W5: Physiology of Vision

Question 1

What are the three ways to interpret light?

Answer 1

Wave of electromagnetic radiation: light is a charge of electric and magnetic fields. Wavelength is measured in nanometres (visible spectrum = 400-700 nanometres)

Rays: since light travels in a straight line at a constant speed (why we can measure peaks and troughs)

Particles: of light are called photons and are described in quanta (which means “discrete packets of light”)

Question 2

Luminance (candelas/m2 )

Answer 2

Light: refers to the scale of intensity along the visible spectrum. Large scale of luminance (“10,000,000 difference btwn starlight & sunlight”).

Question 3

Contrast

Answer 3

light-luminance: contrast btwn min and max luminance values determines visibility, C=(Lmax - Lmin)(Lmax + Lmin) presented as 0-1 or %, nothing is visible if contrast = 0
Theory: Regardless of the luminance, different objects consistently reflect their own percentage of light (“E.g. white paper ~75%, black paper ~5% (i.e. 15:1 ratio) - reason behind color constancy)
^ THEREFORE “Relative luminance is constant regardless of absolute luminance”

Question 4

Process of light

Answer 4

light reflects off surface and onto cornea -> cornea bends light onto retina (does most focusing) -> light travels through aqueous humour, pupil, lens and vitreous humour -> to hit and project image onto retina -> passes ganglion and other cells to the photoreceptors, they respond and send info back to ganglion cells -> retinal ganglion cells synapse into optic nerve at disc, send axons to partially decussitate at the chiasm -> goes along optic tract to terminate in LGN in thalamus -> projects to Striate cortex -> projects to extrastriate areas

Question 5

Cornea

Answer 5

Eye: “transparent window” - allows light to penetrate through the eye. Main lens (“focusing light onto retina”)

Question 6

What are the two characteristics of lenses?

Answer 6

Eye:
1. Curved surface
2. air-cornea boundary: Substance of the lens makes light pass “more slowly than through air” - this bends and reflects light onto retina

Question 7

Aqueous/Vitreous Humour

Answer 7

Eye: fluid which maintains eye pressure and nutrience/fluid within eye that gives its shape

Question 8

Iris

Answer 8

Eye: coloured part, contracts to control the opening/”aperture” of the pupil to reduce light levels when too bright, relaxes to do the opposite effect

Question 9

What is the Retinas range of lumination?

Answer 9

10000000000:1 - starts to damage retina

Question 10

Pupil

Answer 10

Eye: “adjustable aperture” that controls the amount of light that enters the eye - contraction and relaxation of pupil is controlled by the iris

Question 11

How does pupil dilation work?

Answer 11

Eye: controlled by the iris. relaxed pupil is 16 x larger than contracted (not useful).
Constricted pupil = focus increases, dilated pupil = focus decreases

Dilation causes besides light: may increase womens attraction, dilation when excited (e.g. when men see naked women, can reveal poker face)

Question 12

Lens

Answer 12

eye: less control but can adjust for distance unlike cornea. Located between zonules of Zinn.
Light refracts as it hits lens so lens has to redirect light into one point onto retina for sharp image

Question 13

Accommodation

Answer 13

Eye
Focus on close object: ciliary muscles contract = zonules relax = rounds out/fattens lens = more refracting (bend) power = greater focusing strength (light bouncing off closer objects enters the light at diverging angles - requires more bending)
Focus on further object: ciliary muscles relax = zonules contract = widens lens = less refracting (bend) power = lessens strength of focus (requires less focus as light from farther objects travel towards the eye in almost parallel lines)
^curviness of lens adjusts power of lens/length of focus from lens to the light
^focusing strength needs to match optics so it can fit correctly to the size of your eye

Question 14

Emmetropia

Answer 14

Lens: regular vision and accommodation abilities

Question 15

Myopia

Answer 15

Lens: (short sightedness)
eye too long/lens too strong = focus on close objects. Distant objects = too much bending = short focal length = blurred
diverging/concave lens reduces optical power

