ch six: the visual system

Question 1

the tarsier

Answer 1

The tarsier is a small (about squirrel sized) nocturnal primate, found in the rainforests of South Eastern Asia. It is the only fully predatory primate in the world, feeding on lizards and insects and is even known to catch birds in mid flight.

It’s most remarkable feature; however, are its enormous eyes, the largest of any mammal, relative to body size.

If your eyes were proportionally as big as those of the tarsier, they would be the size of grapefruits.

These enormous eyes are fixed in the skull, and can´t turn in their sockets. (they can’t move their eyes, but they can move their neck)

To compensate for this, the tarsier has a very flexible neck, and can rotate its head 180 degrees, just like an owl, to scan for potential prey or predators.

With each eye weighing more than its brain, the tarsier has extremely acute eyesight and superb night vision; it has even been suggested that they may be able to see ultraviolet light.

On the other hand, they seem to have very poor color vision, as is the case with many nocturnal animals (including house cats and owls, for example)

they only have rod receptors / the eyes only have one photoreceptor (nocturnal)

Question 2

the dragon fly

Answer 2

The dragonfly, possibly the most formidable aerial hunter among insects, also has some of the most amazing eyes in the animal world.

They are so big that they cover almost the entire head, giving it a helmeted appearance, and a full 360 degree field of vision.

These eyes are made up of 30,000 visual units called ommatidia, each one containing a lens and a series of light sensitive cells.

Their eyesight is superb; they can detect colors and polarized light, and are particularly sensitive to movement, allowing them to quickly discover any potential prey or enemy.

Some dragonfly species that hunt at dusk can see perfectly in low light conditions, when we humans can barely see anything.

Not only that; dragonflies also have three smaller eyes named ocelli which can detect movement faster than the huge compound eyes can; these ocelli quickly send visual information to the dragonflies’ motor centers, allowing it to react in a fraction of a second and perhaps explaining the insect’s formidable acrobatic skills. Although dragonflies are not the only insects with ocelli (some wasps and flies have them too), they do have the most developed ones.

Question 3

the stalk eyed fly

Answer 3

These small but spectacular creatures are mostly found in the jungles of South East Asia and Africa

They get their name from the long projections from the sides of the head with the eyes and antennae at the end.

Male flies usually have much longer stalks than females and it has been confirmed that females prefer males with long eyestalks.

Males during mating season often stand face to face and measure their eyestalk’s length; the one with the greatest “eye span” is recognized as the winner.

Male stalk eyed flies also have the extraordinary ability to enlarge their eyestalks by ingesting air through their mouth and pumping it through ducts in the head to the eyestalks. They do this mostly during mating season.

Question 4

the night vision

Answer 4

no species can see in the dark, but some are capable of seeing when there is little light

in order to see something, our visual system needs to detect light

light is our visual stimulus - if there is no light there is nothing to see

Question 5

the night vision; two ways to detect light

Answer 5

light can be thought of as:

1) particles of energy (photons)
* photons are the basic units of energy of life

2) waves of electromagnetic radiations
* light gets reflected off of objects and that reflection of light is reflected as electromagnetic radiations

Question 6

light

Answer 6

• vision is based on visible light between 380-760nm
• humans can see approx 350-750nm
• a photon is the smallest possible unit of light energy

Question 7

light; ultraviolet and inferred lights

Answer 7

there are light sources or wavelengths of light that we can’t see but we can measure

ex: ultraviolet

• we can’t see it but we can feel it
• it can cause sunburns and its something we can measure

ex: inferred light
- we can’t see it but we can use it for technology, security, and for medical reasons

Question 8

electromagnetic spectrum; wavelenght of light

Answer 8

(this is what we can see)

continuum of energy produced by electric charges and is radiated as waves

/ wavelength of light: distance between the peaks of the electro magnetic waves

Question 9

electromagnetic spectrum; wavelengths of light (long, med, short)

Answer 9

long wavelengths of lights - reds, oranges, and yellows (longer distance / longer wavelength)

medium wavelengths of lights- greens

short wavelengths of lights - purple and blues (shorter distance between the peaks)

note: the size is based how far the peaks are from one another

Question 10

wavelengths do not have “color”

