly; absent in fovea
    • In dim light, our vision is better on the periphery
      
      • This is b/c no rods in the fovea
  • Cones: Photoreceptors that are specialized for daylight = photopic vision, fine visual acuity and colour (4-5 million) – 3 types
    
    • Location: Smaller and packed in the fovea; larger and loose in the periphery
    • Barely any color vision in the periphery
    • Don’t work in dim light
• Optic disk: 0 cones, 0 rods
• Some animals have mostly rod retinas, (e.g., rats, owls) or only cones (some lizards)
• 3rd photoreceptor for circadian rhythm (specialized ganglion cells)
• Eccentricity: distance from fovea
• Why do have rods when we mainly use cones to see these days?
  
  • our ancestors fished, so having rods help them survive better when fishing at the bottom of the ocean where there is low light

Question 10

• outer segment
• inner segment
• synaptic terminal
• Visual pigment
  
  • 2 molecules
• 2 types of visual pigments
  
  • 3 subtypes for cones
    
    • %
    • wavelength
    • sensitive to what color
    • are they in fovea?
• Can we see color at this stage?
• Potential type 5 - melanopsin = fx?
• • Cats: night vision
    
    • pupil shape
      
      • horizontal dimension implication
      • vertical dimension implication
      • Why Cat’s eyes are “reflective”
• Humans - why we have red eyes?
• process of capturing photon - 3 steps
• graded potentials
• pro and con of graded potential
• # glutamate NT and # photons relationship

Answer 10

Light Transduction by Rod and Cone Photoreceptors

• Outer segment: stack of pancakes = membranes, contain layers of visual pigments
  
  • Visual pigments: Catch photons and transduce light energy to neural signals
• Inner segment: produce visual pigment, and send it to the outer segment
• Synaptic terminal: connects to bipolar and horizontal cells
• • Visual pigments are molecules with 2 parts
    
    • 1. Chromophore (captures photons)
    • 2. Opsin (absorbs light of a specific wavelength)
         * Retinal: derived from Vit A, and becomes beta-carotene
         * Opsin and chromophore are connected
• 4 types of visual pigments:
  
  • Rhodopsin (in rods)
  • 3 subtypes for 3 cones
    
    • S-cones (5-10%) → capture “short” wavelengths (ex. blue)
      
      • Most sensitive to “blue” light
      • 0 in fovea
    • M-cones → capture “medium” wavelengths (ex. green)
    • L-cones (~twice as many as M) -> capture “long” wavelengths (ex. red)
  • NOTE: We can’t perceive the color at this stage yet
  • Potential type 5/Melanopsin: in ganglion cells, sensitive to ambient light
    
    • sends signal to suprachiasmatic nucleus to regulate circadian rhythm
• Daylight colour vision in mammals worse than in many other animals
• Cats: night vision
  
  • Their pupils are shaped like slits (elongated) -> larger aperture in vertical dimension
  • IOW: in horizontal dimension, the slit is narrower -> more acute vision
  • IOW: vertical dimension, let in more light (~5x more light)
  • Cat’s eyes are “reflective”
    
    • Shine bright light at cats’ eye
    • Choroid (layer behind retina): instead of absorbing the excess light, it reflects it back to the photoreceptors so it receives the info
    • Some of the light is reflected out of the eye, so we see the reflection
• Humans
  
  • When we use flash light in photos, the picture may show “red eye”
  • Although our choroid (layer behind retina) is black, it is supplied by many blood vessels
  • We see red b/c the blood in the choroid reflects the light back
  • • Capturing a photon
  • 1. When light hits a photoreceptor, the process of photoactivation/bleaching begins
  • 2. The photoreceptors hyperpolarizes
       * (Unlike other neurons, they don’t depolarize)
  • 3. Photoreceptors send signals in Graded potentials (fewer NT (glutamate) released in synapse) → signal to bipolar
       * (Unlike other neurons, they don’t have all or nothing AP)
       * All or nothing AP: Intensity = frequency of AP
       * Graded potentials provide Fine grain responses to diff lv of light
       * May lose some info; not sig
• # of glutamate at the photoreceptor-bipolar cell synapse is inversely prop to # of photons absorbed by photoreceptor*