Question 16

Hyperopia/hypermetropia

Answer 16

Lens: (long sightedness)
eye is too short w- lens too weak = only focus on far objects. Long focal length = close objects not bent enough.
converging/convex lens (glasses) adds power

Question 17

Presbyopia

Answer 17

Lens: when “lens loses natural elasticity” due to age (lose half by 30, all by 55). Loses accommodation abilities and near point (closest point of focus) distances, making close objects blurry
Treatment: Reading Glasses

Question 18

Astigmatism

Answer 18

Lens: Lens has different focal length/refractive power/degree of sharpness for different orientations (e.g. emmetropic for vertical lines, myopic for horizontal lines)

Question 19

Retina

Answer 19

Eye: “light sensitive membrane at the back of the eye that contains rods and cones”. Incl Fovea and periphery. Lens reflects images onto this surface, for it to be processed and sent to brain via optic nerve.

Question 20

Photoreceptors

Answer 20

Retina: are at the back of the eye as they require good blood supply for transduction. Neural processing begins when light enters receptors. For more info, go to following section

Question 21

Retinal Ganglion Cells

Answer 21

Retinal cells: axons in these cells carry information to visual cortex

Question 22

Blind Spot/Optic disk

Answer 22

Eye: due to “neural bundle” penetrating retina - cannot see in this area due to lack of photoreceptors. But having two eye with blind spots on different parts resolves the issue.

Question 23

Filling-in effect of the blind spot/optic disc

Answer 23

Retina: Theres a filling in process of the blind spot which may a result of: filling in spot with surrounding stimuli or expands surrounding stimuli to fill in spot.

Question 24

What are the types of photoreceptors in the eye/retina?

Answer 24

Rods & Cones

Question 25

Rods

Answer 25

Photoreceptor
High sensitivity, suited to night vision - rod activity increases with light intensity - don’t help in daylight as they’re responding fully during lower intensities.
Most responsive to green light
Located across the periphery, none in fovea (central part of eye)

Question 26

Cones

Answer 26

Photoreceptor
three types that respond to different wavelengths - each cone is named after colour of wavelength it detects (not even amount of cones)
all three contribute to colour vision - lack of red or green cone = colour blindness.
Low sensitivity, suited for daylight.
All cones are most responsive to yellow light.
Cones are mainly located on the fovea (central area of eye) light hits fovea when looking at object in front of you) still some are scattered across the periphery (rest of eye)
No blue cones in fovea

Question 27

What are the three types of cones?

Answer 27

photoreceptors
Red/long-wave cones: tuned to long light wavelengths
Green/mid-wave cones: tuned to mid length light wavelengths
Blue/short-wave cones: tuned to short light wavelengths

Question 28

Adaptation

Answer 28

Photoreceptors: ‘neurons respond to 1000:1 light intensity range at once’. Adaptation allows eyes to respond to a much larger range of light - cones adjust in 10 mins; rods 30-40 mins
Alternate between cones and rods depending on amount of light available (e.g. need to switch to rods when entering cinema as theres not enough light for cones - but rods need time to adapt from being overstimulated from sun)
Scotopic Vision: when only rods are used to see in dark
Photopic Vision: when rods dysfunction due to overstimulation from bright light
Mesopic Vision: when both rods and cones are used to see

Question 29

Dark Adaptation

Answer 29

Photoreceptors: cones recover from bleaching first (5 min) and are used to see the dim light - fully recovered after another 5 mins. Rods then overtake (at 10 mins) as they recover (fully in 30 mins). The opposite process is light adaptation
Dark Adaptation Curve: is the two connected curves that show recovery of cones and rods after bleaching

Question 30

Purkinje Shift

Answer 30

photoreceptors - adaptation: (due to cones sensitivity to yellow light and rods sensitivity to green light) red petals appear darker and green stem appears lighter - this reverses (red = lighter, green = darker) over time (e.g. next day) due to adaptation. This effect is why army forces sleep in red light - prevents rod bleaching allowing them to see when on the attack in the middle of the night

Question 31

Retinal Ganglion Cells

Answer 31

Retina: final layer of cells in retina which carry information to visual cortex. There are two types: large and small - their properties differ and relate to size.
Cell fires if light hits more of the on region of its receptive field

Question 32

What are the two types of retinal ganglion cells?