Answer 10

• wavelengths of lights DO NOT have color properties
• certain objects reflect specific wavelengths of light, and these wavelengths create a pattern of firing in photoreceptors
• these different wavelengths of light are interpreted by the brain as colors

Question 11

wavelengths of color; sensation vs perception

Answer 11

perception is our own reality; its how u see reality; how ur brain interprets the stimuli has coming to it; how we perceive it and how we process the sensations coming in

we might all have the same sensations but our brains might interpret those sensations differently (thats how we get perception)
* in this case we process the different electromagnetic waveforms as different colors

notes:

magic of our perceptual system!

how the nervous system transforms wavelengths into the experience of color is still unresolved

Question 12

the eye

Answer 12

cornea -> pupil -> lens -> retina -> optic nerve

we need light in order to see because our eyes are designed in a way that light will be projected right on the retina

ex:

the wavelengths of light from the item will be reflected into the eye, it will be focused on to the retina by the cornea,

Question 13

retina & fovea

Answer 13

/ retina: layers of cells at the back of the cell

/ fovea:

Question 14

cornea

Answer 14

/ cornea: a fixed lens and it responsible for about 80% of the focusing power on to the retina

• this is also what is changed in eye surgery (the shape of the cornea)

Question 15

iris

Answer 15

/ iris: the colour of the eye

Question 16

pupil

Answer 16

pupil can dilate (gets smaller or bigger)

pupil basically lets in more / less light

more light -> the better vision you’ll have

Question 17

lens

Answer 17

changeable structure / changed by ciliary muscles that are around it

lens mostly works to focus sort of really up close personal vision stimuli
- ex: when you bring your fingers to ur nose you can acc feel ur lens straining (its trying to focus that object so its changing shape to get that object focused right on mostly on the fovea)

lens does focus but its abt 20% of the focusing power (most of the focusing power of light onto the retina comes from the cornea - and the other 20% comes from the lens being able to move and change positions )

Question 18

the eye; how light passes by

Answer 18

so when light comes through and is reflected onto the retina, the retina will go through all the layers of the retina and will activate these photoreceptors

photoreceptors will activate the bipolar cells and ganglion cells and then it will get sent out through the optic nerve

Question 19

optic lens (the blind spot)

Answer 19

optic nerve will go out through each eye and will go to process visual information in the brain

the blind spot - no photoreceptors / mainly because thats just an area where these bundles of axons coming from the retina are leaving the eye

Question 20

retinal cells

Answer 20

millions of photoreceptors line the retina: rods & cones

• photoreceptor layer (back of the eye)
• bipolar cell layer
• ganglion cell layer (very front layer of the retina)

–
we have light that comes in through the eye and has to travel through all these layers of the retina

this means that some light wont get through / some might will get lost / bounce through and some of it will miss photoreceptors / but what we hope is that most of the light will active these photoreceptors

when they become active then they can activate the ganglion cells and bipolar cells..

Question 21

retinal cells; ganglion cells

Answer 21

it is the ganglion cells that if they become active, then they will send out information in the brain that there’s light in our environment

Question 22

the fovea

Answer 22

• light is focused on the fovea
• rich in cone receptors (fovea is the most important part of the retina because it contains ONLY cone receptors)

/ cone receptors: photoreceptors that are specialized in colour and detail

• our ability to see well is dependent on what gets projected on the fovea
• cones actually needs a lot of light in order for them to get active (thats why retina is a little bit indented/thinner)
• specialized for seeing fine details and colours (cones)
• the thin layer of ganglion cells reduces the distortion of the light passing through
• the thin retina (fovea) allows more light to get through

Question 23

photoreceptors; transduction

Answer 23

convert light into nerve impulses - TRANSDUCTION

photoreceptors are the specialized cells that we have that are specialized for taking in light stimuli (diff wavelengths of light)

• and they do so in this OUTER SEGMENT OF THE NEUTRON that contains specialized disc
• these specialized disc is what essentially makes them different than any other neuron
• any other neurons would typically have dendrites but they don’t have dendrites because they are picking up light and not neurotransmitters

Question 24

photoreceptors; specialized cells in transduction

Answer 24

/ specialized cells: the receptors for transduction

transduction is when we take some outer stimulus (light / sound / mechanical changes etc) and it takes in that external stimuli and translates it to something that the brain can understand which is a action potential

Question 25

transduction

Answer 25

/ transduction: process of converting light into electricity

Receptors in the retina contain light sensitive photopigments made from opsin and retinal

• Opsin: a long protein strand
• Retinal: a light sensitive molecule (if light hits it it basically produces a firework of activation)
• Vitamin A and retinal (eating carrots keep your vision healthy for a long time)

Question 26

visual receptors; when does transduction occur?

