B&B Vision Flashcards
How many muscles control the position of the eyeball in the socket and what cranial nerves are they innervated by?
6
lateral (VI) & medial (III) rectus
superior (III) & inferior (III) rectus
superior (IV) & inferior (III) oblique
In bright light, what contracts to restrict the pupil?
The circular sphincter muscle of
the iris
In dim light, what contracts to widen the pupil?
The radial dilator muscle
Describe the muscles that contract/relax when focusing near
Ciliary muscles contract
Zonular fibres relax
Lens become more spherical
Eyes converge and pupils restrict
Describe the muscles that contract/relax when focusing far
Ciliary muscles relax
Zonular fibres contract
Lens become less spherical
Eyes diverge and pupils dilate
What is the cause and symptom of Myopia?
Myopia causes nearsightedness so close objects are clear but far objects are blurred. This happens because the cornea/lens is too powerful or the eyeball is too long. The image is focused in front of the retina
What is the cause and symptom of Hyperopia or Presbyopia?
Hyperopia causes farsightedness so far objects are clear but close objects are blurred. This happens because the cornea/lens is too weak or the eyeball is too short. The image is focused behind the retina.
How do you correct Myopia?
Concave corrective lens
negative
How do you correct Hyperopia?
Convex corrective lens
positive
What is an astigmatism?
Caused by the irregularities in the curvature of the cornea
How do you correct an astigmatism?
Need cylindrical (not spherical) corrective lens
What is retinal isomerization?
There are cis/trans isomers of the retinal
When a photon of light is absorbed in the visible range, 11-cis retinal can isomerize to all-trans retinal
It goes from being kinked to straight
Purple to bleached
Can also be reversed but enzymes are needed
What is phototransduction?
Process by which photons are converted to electrical signals. Occurs in photoreceptors
Describe the process of phototransduction
eg phosphoryltion and completion and stabilisinf g protein
1) Phosphorylation of metarhodopsin II (R)
2) Completion of phosphorylation and stabilising
(“capping”) by arrestin (Arr): inactivation of R
3) Inactivation of G-protein (transducin) and
phosphodiesterase (PDE) by a GTPase activating
protein (GAP)
4) Regeneration of cG by guanylyl cyclase (GC):
depends on low [Ca2+] and requires GTP
5) cG-gated channels re-open, Na+
influx
Describe the process of brightness (or light) adaptation?
1) During illumination, cGMP gated channels close and
intracellular Ca2+ concentration falls (because of activity of Na/Ca pump)
2) Ca2+ no longer inhibits guanylyl cyclase, which recycles cGMP from GTP
3) cGMP is replaced quickly, and cGMP gated channels reopen, keeping the light response relatively small and brief
What is the distribution of rods and cones?
Receptor density decreases towards periphery
Cone density peaks in the fovea and falls off rapidly
Rod density peaks at ~18° eccentricity
Acuity declines with receptor density
Compare rods and cones
Cones mediate photopic vision and rods mediate scotopic vision
Cones are for day time light levels and rods are for low light levels
Cones are for a range of colours (trichromatic) and rods are for shades of grey
Cones are densest in the fovea and there are no rods in the fovea
The cone density falls off sharply in periphery but rod density rises near periphery
Cones have a low density to light and rods have a high sensitivity to light
Cones have a quick recovery in the dark and rods have a slow recovery in the dark
What is the receptive field (RF)?
The area on the receptor surface where an appropriate
stimulus causes a change in neuronal activity.
In the visual system, it is the region on the retina
where illumination influences a cell’s response.
What are bipolar cells?
They mediate the path of visual information between photoreceptors and ganglion cells
They receive direct excitory input from the photoreceptors in the centre and indirect inhibitory input from those in the surround
On/off bipolar cells
If you shine a light onto the center of a on-center cells do you get action potentials?
Yes
If you shine a light onto the center of an off-center cells do you get action potentials?
No
Take light away and then you get APs
If you shine a light onto the outside of a on-center cells do you get action potentials?
No
Take light away and then you get APs
If you shine a light onto the outside of an off-center cells do you get action potentials?
Yes
In your eye, where do the optic nerves meet?