Answer 32

Retinal ganglion cells:
There are two types: large and small - their properties differ and relate to size.
Ganglion Cell Types:
M Cells: magnocellular cells are large. Carry “dynamic aspect” info (e.g. movements). Hard to study as there is no functional organization
P Cells: parvocellular cells are small. Carry color info by differentiating signals from red and green cones (e.g. red excites and green inhibits some p cells)
Lateral Geniculate Nucleus: lesioning this area allows us to examine the functions of P and M cells as they “project” to different areas of the nucleus.

Question 33

Receptive Field of ganglion cells

Answer 33

Retinal Ganglion Cells:

Fovea: small RF’s = more cortex devoted to these RF’s = better acuity

Periphery: large RF’s = less cortex devoted = worse acuity (why peripheral field is blurry)
^if we were to magnify peripheral vision, its acuity would match foveas

Cortical magnification: more cortex is devoted to processing foveal info

Question 34

What are the three pathways that send visual information to the LGN/Thalamus to be projected to V1 and Extra striate areas?

Answer 34

Optic nerve - optic chiasm (partical decussation) - Optic tract

Question 35

Optic Nerve

Answer 35

Optic Nerve:
starts the journey from blind spot to visual cortex.
optic nerve stops at chiasm

Question 36

Optic Chiasm

Answer 36

Optic Nerve: area where optic nerves from each eye combine and the fibers/axons “partially decussate”: axons from inner/nasal side of retina cross over and connect to the contralateral side of brain while axons on the outer/temporal side of retina connect to the ipsilateral side of brain - left bunch of fibers carry info about right visual field and vice versa.

Question 37

Optic Tract

Answer 37

Optic Nerve: starts after optic chiasm - why one loses half a visual field if they cut the tract and the whole field if they cut the nerve (i.e. Hemianopia)

Question 38

Lateral Geniculate Nucleus

Answer 38

signals partially decussate at chiasm to be sent indv to LGN layers
main relay station in thalamus between the retina and visual cortex - mostly received input from cortex (top-down processing) - ‘why people suggest it may filter what info goes into cortex’ (i.e. attention)

Question 39

Describe the layers of the LGN?

Answer 39

6 layers within LGN - 3 dedicated to each eye for the axons to terminate in
Magnocellular Layers: bottom 2 layers of LGN containing M ganglion cell fibers (large) - layer 1 = contralateral, layer 2 = ipsilateral
detects motion and coarse features (not colour or depth - evident from lesions)
Parvocellular Layers: top 4 layers of LGN containing P ganglion cell fibers (small)
Layers 3 and 5 = ipsilateral, layers 4 and 6 = contralateral
^layers 5 and 6 appear to be excessive layers - unsure about their function
detects colour and fine features
Contralateral Layers: M & P layers that receive information from the right eye
Ipsilateral Layers: M & P layers that receive information from the left eye
*pattern of M & G Layers: contra, ipsi, ipsi, contra, ipsi, contra

Question 40

What cells lie between each layer of LGN?

Answer 40

Koniocellular Cells (K Cells): 10000 small cells between the LGN layers. Involved in blue-yellow comparison component of vision - as K cells receive ganglion axons from ganglion cells that receive blue cone input

Question 41

Retinotopic Mapping

Answer 41

LGN: “cells from adjacent side of retina will project to cells on adjacent side of LGN layers - orderly map reflecting the visual world.” Every layer has this organization - therefore each LGN has 6 maps for the ipsilateral side of visual field (right LGN = left visual field, left LGN = right visual field).
Lesion studies show that the M and P layers hold different information and the area of the cortex that is damaged is reflected in the small area of visual field where vision is lost.
^depth and large shape modification is detected by both as abilities remained after lesion

Question 42

Dyslexia

Answer 42

LGN: may be due to smaller magnocellular (motion perception) layers in LGN and reduced activation within these layers.