Answer 26

note: transduction occurs when retinal absorbs 1 photon of light

> outer segment

• where light acts to create electricity
• stack of discs

–
inside each disc is thousand of tiny visual pigment molecules
- these visual pigment molecules can contain the OPSIN STRAND and the light sensitive molecule retinal

thousands of visual pgment molecules and hundreds of disc - all containing 1 light senstitive retinal molecule

if this retinal molecule has light on it, it will explode and the entire cell will become active

Question 27

isomerization

Answer 27

The whole process of transduction / of light hitting the retinal molecule - isomerization

Transduction begins when 1 photon of light is absorbed by the light-sensitive retinal

when light hits that retinal molecule what happens to the opsin strand?

• it changes shape / and at the point that it changes shape, this retinal molecule cannot be active again

Isomerization occurs when retinal changes shape, sticking out from the opsin
- while this whole thing has changed shape, it can no longer be active again

Triggers transformation of light into electricity in receptors

isomerization is basically the beginning of transduction

• the cell is beginning to absorb the light and process the light
• the photoreceptor has not produced an action potential yet
• but when it does THAT IS ACTION POTENTIAL

Question 28

visual pigment bleaching

Answer 28

Retinal separates from the opsin

The retina then become lighter in colour called visual pigment bleaching

• when this process of isomerization occurs, the retina becomes bleached
• the bleach appearance basically means that the cells have gone through isomerization. they are active and they can no longer be active at that time

Question 29

visual pigment bleaching; what happens when light is constant?

Answer 29

WHAT HAPPENS WHENLIGHT IS CONSTANT and all the photoreceptors are bleached?
- when we wake up in the morning and we see light, basically all of our visual pigment molecules will go through isomerization and we wouldn’t be able to see light anymore (light would hit our retina and wed be blind all day)
–
researchers have found that overtime as the light is mainatned, they find that some visual pigment molecules will start to go through a visual pigment regeneration

Question 30

visual pigment regeneration

Answer 30

researchers have found that overtime as the light is mainatned, they find that some visual pigment molecules will start to go through a visual pigment regeneration

–
As light remains on, more and more of the retinal is detached, but more and more are regenerated
- regeneration process becomes a cooperative system (some goes thru isomerization and some goes thru regeneration)

Molecules that split apart are then put back together

Opsin and retinal are rejoined

• they become joined again and the retina starts to become pink or more red and it will go back to its regular shape
• that cell that will get back to this point will be able to do isomerization and go through that process of transduction again

it takes a about 6 mins for our cones to regenerate

Question 31

rod vs cone vision; duplexity theory of vision

Answer 31

duplexity theory of vision - cones and rods mediate different kinds of vision

Question 32

cones

Answer 32

photopic
(daytime vision)

• found mostly in fovea
• high acuity (they process details)
• color vision (they process the diff wavelengths of light that come in)
• needs lots of light
• 6 million
• no convergence

Question 33

rods

Answer 33

scotopic
(nighttime vision)

• found mostly in periphery (not found at all at fovea)
• low-acuity (they dont see detail)
• gray-scale vision (no colour)
• needs little light (very sensitive thats why)
• 120 million
• more convergence

Question 34

rod and cone distribution

• what does fovea contain?
• what’s in our periphery>

Answer 34

FOVEA contains only coned (~1% of all cones), while the PERIPHERY contains both rods and cones

in the fovea we have a high density of cones ONLY RED CONES (abt 150,000) then it drops off to about 10,000 cones into the periphery
- it stays pretty stable in the periphery

right next to the fovea, we have the highest density of rod receptors
- rod receptors are also good to process motion

Question 35

greater sensitivity in rods; convergence

• what is convergence? what does it do?
• which one has greater convergence?