Question 11

• 2 paths neuron pass info
  
  • Path 1
  • Path 2 - H
    
    • controlled by
    • fx
    • Ex. photoreceptor 1 and 2 both send info to bipolar -> ganglion → what does horizontal cell do?
• 2 fx of amacrine cells

Answer 11

• 2 paths neuron pass info
• 1. Vertical path: photoreceptor -> bipolar -> ganglion cell
• 2. Lateral path for lateral inhibition
     * Controlled by Horizontal cells and amacrine cells
     * This allows various regions of the retina interact via lateral inhibition
     * Ex. photoreceptor 1 and 2 both send info to bipolar -> ganglion
     * horizontal cell is connected photoreceptors 1&2, each w/ a diff synapse
     
     • Ex. synapse A to connect PR 1; synapse B to connect PR 2
     • Ex. synapse A = activate; synapse B = inhibit
     • When the central and peripheral PR (ex. PR 1&2) are both sending info, the horizontal cell can inhibit the peripheral signal and activate the central signal
• Amacrine cells Fx: contrast enhancement, temporal sensitivity (detect changes in light patterns over time)

Question 12

• Bipolar cells
  
  • 3 sources of input
  • output to?
• NAME 4 types of bipolar cells
  
  • diffuse bipolar cell
    
    • input source
    • characteristics
    • Any in fovea?
    • common among?
  • Midget bipolar cells
    
    • input
    • output location
    • 1 to 2 relationship
• Retinal periphery vs fovea on visual acuity vs sensitivity
• Each foveal cone is connected to 2 bipolar cells
  
  • 2 types
  • process
  • Graded potentials

Answer 12

• Bipolar cells: receive input from rods OR cones (not both) and horizontal cells; then pass signals to ganglion cells
• Diff types of bipolar cells
• 1. Diffuse bipolar cell: Receive input from multiple photoreceptors (rods & peripheral cones)
     * good visual sensitivity (perceive more); shit visual acuity (TMI)
     
     • NOTE: none in Fovea
     • Happens mainly in the rod pathway
• 2. Midget bipolar cells: Receive input from a single cone in fovea (cones connect to two bipolar cells)
     * NOTE: never rods; only in fovea
     * 1 to 2 relationship: 1 PR in the fovea is connected to 2 midget bipolar cells
• IOW:
  
  • retinal periphery: high deg of convergence -> high sensitivity to light but poor acuity
  • fovea: low deg of convergence -> high acuity, poor sensitivity
• • Each foveal cone is connected to 2 bipolar cells
    
    • 3. ON bipolar cells (graded potentials)
         * When there is light shining on photoreceptor -> PR sends into to ON bipolar cell -> activates it (i.e. depolarizes)
         * NOTE: light does not directly activate the ON bipolar cell, only indirectly
    • 4. OFF bipolar cells
         * When there is light shining on photoreceptor -> PR sends into to OFF bipolar cell -> deactivates it (i.e. hyperpolarizes)
• IOW: they respond differently to the same photoreceptor input

Question 13

• Ganglion cells - input, output
• 5 types (names
  
  • input
  • output
  • looks like
  • %
  • aka
  • NOTE: #5: only fx, input source
• 2 paths
  
  • each path: connections (2)
• P ganglion vs M ganglion
  
  • response type
  • color sensitivity
  • receptive field size
  • spatial resolution
  • info it collects

Answer 13

Communicating to the Brain via Ganglion Cells

• Ganglion cells: receive visual info from photoreceptors via intermediate neurons. (bipolar cells and amacrine cells), then send info to the brain
• 1 P ganglion cells (small ganglion cells): receive input from midget bipolar cell and send info to parvocellular in the lateral geniculate nucleus
  