Question 43

What is the role of receptive field of retinal ganglion cells?

Answer 43

To group retinal output
determines important/useful info (about CHANGES in visual field) to send to brain

Question 44

Retinal Ganglion Cells’ receptive fields

Answer 44

determine important information (edges and changes in pattern of firing) to reduce amount sent along the optic nerve and to the cortex (measure using microelectrode while flashing light in eye). Ganglion cells also indicate shades of things by whether its inhibited or excited at the edge (e.g. excited on-centre cells at edge suggest the rest of the area is a light colour and inhibition suggests dark colours).

Question 45

Centre-surround Antagonistic Receptive Field

Answer 45

Ganglions RF’s: small area of retina that will cause increase or decrease in firing of ganglion cells depending on how much light falls on each concentric area.
On Region: rate of action potential firing increases when light hits this area and decreases when it hits the off region. On region is seen as a mexican hat as light hitting the centre causes Maximum Excitation, and light hitting the brim/surround area causes Maximum Inhibition. This pattern becomes an upside down mexican hat for the off region
Off Region: rate of action potential firing decreases when the light hits this area and decreases when light hits the surroudning/on region.
On Centre: firing rate increases when light hits the centre, decreases for surround.
Off Centre: firing rate increases when light hits surround, decreases for centre
* these cells are required to signal decreases in light intensity w/o going back to baseline level
Excitatory Input: when stimulation of a certain area increases firing of cells in that region
Inhibitory Input: when stimulation of the adjacent area decreases the firing rate in the opposite region
*Spontaneous activity: no light or light over whole RF
Response: if small spot of light hits either region entirely or more than the other

Question 46

What are the reasons for ganglion cells RF?

Answer 46

so ganglion cells can indicate changes/edge of image to define object boundaries which is crucial when interacting with objects (RF’s on lighter edge = less inhibition (from dark bar covering portion of off region) = perceived as lighter luminance - darker edge is created via opposite of this process)
Ganglion cells also indicate shades of things by whether its inhibited or excited (e.g. excited on-centre cells suggest light colour and inhibition suggests dark colours).
Allows us to depend on contrast of luminance instead of absolute luminance (reason for colour constancy) - to detect large luminance range. Higher contrast = higher cell activity

Question 47

Why do we have on-centre cells and off-centre cells?

Answer 47

Reason for two sets of concentric field cells: to easily detect dark and light spots by not exceeding much further beyond the spontaneous rate

Question 48

Why do they call it ANTAGONISTIC receptive fields?

Answer 48

ganglion cells: Antagonistic because stimulating surrounding regions causes inhibition of firing rate. Contributes to visual illusions

Question 49

List visual illusions caused by faults in retinal ganglion cells

Answer 49

Herman/Herring grid
Scintillating Grid
Craik– O’Brien– Cornsweet illusion
Simultaneous lightness contrast illusion

Question 50

Herman/Herring Grid

Answer 50

Ganglion cells: illusory presence of grey dots at the “intersections” of the picture which your eye is not directly focusing on. Grey dots = intersections have more lateral inhibition/light stimulating off regions of on-centre cells = reduced firing at intersections in comparison to the firing along the street = signals to brain that reduced firing is due to reduced luminance. The point of focus remains white as the foveas receptive fields are too small to cover the intersection.
*this theory has some compelling evidence

Question 51

Scintillating Grid

Answer 51

Ganglion illusions: dots at intersections scintilate/flash as you move your eyes across the screen due to simultaneous contrast effects \/.
Simultaneous Contrast Effects: …All dots are same shade of white but black background alters in shade across space (making circles on darker side lighter than others) - differences of light intensity hitting the surround area (as you trace along the surround of the field) of these on centre fields cause the ganglion cells to perceive these dots as changing in overall intensity and therefore appear to be flashing.