Answer 35

convergence is essentially when we have one more neuron all converging their inputs into one neuron

• if we have convergence then we have multiple photoreceptors all sending inputs to one cell
• all this input can sum together to produce an action potential into ganglion cells

Ganglions cells on average receive input from 120 rod receptors

In other words, rods have greater CONVERGENCE

Remember that the more excitatory NTs there are at the synapse, the more likely the neuron will fire

Question 36

convergence in the retina

• rods vs cones

Answer 36

cones -

• high acuity = low convergence
• they are non convergent in a way
• 1 cone receptor activates 1 bipolar cell that activated 1 gagngliuon cells and sends it out from the retina to the rest of the visual system

rods -

• low acuity = high convergence
• 50-100 rod recepters converges onto a few bipolar cell and then converges onto one ganglion cell

-

Question 37

non convergence

Answer 37

it is responsible for our ability to see details

cones have a 1 to 1 correspondents in the visual system because the want to be able to see where they find changes in detail in our visual field

whereas rod receptors, because they converge, that contributes to their high sensitivity (they can receive little amounts of light and still tell the visual system that light is there)
- the problem is high convergences leads to less detail

Question 38

color and reflectance

• color of objects are determined by what?

Answer 38

color of objects are determined by the wavelengths of light that reflect back into our eyes
- some wavelengths are reflected more than others

Question 39

reflectance and transmission; selective reflection

Answer 39

/ selective reflection: some wavelengths are reflected more than others

Question 40

reflectance and transmission;; selective transmission

Answer 40

/ selective transmission: only some wavelengths pass through the object or substance

Question 41

spectral sensitivity

Answer 41

how sensitive are rods and cones to different wavelengths of light along the color spectrum?

–
/ spectral sensitivity: our sensitivity to light at each wavelength

Question 42

measuring spectra sensitivity

Answer 42

use flashes of MONOCHROMATIC LIGHT

- light that contains one single wavelength

Question 43

spectral thresholds

• where is threshold the highest?
• where is threshold the lowest?
• which needs more/ less light?

Answer 43

By presenting variations of the wavelengths in a psychophysical experiment we can get a graph of each individuals THRESHOLD

(see diagram) 6b

Question 44

spectral sensitivity

Answer 44

we can convert threshold to sensitivity

(see diagram) 6b

Question 45

spectral sensitivity (cones vs rods) (in wavelengths)

Answer 45

CONES are more sensitive to longer wavelengths (peak at 560nm)

RODS are more sensitive to shorter wavelengths (peaks at 500 nm)

(see diagram) 6b

Question 46

night vision

Answer 46

in the dark, we rely more on rods for vision

this explains why we are more sensitive to blues and greens typically seen as “night vision”

Question 47

purkinje effect

Answer 47

Johannes Purkinje (1852)

Flower in light (A), dusk (B), and dark (C)

• A (mostly cones)
• B (rods and cones)
• C (mostly rods)
• see picture 6b

If you dark adapt your eyes, you will notice that the blue flower is brighter than the red

Question 48

visual receptors; when does transduction occur?

Answer 48

transduction occurs when retinal absorbs 1 photon of light

Question 49

absorption spectrum

Answer 49

We can see certain wavelengths in dim lighting conditions as a consequence of rhodopsin’s ability to absorb them

Question 50

three kinds of cones; absorption spectra

Answer 50

The differences in spectral sensitivities between rods and cones is due to the absorption spectra of the visual pigment molecules - 3 for cones and only 1 for rods

Question 51

how do we perceive colors? ; trichromatic theory

Answer 51

/ trichromatic theory: color vision depends on activity of three different color receptor types
> red, green, blue

Question 52

how do we perceive colors? opponent-process theory

Answer 52

/ opponent-process theory- color vision is related to opposing responses by blue - yellow, and red - green

Question 53

trichromatic coding of colors

Answer 53

All colors can be coded using a pattern of activation across each of the three different cone receptors

Colour perception is based on the pattern of activity of these three receptors – cones

Activation indicated by the SIZE

*see picture 6b

Question 54

types of color deficiency; pratanopia

Answer 54

Protanopia - missing the long wavelength (red) pigment

~1% of males affected (.02% of females)

Question 55

types of color deficiency; deuteranopia

Answer 55

Deuteranopia -missing the medium wavelength (green) pigment

~1% of males affected (.01% of females)

Question 56

types of color deficiency; tritanopia

Answer 56

Tritanopia - possibly missing the short wavelength pigment?