  • P dendritic trees are small
  • 70% of ganglion cells in human retina
  • aka Midget ganglion cell (in the Midget pathway
    
    • [EL1]Terrible and confusing naming = but it is what it is
• 2 M ganglion cells: receive input from diffuse bipolar cells and feeds magnocellular (magno = large) layer of lateral geniculate nucleus
  
  • M ganglions look like huge umbrellas
  • 10% of ganglion cells
  • (aka parasol ganglion cell: used in the parasol pathway & dendrites = tree-like)
• 3. ON-center ganglion cell - later
• 4. OFF-center ganglion cell – later
• 5 Koniocellular layers: a type of ganglion cell located b/w magnecellular and parvocellular layer in LGN
  
  • Some receive input from S-cones and form the ancient “blue-yellow path”
  • Some receive input from other ganglion cells to form “nonblue”cells
• 2 pathways
• Midget pathway: only has cones
  
  • 1. PR Cones only connect to midget bipolar cells
       * 1.In the fovea, 1 cone is connected to 2 bipolar cells; a bipolar cell maybe connected to few cones tho
       * 2. Midget bipolar cells are connected to few P ganglion cells
       
       • In the fovea, midget bipolar cell is only connected to 1 P ganglion cell
  • Parasol pathway: bigger network
    
    • 1. Many photoreceptors are connected to a diffuse bipolar cell
    • 2. Many bipolar cells are connected to a parasol/M ganglion cell
• P ganglion cells
  
  • Sustained responses: when activated by light, their response is constant/keeps going
  • Sensitive to color: indirectly connected to 1 type of cones (S,M,L) via bipolar cell
  • Smaller receptive field: only connected to 1 bipolar cells (or only few)
    
    • The bipolar cells they are connected to 1 photoreceptor (or only a few)
    • IOW: they have a small receptive field/ window (to see the outside world)
      
      • Receptive field: The region in space (i.e., the visual field) in which stimuli will activate a neuron
  • Finer spatial resolution
    
    • Since they have a smaller receptive field -> see things more in detail
  • P cells: provide info on “contrast” in retinal image
• M ganglion cells:
  
  • transient responses: short-lived responses
    
    • when activated by light, they respond briefly, then deactivates
  • ’insensitive’ to colour: collect info from all sorts of cones (S,M,L)
  • larger receptive fields/window (to see the outside world)
    
    • Since they are connected many bipolar cells, and these bipolar cells are connected to many photoreceptors -> large receptive fields
    • Collect info from more areas of the visual field
  • coarser spatial resolution
    
    • Since they have a larger window, they can’t see as much details
  • M cells: provide info on how the image change over time

Question 14

• receptive field
• Ganglion receptive field shape
• Kuffler mapped the receptive fields of individual ganglion cells in cats
  
  • 2 main findings
• ON-centre ganglion cell
  
  • light on centre
  • light on surround
  • Disinhibition
• OFF-centre ganglion cell
  
  • light on centre
  • light on surround
• Centre-surround organization: 2 features
  
  • contrast
  • Example of ???
• *

Answer 14

• Receptive field: region on the retina (and corresponding region in visual space) where visual stimuli influence the neuron’s firing rate
• Photoreceptors have round receptive fields
• Kuffler mapped the receptive fields of individual ganglion cells in cats
  
  • shined a light on cat’s retina
  • 1 He found Ganglion cells have concentric (round and 2 parts) receptive fields
  • Each part responds diff to light: increase/decrease firing rate
  • • ON-centre ganglion cell (NOT called ON ganglion cell)
  • The “ON” fx only applies to the centre area
    
    • If the light spot is on the centre area (“+”) -> activates the ganglion cell
    • If the light spot is on the Surround area (“-“) → inhibited (fewer AP)
    • Disinhibition: It then shows a burst of AP after the inhibition
    • (sort of like you held your breath when you detect light; then you breath rapidly after the light is gone)
      
