ch six: the visual system Flashcards
the tarsier
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)
the dragon fly
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.
the stalk eyed fly
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.
the night vision
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
the night vision; two ways to detect light
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
light
- 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
light; ultraviolet and inferred lights
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
electromagnetic spectrum; wavelenght of light
(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
electromagnetic spectrum; wavelengths of light (long, med, short)
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
wavelengths do not have “color”
- 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
wavelengths of color; sensation vs perception
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
the eye
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,
retina & fovea
/ retina: layers of cells at the back of the cell
/ fovea:
cornea
/ 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)
iris
/ iris: the colour of the eye
pupil
pupil can dilate (gets smaller or bigger)
pupil basically lets in more / less light
more light -> the better vision you’ll have
lens
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 )
the eye; how light passes by
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
optic lens (the blind spot)
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
retinal cells
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..
retinal cells; ganglion cells
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
the fovea
- 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
photoreceptors; transduction
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
photoreceptors; specialized cells in transduction
/ 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
transduction
/ 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)
visual receptors; when does transduction occur?
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
isomerization
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
visual pigment bleaching
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
visual pigment bleaching; what happens when light is constant?
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
visual pigment regeneration
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
rod vs cone vision; duplexity theory of vision
duplexity theory of vision - cones and rods mediate different kinds of vision
cones
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
rods
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
rod and cone distribution
- what does fovea contain?
- what’s in our periphery>
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
greater sensitivity in rods; convergence
- what is convergence? what does it do?
- which one has greater convergence?
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
convergence in the retina
- rods vs cones
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
-
non convergence
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
color and reflectance
- color of objects are determined by what?
color of objects are determined by the wavelengths of light that reflect back into our eyes
- some wavelengths are reflected more than others
reflectance and transmission; selective reflection
/ selective reflection: some wavelengths are reflected more than others
reflectance and transmission;; selective transmission
/ selective transmission: only some wavelengths pass through the object or substance
spectral sensitivity
how sensitive are rods and cones to different wavelengths of light along the color spectrum?
–
/ spectral sensitivity: our sensitivity to light at each wavelength
measuring spectra sensitivity
use flashes of MONOCHROMATIC LIGHT
- light that contains one single wavelength
spectral thresholds
- where is threshold the highest?
- where is threshold the lowest?
- which needs more/ less light?
By presenting variations of the wavelengths in a psychophysical experiment we can get a graph of each individuals THRESHOLD
(see diagram) 6b
spectral sensitivity
we can convert threshold to sensitivity
(see diagram) 6b
spectral sensitivity (cones vs rods) (in wavelengths)
CONES are more sensitive to longer wavelengths (peak at 560nm)
RODS are more sensitive to shorter wavelengths (peaks at 500 nm)
(see diagram) 6b
night vision
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”
purkinje effect
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
visual receptors; when does transduction occur?
transduction occurs when retinal absorbs 1 photon of light
absorption spectrum
We can see certain wavelengths in dim lighting conditions as a consequence of rhodopsin’s ability to absorb them
three kinds of cones; absorption spectra
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
how do we perceive colors? ; trichromatic theory
/ trichromatic theory: color vision depends on activity of three different color receptor types
> red, green, blue
how do we perceive colors? opponent-process theory
/ opponent-process theory- color vision is related to opposing responses by blue - yellow, and red - green
trichromatic coding of colors
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
types of color deficiency; pratanopia
Protanopia - missing the long wavelength (red) pigment
~1% of males affected (.02% of females)
types of color deficiency; deuteranopia
Deuteranopia -missing the medium wavelength (green) pigment
~1% of males affected (.01% of females)
types of color deficiency; tritanopia
Tritanopia - possibly missing the short wavelength pigment?
Extremely rare, less than .01% of males
opponent-process theory
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)
opponent-process from cones
Blue-Yellow and Red-Green mechanisms can be created by excitatory and inhibitory inputs from the trichromatic cone receptors
trichomatic coding & opponent coding
see diagram
126 million receptors but only 1 million ganglion cells
Rods converge more than cones
retinal ganglion RFs
Ganglion neurons have center-surround receptive fields
Respond best to small spots of light
opponent neurons
opponent center- surround receptive fields
see picture 6b
(yellow on, blue off / blue on, yellow off / red on, green off / green on, red off)
two theories together
see diagram 6b
colour in the brain
- what is cerebral achromatopsia?
Perhaps there is a “colour area” in the brain?
Cerebral achromatopsia – colour blindness due to damage to the cortex
Good visual acuity
Motion perception
the visual system
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!
geniculo-striate pathway
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
LGN (lateral geniculate nucleus)
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
organization of LGN
/ 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??
LGN neuorons
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
difference of the receptive field of retinal cell vs LGN
Lgn cells are bigger
receptive field will cover a lot more visual field because its in a ganglion cell level
retinotopic organization
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
RFs of V1 Neurons
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”
direction-specific RFs
complex cell vs end-stopped cells
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)
organization of striate cortex
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
three terms for the primary visual cortex
1) primary visual cortex
2) V1
3) striate cortex
— they all mean the same
hypercolumns (location column)
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”
cortical magnification
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
visual association areas
*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
dorsal and ventral streams
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
Patient DF: visual form agnosic
~ Goodale et al (1991)
they were the first one who demonstrate the what and how system
- what system is important for humans
patient RV: optic ataxic
- brain damage in which stream?
Brain damage to the dorsal stream (where/how)
Can identify objects
Cannot perform the correct grasp to interact with objects
doube dissociation
visual agnosia vs optic ataxia
visual agnosia: damage to the ventral stream
perception: no
action: yes
optic ataxia: damage to dorsal stream
perception: yes
action: no