Extremely rare, less than .01% of males

Question 57

opponent-process theory

Answer 57

Ewald Hering (1834-1918)
Adapting to a red field generated a green afterimage (and vice versa)
Adapting to a blue field generated a yellow afterimage (and vice versa)

Question 58

opponent-process from cones

Answer 58

Blue-Yellow and Red-Green mechanisms can be created by excitatory and inhibitory inputs from the trichromatic cone receptors

Question 59

trichomatic coding & opponent coding

Answer 59

see diagram

126 million receptors but only 1 million ganglion cells

Rods converge more than cones

Question 60

retinal ganglion RFs

Answer 60

Ganglion neurons have center-surround receptive fields

Respond best to small spots of light

Question 61

opponent neurons

Answer 61

opponent center- surround receptive fields

see picture 6b

(yellow on, blue off / blue on, yellow off / red on, green off / green on, red off)

Question 62

two theories together

Answer 62

see diagram 6b

Question 63

colour in the brain

• what is cerebral achromatopsia?

Answer 63

Perhaps there is a “colour area” in the brain?

Cerebral achromatopsia – colour blindness due to damage to the cortex
Good visual acuity
Motion perception

Question 64

the visual system

Answer 64

the pathway from the eye (retina) to the visual cortex

• optic nerve leaves the eye (blind spot) and then goes to a nucleus called LGN in the thalamus, and then the visual information goes thru these radiations to the primary visual cortex in the occipital lobe
• after that it goes into two directions / goes up to the parietal lobe or down towards the temporal lobe
• see picture 6c

Notice how the information flows through the thalamus (LGN), to the primary visual processing areas (V1) and then beyond!

Question 65

geniculo-striate pathway

Answer 65

Each eye receives input from the left and right visual fields
- we have the left visual field (NOT THE LEFT EYE that gets processed in the right occipital cortex (contralateral organization)

Some retinal fibres cross over at optic chiasm and project to lateral geniculate nucleus

Approx. 10% of the fibers from the optic nerve reach the superior colliculi (SC)
- not all of the info coming from the retina or LGN

The SC is responsible for eye movements and other visual behaviours

• below the thalamus in the tectum
• processing really basic eye movement / orienting our attention AUTOMATICALLY to some type of movement motion / visual stimulus that usually commonates an eye movement

The LGN filters the visual input from each eye and sends it to the primary visual areas (V1)

Each V1 hemisphere in receives input from the contralateral visual field
- still getting info from both EYES, but only 1 VF

Question 66

LGN (lateral geniculate nucleus)

Answer 66

The LGN is thought to be where visual info is regulated (filtered)

• part of the thalamus
• collection of nuclei

Notice not a lot of info is passed from the LGN to cortex
- it doesnt give all of the information to the primary visual cortex (~40%)

Only 4 out of 10 nerve impulses are transmitted to V1

It also organizes information

• LGN filtering out that information somehow
• youre telling your visual cortex what it is you want to see / what you want to process
• everything else should sort of be ignored

LGN is sort of an attentional filter because it allows relevant information to get to the visual primary cortex

Question 67

organization of LGN

Answer 67

/ magnocellular channel
- motion, contrast

/ parvocellular channel
- colour, fine details

–
layers 1 & 2: process information from rod receptors and follow the magnocellular layer
- magno is fast but light / not much detail

layers 3-6: processing imformation from the parvocellular layer (cone receptors)
- parvo has more layers because cones carry more information (light and details)

LGN is also organized according to eye (one layer will be the left and ine layer will be the right and so on)
- we have to keep it separate because??

Question 68

LGN neuorons

Answer 68

LGN neurons have the same receptive fields as the retinal ganglion cells

• Center-surround
• if light is presented into the centre of the excitatory receptive field, then that neuron fires a lot and once the inhibitory region is covered in light it starts to slow down, and if all of it is covered, it stops

Respond best to small spots of light

Question 69

difference of the receptive field of retinal cell vs LGN

Answer 69

Lgn cells are bigger

receptive field will cover a lot more visual field because its in a ganglion cell level

Question 70

retinotopic organization

Answer 70

LGN is organized according to what informnation is projected on to the retina (retinonotopic)

/ retinotopic: it is spacially organized according to the photoreceptors that are active on to the retina

> retinotopic maps

Each place on the retina corresponds to a place in the LGN and a place in the visual cortex