      • Fig legend
        
        • x-axis = time
        • vertical lines = when AP happens
        • Yellow = light spot is on
  • OFF-Centre Ganglion cell (opp)
    
    • When light spot is on the centre -> fewer AP
      
      • After the light spot is gone -> burst of AP (disinhibition/sigh of relief)
    • When light spot is on the surround area -> more AP
  • • Centre-surround organization: 2 features
• 1. responsive to light spots that have a specific size
     * IOW: retinal ganglion cells act as a filter
     
     • Responds best to stimuli that are just the right size
     • Responds less to stimuli that are larger/smaller
• 2. They are particularly sensitive to differences in light intensity – contrast
     * Ex. For ON-centre ganglion cells:
     
     • Stronger response when there is light in centre, no light in the surround area
       * Ex. OFF-centre ganglion cells
     • Stronger response when there is NO light in centre, light in the surround area
       * Both ON-OFF centre ganglion cells
     • weaker response when there is light in centre, AND surround area; or NO light in both areas
       * IOW: less ‘interested’ in ambient light intensity (when there’s light everywhere, centre and surround area) (less useful info)
       * More interested in contrast
     • Contrast: diff in light intensity b/w object and background or light and dark parts of the object
  • Thus, this shows Lateral inhibition through lateral pathway

*

Question 15

• Mach bands
  
  • what is the illusion?
  • what is the reality?
  • which part of the brain created the illusion?
  • Describe the red curve
    
    • area A
    • area D
    • area G
    • Area B
    • Area C
  • Area D vs G: They both hv activation and inhibition, so shouldn’t we see the same shade
  • Ganglion cells hv this weird b
    
    • 2 benefits

Answer 15

• WTF do they do these strange things?
• Mach bands: illusory edges
  
  • This is a continuum of grey shades
  • But somehow w/in each rectangle/shade, it seems the left edge is brighter than the right edge area
  • In reality, it is the same shade throughout the whole shade/rectangle
• This illusion is created by your brain, esp the ganglion cells
  
  • Circles A -> G: a few concentric receptive fields of ganglion cells projected in our visual field/real world
  • MP: Collect info from all
  • Curve: how light the diff regions of mach band is
  • Diff steps -> Diff shades on mach band
  • 2 curves
    
    • Blue: step fx, the actual luminance from the mach band
    • Red: what we perceive
      
      • w/in each rectangle, the left edge is brighter than the right edge area
      • This is due to the ganglion cells
        
        • Area A: uniformly covered by less light
        • Area D: uniformly covered by more light
        • Area G: uniformly covered by most light
        • • Area B: part of the receptive field is overlapping into the next region (which has more light)
            
            • If you average the light in Area B, it should have more light than Area A
            • But why is point B lower than point A on the graph?
            • This is b/c the light in the next region covers the inhibitory area in the ganglion cell (“-“) -> more inhibition in cell B -> perceive as darker
            • Similar logic for area E
        • Area C: part of the receptive field is overlapping back into the prev region (which has less light)
          
          • If you average the light in Area C, it should have less light than Area D
          • But why is point C higher than point D on the graph?
          • This is b/c there is less light in the prev region that is in the inhibitory area in the ganglion cell (“-“) -> less inhibition in cell C -> perceive as lighter
          • Similar logic for area F
        • Area D vs G
          
          • They both hv activation and inhibition, so shouldn’t we see the same shade
          • No, the centre area matters more
            
            • Specifically, D has less light, and is activated less compared to G
    • • Ganglion cells hv this weird b that creates illusions
    • Benefit: we can clearly see there are 2 shades
    • IOW: it helps us detect edges more clearly
      
      • Ex. see textures, see objects vs background
      • LHS: ganglion only filter -> see exaggerated edges
        