In the chiasm
In the chiasm, hemidecussation takes place. What is this?
Half of the optic nerve fibers, from the temporal half of the retina eg, stay on the same side of the brain while the other half from the nasal part eg, cross over to the other side of the brain so the left half of the visual field is mapped on the right side of the brain vice versa
Describe the Lateral geniculate nucleus LGN
Consists of 6 layers
Receives input from both eyes
Each layer receives input from only 1 eye
Layers 1,4,6 are contralateral
Layers 2,3,5 are ipsilateral
At this point info from 2 eyes is still separated
Layers 1,2 are magnocellular
Layers 3-6 are parvocellular
The receptive fields are similar to retinal ganglion cells
Describe the primary visual cortex V1
Also known as the striate cortex or area 17
Consists of 6 layers
Contains retinotopic map
Receives inputs from both eyes
Input primarily arrives in layer 4
The left and right eye inputs in layer 4 segregate into ocular dominance columns
Cells outside layer 4 are mostly binocular
This means they receive inputs from both eyes
Are the V1 neurons sensitive to the orientation of a bar of light?
Yes
Only preferred orientation gives APs (diagonal) - Hubel and Wiesel 1962
What is a simple cell receptive field?
Adjacent stripes of on and off responses
What are orientation columns?
In the visual cortex
Neurons with similar orientation preferences are clustered
What are the different types of retinal ganglion cells?
Bistratified
Midget
Parasol
What layers do the different types of retinal ganglion cells project to?
Then what do those layers project to?
Bistratified - Koniocellular - blobs in layers 2/3
Midget - Parvocellular - 4c beta
Parasol - Magnocellular - 4c alpha
What is the Trichromatic theory?
All colours can be represented as a combination of 3 colours
More than one cone type needed to distinguish variations in brightness from variations in wavelength
Theory does not explain why some colours blend
There is differential sensitivity of cones to light wavelengths
red = long, green = medium, blue = short
What is the opponent process theory?
There are 2 classes of of processes
1) Spectrally opponent (Colour)
2) Spectrally non-opponent (B+W)
yellow,blue,red,green could be thought as primary colours
Match with antagonistic centre-surround receptive field organisation (bipolar cells, retinal ganglion cells, LGN, cells)
This theory is better than trichromatic
What does a blue S cone connect to?
What does Yellow L+M cone connect to?
What does a green M cone connect to?
What does a red L cone connect to?
S cone on-bipolar (rare) on the bistratified S cone on cell
L+M cone off-bipolar on the bistratified S cone on cell
M cone on/off bipolar on the midget cell system in the fovea
L cone on/off on the midget cell system in the fovea
Where are the colour selective LGN cells found?
Parvocellular and Koniocellular layers
True or false
Magnocellular layers are ‘colourblind’
True
eg spectrally non-opponent
Where do the blobs in V1 project to?
The thin stripes in V2
Where do the interblobs in V1 project to?
Pale stripes in V2
Where does the information from the magnocellular layer in V1project to?
Thick stripes in V2
The thick stripes in V2 are the beginning of 1 of 2 streams of the visual system. What is this stream and where does it lead?
Dorsal stream
MT or V5
Where do the pale stripes and thin stripes of V2 project?
Ventral stream which starts in V4
The dorsal stream is otherwise known as the parietal stream. Is this the Where or the What stream?
Where
The ventral stream is otherwise known as the temporal stream. Is this the Where or the What stream?
What
eg what objects are
Does V4 respond to non-cartesian gratings?
Is is specialised in colour representation?
Yes
Also respond to radial gratings and windmill patterns
Yes
What is a double-opponent cell?
eg in the centre it has an ON response to red AND an OFF response to green and in the surround it has an OFF response to red and an ON response to green
Colour constancy occurs in V4
What is it?
Subjective constancy
Ensures that the perceived colour of objects remains relatively constant under varying illumination conditions
eg white and gold dress, blue and black dress
What is the cause of lateral inhibition in the retina?
Horizontal cells
What kind of vision loss do you get when you cut the left optic tract after the chiasm?
Loss of the right half of the visual field
Which theory of colour vision explains why certain colours dont blend?