Damage to a piece of the map will result in blindness in the corresponding portion of the visual world

Question 71

RFs of V1 Neurons

Answer 71

SIMPLE CORTICAL CELLS- have side-by-side opponent receptive fields
Monocular receptive fields
- made up of LGN neurons
- not centre surround they are rectangular / rectangular centre excitatory and inhibitory regions
- they want lines

Neuron responds maximally when the stimulus is in the preferred orientation

• Orientation selectivity
• Different neurons respond to different orientations

V1 neurons as“feature detectors”

Question 72

direction-specific RFs

complex cell vs end-stopped cells

Answer 72

COMPLEX CELLS- respond best to bars at a particular orientation that move in a specific direction
- why do we want to have a cell that responds to lines that move? how will that benefit us? - they keep track of the features moving in different directions in our visual world so that the objects stay complete objects even if we move around or that object move around

END-STOPPED CELLS - fire when lines of a specific length move in a particular direction

• Also respond to moving edges or angles (corners)
• cells that respond in moving in a particular direction but having an end to them
• cells used for creating complete boundaries between objects (if theres no corner or end to these lines it will just be a continuous line forever)

Question 73

organization of striate cortex

Answer 73

The primary visual cortex (i.e., V1) is the first cortical region that receives visual information from the retina

Often referred to as striate cortex because of distinct stripe pattern

6 layers- layer 4 receives sensory inputs
- Specifically, layer 4Cα (magnocellular), 4Cβ (parvocellular)

layers 4C a or b - processing info from the rods (magnocellualr) and cones (parvocellular) keeping them separate

Question 74

three terms for the primary visual cortex

Answer 74

1) primary visual cortex
2) V1
3) striate cortex

— they all mean the same

Question 75

hypercolumns (location column)

Answer 75

Each region of visual space is represented by a V1 module

Blobs are sensitive to color, but not orientation or form

Color information sent to V4/V8

Neurons outside blobs respond to orientation and form but NOT color

notes:

Each column contains a full set of orientation columns

It is not always the case however, that the orientation columns line-up, sometimes they can be arranged as a “pin wheel”

Question 76

cortical magnification

Answer 76

Fovea accounts for ~ .01% of the area of the retina
- its represented by about 10% of the visual cortex

However, 10% of visual cortex is devoted to the fovea!

notes:

Used brain imagining

Participant saw light either in a small area of the fovea or larger area of periphery

Question 77

visual association areas

Answer 77

*see picture 6c

V1 essentially goes out to V2 and out to V3 and then V4 which is a color prorcessingh area and then we get to MT which is jisy a motion processing area

and then eventually infromation goes in different directions and information goes further and goes to sort of upwards towards the parietal cortex (dorsal stream) or down to the inferior temporal cortex

Question 78

dorsal and ventral streams

Answer 78

Dorsal Stream: pathway from primary visual cortex
- The “where” pathway (location and movement), or
- The “how” pathway for control of behavior (e.g., reaching)
- process coming from rod receptors and that magnocellular pathway because its processing motion informiartjon for like where things are
- this system works fast and its a system that is constanmtky updating and working / so it can update your behavipurs
- vision for action pathway (where things are and how to interact with them even if we dont know whats coming)
- it is unconcious
–
Ventral Stream: pathway from primary visual cortex to ventral prestriate coretex to inferotemporal cortex
- The “what” pathway (color and shape)
- The pathway for conscious perception of objects
- for object recognizition (memory for onjects)
- our vision for recognition or preception pathway
-conatoins information what that object is (color / layout / what we have in memory stored for those objects)
- it is very conscious

these two streams are independent pathways even though they consistently interact together

they can work indepednently and they can also function without one another

Question 79

Patient DF: visual form agnosic

Answer 79

~ Goodale et al (1991)

they were the first one who demonstrate the what and how system
- what system is important for humans

Question 80

patient RV: optic ataxic

• brain damage in which stream?

Answer 80

Brain damage to the dorsal stream (where/how)

Can identify objects

Cannot perform the correct grasp to interact with objects

Question 81

doube dissociation

visual agnosia vs optic ataxia

Answer 81

visual agnosia: damage to the ventral stream

perception: no
action: yes

optic ataxia: damage to dorsal stream

perception: yes
action: no