        • IOW: perception begins in the retina
    • IOW: this reduces the amount of info that needs to be transmitted from the retina to the brain
      
      • Info from 90 million rods 5 million cones is compressed into 10 million ganglion cells
      • Visual system is interested in contrast/change
        
        • Ex. the sky is a uniform blue (boring) -> Ganglion cells do not transmit as much info about that
    • Therefore ganglion cells help compress visual info

Question 16

• sensitivity
• 4 mechanisms important for dark and light adaptation
• What is 5th mechanism that doesn’t adapt but is the most important?
• 1 pupil
  
  • light vs dark
  • how much sensitivity does this improve
  • Pro
  • Con
• 2 photoreceptors
  
  • cones - which type of vision?
  • rods - which types of vision?
  • low threshold = ?
  • overal trend
    
    • explanation
  • What does the purple curve represent?
    
    • What does the notch mean?
  • What does the red curve represent?
    
    • What does this imply their vision in daylight vs darkness?
  • What does the purple curve represent?
    
    • What does this imply their vision in daylight vs darkness?
  • How does light and dark adaptation work?
• 3 photopigment replacement
  
  • abundance of photopigments - 2 aspects
  • define bleached
  • How is it reversed - popcorn analogy
  • 1 Photopigments and photons in daylight
  • 1 Photopigments and photons when you walk into the daylight
• 4 Adaptive mechanisms in retina circuitry
  
  • Ppl w/ only rods retinas & sunglasses
  • Ppl w/ duplex retinas & sunglasses
    
    • Why do their ganglion cells here do not receive lots of signals from the rod
• 5 Neural circuitry of the retina
  
  • What is the neural circuitry interested in?
  • Ex. ganglion cells

Answer 16

1 The Pupil in Light vs. Dark Conditions

Dark and light adaptation

• Sensitivity: the ability to see diff lv of light
• The human eye can deal w/ low light and lots of light
• 4 mechanisms important for dark and light adaptation:
  
  • Pupil dilation
  • Photoreceptors
  • Photopigment replacement
  • Adaptive circuitry
• 5th mechanism that doesn’t adapt is the most important one:
  
  • Neural circuitry of the retina
  • It Filters out contrast information, ‘ignores’ absolute luminance levels

• When it is dark, pupil widens (d= 8mm) -> more light enters -> less acuity (blurry)
• When it is light, smaller pupil (d = 2mm) -> less light enters
  
  • Small vs big: 4x difference
  • Area ∝ d^2
  • Small vs big area: 16x difference; big area lets through 16x more light
  • IOW: 16-fold improvement in sensitivity
• This is a quick fix in sensitivity, doesn’t contribute sig to out sensitivity
• We need time to adjust to lighting differences b/c the other mech are slower

• Cones – for photopic vision
• Rods – for scotopic vision
• Psychophysics
  
  • Showed that there are 2 types of photoreceptors
• X-axis: time ppl spend in darkness
• Y-axis: absolute threshold for luminance
  
  • IOW: can you detect light
  • The lower = the better/more sensitivity
• Overall, the curves decrease
  
  • Makes sense: the more time you spend in the dark, you can see more details in the dark (more sensitive/better)
• Purple = duplex retina (has rods and cones)
  
  • 0-5 min: drops, then slows down
  • 7-30 min: accelerates the drop
  • Notch = indicates there are 2 types of mechanisms/photoreceptors
  • # 1 Red: some ppl have cones only retina
    • curve levels off at a higher threshold
    • IOW: they can see clearly when there’s light
    • Shit vision in the dark
  • # 2 Blue: Only rod retina
    • IOW: they see perfectly in the dark
    • Shit vision in daylight
• How does light and dark adaptation work?
  