Opponent process theory
In which brain area do neurons first respond to the colour of objects independent of illumination?
V4
Where does the separation into two visual systems begin?
In the retina
3 types of retinal ganglion cells
Becomes more obvious in V2
What area of the brain is critical for the detection of the direction of motion?
MT/V5
What are LIP neurons for?
They are thought to carry signals appropriate for identifying salient targets
And for for guiding saccadic eye movements
What is an insect’s eye made out of?
Ommatidia - 1 is an ommatidium
First visual processing unit
What does an ommatidium consist of?
8 photoreceptor cells Bristle cells Secondary and tertiary photoreceptor cells Cell body Rhabdomere
Where does the phototransduction process on an insect take place?
The hundreds of microvilli on the rhabdomere which is part of the ommatidia
Describe the phototransduction process in insects
Activation by light of rhodopsin (same as mammals)
Couples to a g protein (same) called Gq has a,b,y subunits
The alpha subunit takes upon a GTP
This activated Gqa interacts with a phospholipase PLC
This PLC breaks down PIP2 into IP3 +H+ + DAG
DAG breaks down into either 2-MAG + FA or 1-MAG + PUFA
Next stage is still somewhat unknown
Do know that the 2-MAG interacts with the TRP and TRPL receptor channels
Channels open
Influx of Ca and Na ions
Net result is depolarisation of the the photorecptor
Contrasts that of a vertebrate (hyperpolarisation)
What is a signalplex?
The light response in fly photoreceptor cells is mediated by a series of proteins that assemble into a macromolecular complex referred to as the signalplex
What is the central player in a signalplex?
inactivation no afterpotential D
(INAD)
A protein consisting of a tandem array of five PDZ domains.
At least seven proteins bind INAD, including the TRP channel, which depends on INAD for localization
to the rhabdomere
Describe the fly visual system
Retina - made up of ommatidia
8 Rhabdomere photoreceptor cells
Optic lobe
Consists of lamina, medulla, lobula and lobular plate
Number of neurons much more limited than a vertebrates
Limited number of cell types
All fairly well characterised
eg the lamina has lamina monopolar cells, centrifugal cells,T1 cell
Why study the fly? esp fruit fly
Drosphola
Easily manipulated genetically
Short life span so can get through many generations
What is GAL4?
Yeast transcription factor
Can express specifically in a group of neurons
And in a time specific manner to express a gene of choice
Outer photoreceptors R1-6 express the what opsin?
Rh1
What do R7 photoreceptors express?
Either Rh3 or Rh4
What do R8 photoreceptors express?
Rh5 or Rh6
Are flies able to see polarised light?
Yes
Can do this because of the responses of R7 and R8 neurons in the dorsal part
And a different distribution in the ventral part
What 2 cells are probably the most important in fly motion vision?
T4 and T5
Describe the motion processing in the optic lobe of the fly
Lamina cells- L1 and L2
L2 connects to Tm1 and Tm2 - These connect to T5 cells
L1 connects to Tm3 and Mi1 - These connect to T4 cells
These are motion sensitive cells - sit on lobula plate
Do L2 neurons in the medulla carry information about the direction of motion?
No
However, brightness decrements (light -OFF) induce a strong increase in intracellular Ca2+
But brightness increments (light-ON) induce only small changes
This means L2 transmit brightness decrements to downstream circuits that then commute the direction of image notion
Direction sensitive retinal ganglion cells receive a specific patter of input from what?
Starburst amacrine cells
ON/OFF types
Starburst amacrine cells receive a specific input from what cells?
Cone bipolar cells on to dendritic field
Which cortical area is the gateway to the dorsal stream and essential for motion vision?
MT/V5
How do we know that MT is critical for motion vision?
From lesion studies in monkeys and microstimulation in monkeys
How does a fly photoreceptor respond to light?
Depolarisation through Na+ and Ca2+ influx
Which are the motion sensitive output cells of the fly optic lobe?
T4 and T5 cells on the edge of the lobula plate
Which cells in the mouse retina play a role in motion detection analogous to the T4 and T5 cells of the fly?
ON/OFF starburst amacrine cells