Question 52

Craik– O’Brien– Cornsweet illusion

Answer 52

Retinal ganglion illusion: change in shade along the edge along middle of photo makes the two sides appear to be different shades - ganglion cells assume whole side is lighter due to excitation/inhibition of cells along the edge

Question 53

Simultaneous lightness contrast illusion

Answer 53

Retinal ganglion illusion: two center boxes of same luminance appear different due to different contrasts between background shades (box with darker background appears lighter as darker background causes less inhibition on portion of off region)

Question 54

Stritate/Primary Visual Cortex

Answer 54

aka V1, striate cortex (appears stripey - diff layer types) and area 17 (brodmann).
Largest visual area of brain - essential to vision.
Located on occipital lobe - posterior portion of brain
“receptive field properties of the cells change in V1”

Question 55

what does the V1’s stripey appearance suggest?

Answer 55

Stripey appearance under microscope suggest different layer types:
3, 4A 4Ca 4Cb receive LGN info - divided into contra and ipsilateral input and further categorized further into P, M and K layer input
layer 6 sends info to LGN (top-down)
L2 L3 L5 L6 send info to higher visual areas

Question 56

Orientation selectivity of the V1

Answer 56

Tuning: Cortical cells are “tuned” or respond maximally to particular features of stimulus and particular orientations of these features - e.g. some cortical cells are tuned to particular lines in particular orientations and respond slight less to lines at similar degrees. = tuning curve
*initially thought spots were the stimuli but crack along spot lead to realisation that cortical cells respond to lines rather than spots of light
Bandwidth: is the width of the tuning curve - width has an inverse relationship with specificity of tuning (i.e. more selective tuning = narrow bandwidth)
Filters: Is another word for tuned cells as it works as a band-pass filter to “filter out” the stimuli that it doesn’t respond to

Question 57

retinotopic mapping

Answer 57

V1 Organization: neighbouring cells also have overlapping receptive fields - reflecting retinotopic projection (image of visual field is reflected onto retina and each V1) and each V1 is dedicated to the ipsilateral visual field - but most cells are in fovea dedicated to resolving centre line between each field

Question 58

Cortical Magnification Factor

Answer 58

V1 Organization: “calculation of amount of cortex given to each 1deg2” - The further the area goes into the peripheral field, the less cortex is devoted to it. Central region of vision between each field has “high magnification” or excessive amount of cortical cells/cortex devoted to it (10% weather than 0.5%). *This shows that V1’s visual map is highly distorted
^this allows high-res in central vision - prioritized to prevent brain weighing tonnes

Question 59

Orientation Columns

Answer 59

V1 Organization: spatial organization of tuned cortical cells - each column is dedicated to one orientation within the same receptive field and neighbouring columns are tuned to the next degree and so forth. Shows that firing of a column of cortical cells makes us see a line of a particular orientation in a certain part of our visual field - also applies to ball = firing of round cells, dogs and other things!
The collection of these columns that responds to one receptive field is referred to as a Hypercolumn and takes up 1mm of cortex. The amount of visual space/receptive field that a hypercolumn has varies (smaller and more hypercolumns for central area, larger areas for peripheral vision as there are less hyper column for these).

Question 60

What makes tuning work?

Answer 60

Simple cells

Question 61

Simple Cells

Answer 61

Simple Cells (How tuning works): simple cells are cells whose response is determined by how much light falls on the on and off regions”
LGN cells receptive fields cannot discriminate between different orientations - responds to all orientations the same - moderate excitement as line covers whole on centre region and portions of the off centre regions at all angles
^ this is why we need V1 cells to be tuned to orientation and other aspects of stimulus features to detect bars, edges and shade

Question 62

How to bar detectors and edge detectors work?