  • Receptor adaptation works at different rates/to different extends for rods and cones
  • IOW: You have 2 types of photoreceptors: rods adapt to dark; cones adapt to light

• photopigment replacement: The same photoreceptor may have different light sensitivity depending on how much it is adapted to darkness (or lightness)
• This is explained by the abundance of photopigments
  
  • The total # of photopigments in the photoreceptor is constant
  • BUT, the # of bleached of photopigments changes
    
    • Some photopigments are bleached
      
      • “Bleached”: a photon interacts w/ the photopigment, and changes its molecular structure
        
        • Ex. heat up a corn -> popcorn
        • Ex. for photopigments – you can stuff the popcorn back into a corn
      • NOTE: it takes time to reverse the bleached photopigment
  • IOW: in daylight, there are many photons -> they bleach many photopigments
    
    • -> Very few non-bleached photopigments left to detect photons
    • Since there is more than enough photons, the photopigments do not need to interact w/ all the photons for us to see
  • When you go into darkness, most of your photopigments are bleached
    
    • You need to interact w/ as many photons in order to see
    • But you have very few photons left that can interact w/ photopigments
    • IOW: It takes time for bleached photopigments to reverse the structure
    • Over time, as there are more photon-sensitive photopigments (reversed photopigments), the same photoreceptor becomes more sensitive
• Thus, the # of photopigments that can interact w/ photons varies depending the light lv

• This has to do w/ rods and cones
• Ppl w/ only rods retinas
  
  • hv to wear sunglasses when they enter broad daylight from a dark env
  • This is b/c rods are very sensitive, and they are overstimulated in broad daylight
• Ppl w/ duplex retinas
  
  • DON’T need sunglasses when they enter broad daylight from a dark env
  • Why?
  • The ganglion cells here do not receive lots of signals from the rods
    
    • Pathway: rods -> bipolar cell -> AMACRINE -> ganglion
  • the amacrine cells (that connect the ganglion cells and bipolar/rods) inhibit the signals
    
    • IOW, during daylight these special amacrines are inhibited
    • At night, these amacrines are active

(5) Neural circuitry of the retina accounts for why we are not bothered by variations in overall light levels

• Not an adaptive fx; it’s always there
• The visual system regulates the amount of light entering the eye, and by ignoring whatever variation in overall light level is left over
  
  • Luminance contrast vs. absolute luminance level
  • IOW: it is interested in the light contrast, not the light levels
    
    • Ex. in the ganglion cells, the contrast of light (light spot in the centre, no light in the surround area) is more important
    • Not really the overall light lv (ex. when light is on the centre and surround area = less signals)

Question 17

• Retinitis pigmentosa
• 4 step process
• where does it start first
• what does it attack first
• Chart legend
  
  • pink?
  • white?
  • black dot?
• Why does the pink look irregular?
• Where does the Black ring of blindness progress towards?
• age-related macular degeneration
  
  • 3 stages

Answer 17

Retinitis pigmentosa: a retinal disease

• Retinitis pigmentosa: genetic diseases that involve the progressive death photoreceptors and degeneration of the pigment epithelium
• No cure
• Process
• 1. Immune system attacks Choroid (layer behind in retina)-> becomes inflamed -> then attacks the retina -> cell death
• It starts in the periphery
  
  • Ppl may not notice it as our visual system fills in the missing info (like it fills in the missing info for the blind spot/optic disk)
• Retinitis pigmentosa affect the rods first
  
  • They lose vision in dark env (ex. driving at night becomes difficult)
• Polar charts: Indicates the visual field
  
  • Normal person
    
    • Pink = can see
    • White = can’t see
    • Black dot = blind spot
    • Why does the pink look irregular? Your eyebrows, nose, and cheeks cover your vision (hv individual diff)
  • Ppl w/ retinitis pigmentosa
    
    • Black ring of blindness: covers the periphery
      
      • The ring progresses inward -> blind
• age-related macular degeneration (AMD)
  
  • Aging affects macula
  • 1 AMD gradually destroys sharp central vision
  • 2 Difficult to read, drive, recognize faces
  • 3 Then blindness