Answer 62

Simple cells (how tuning works)
Bar Detector: On and off regions are parallel bars so as the angle slightly alters, slightly more of the off region is being excited which causes these cells to respond slightly less.
How to produce own on/off regions = take output from some cells with on centre receptive fields and others with off - centre receptive fields tuned to the particular degree of stimulus
Edge Detector: V1 cells have another layout where one half of the field is the on region and the other half is the off region - this allows detection of edges

Question 63

What are Complex Cells

Answer 63

Complex Cells: also respond to specific orientations of lines but do not have on-off regions - on and off responses can occur at every point of field (therefore fields do not suggest type of tuning) referred to as phase insensitivity.

Question 64

How do you create complex cells?

Answer 64

Since complex cells are phase insensitive, it can be created by “wiring together simple cell outputs” that respond to the same orientation in different receptive fields so that it will respond to a particular line in a general region of RF’s via the OR operation: ‘the complex cell will fire if it receives input from this cell OR this cell OR this cell”

Question 65

What is phase insensitivity and what type of cell has this property?

Answer 65

Phase Insensitivity: refers to the fact that complex cells are insensitive to positioning/phase of stimulus in receptive field as they lack on and off regions - therefore maintaining the same response regardless of position in field.

Question 66

Hypercomplex Cells

Answer 66

(in V1) are tuned to bars of a specific orientation and length - response declines (rather than remaining constant due to inhibitory surround) if the bar exceeds the RF.
*Making simple cells w/ LGN cells, complex cells w/ simple cells and hypercomplex cells w/ complex cells suggest cells get more complex as you progress along the visual pathway.

Question 67

What are trigger features?

Answer 67

Refers to the types of stimulus features that produce a response in a tuned cell, signaling the “presence of a particular state of affairs”
Trigger features also become more specific as you progress along the visual pathway (from spots to angle of lines to angle and length of lines).
Cells in V1 (incl hypercomplex) are tuned to some/all following var’s: particular direction, speed, distance and colours

Question 68

Neural Doctrine

Answer 68

trigger features: high level cortical cells (e.g. hypercomplex cells) elicit perceptual elements in the way that stimulus elicits a response from receptors. Therefore your perception is just a reflection of what cortical cells are active.

Question 69

Face Cells

Answer 69

extremely complex, only fire to normally structured faces (not distorted). Located in an inferior temporal area in humans (couple synapses before V1).

Question 70

Grandmother Cell Hypothesis:

Answer 70

cells are so specialized that they can be tuned to a particular person. Their tuning is general enough that it isn’t tuned to particular features of the individual (e.g. hair and clothing) or a particular RF.
Different ideas on how it relates to perception: perception is a result of these cells firing (like neural doctrine) or perception is the firing of these cells
Criticisms: then we would need a cell for every object ever, cells that trigger these diff percepts have same structure so what causes their specificity

Question 71

Binocularity and Ocular Dominance in V1

Answer 71

Monocular cells in retina and LGN (process each eye info separately) - also travel to V1. are arranged by info from each eye into Ocular Dominance Columns
Binocular Cells: cells that are excited by one or both eyes - allows us to estimate more complex things like distance - 70% of cells in V1, most cells in extrastriate areas.

Question 72

Extrastriate Cortex

Answer 72

30 visual areas beyond V1
People claim areas are specialised to a single stimulus features (e.g. colour or motion) but they process multiple (e.g. V5 controls simple motion and depth)
V4 (colour change over space), V5/MT (simple motion + depth), MST (complex motion)
As we go deeper into areas, tuned stimulus becomes increasingly specific, RF’s get larger

Question 73

pathway along striate and extrastriate cortex

Answer 73

M, P & K streams project to V1 -> V1 projects to other areas -> these areas feedback laterally to other extrastriate areas and send descending feedback to V1/2

Question 74

what type of mapping does V1 and extrastriate areas have

Answer 74

All areas have retinotopic mapping: neighboring cortical cells have neighbouring RF’s on retina - this suggests that each area has their own visual maps