2.0 Visual System Flashcards
What is the wavelength of visible light?
390nm → 700nm<br></br>(violet) → (far red)
What are the 4 variables that the visual system can analyse?
1) Intensity<br></br>2) Wavelength<br></br>3) Space<br></br>4) Time
Illuminance vs Luminance
<b>Illuminance</b> = Quantification of light from light source (lux)<br></br><br></br><b>Luminance</b> = Quantification of light reflected from objects (cd/m²)
Define reflectance:
A property of a material that dictates the proportion of reflected light from an illuminating source
Define constrast
Contrast is defined as the ratio of incremental light intensity to the mean background intensity (∆I/I)
What is pointspread function?
<b>Pointspread function = blurred image of a point source on the retina</b><br></br><br></br>Diameter of pointspread function = wavelength of light/diameter of aperture
4 factors which may limit the optical quality/visual resolution:
1) Diffraction<br></br>2) Chromatic aberration<br></br>3) Spherical aberration<br></br>4) Glare
Define diffraction
Spreading of waves as they pass through an apperture.<br></br>Diffraction causes pointspread function
Define Spherical aberration
<b>In a spherical surface, rays towards the edge are refracted more</b>
Define Chromatic aberration
<b>Different colours are refracted by different amounts (thus focused at different depths)</b><br></br>Human eye = well focused for green. Poor focus for blue
Define Glare
Small particles in optical media scatter light in all directions → ↓ contrast of image
Define Emmetropic
Object at infinity is sharply focused
Define Ametropic
Inability to sharply focus at an object at distance infinity
Define Myopia
Short sight (inability to focus on distant objects)<br></br>Light is focused at a point before the retina → ↑ pointspread function
Define Hypermyopia
Long sighted. Inability to focus on nearby objects
Define Presbyopia
A progressive ↓ in the ability of the eye to focus on near objects<br></br><br></br>Caused by ↓ accomodation because of ↓ lens elasticity
Define Grain (receptor spacing)
To see two points as separate points, receptor spacing needs to be at least 1/2 the width of the pointspread function<br></br><br></br>(this is usually achieved in the fovea)
What is the receptor spacing in fovea?
0.5 arc mins apart
What is cataracts?
Clouding of the lens
What is the power of a lens?
The strength of the lens<br></br><br></br>Expressed in <b>dioptres</b>
How are the lens and cornea supplied with metabolites?
They are avascular. Supply is via the aqueous humour<br></br><br></br>(secreted by ciliary body and drains in the canal of schlemm + trabecular meshwork)
What is glaucoma?
A reduction in the outflow of aqueous humour → ↑ intraocular pressure
What is accommodation?
<b>Lens changes its focal length to focus on objects at different distances</b><br></br><br></br>Due to combination of:<br></br>1) Radial elastic ligaments (suspensory ligaments)<br></br>2) Circular ciliary muscle<br></br><br></br>Near reflex accomodation requires:<br></br>1) Constriction of pupils<br></br>2) Convergence of the two eyes (to fixate on new target)
What are the two muscles that control pupillary size? <br></br>What is their innervation?
1) Sphincter pupillae (PSNS)<br></br><br></br>2) Dilator pupillae (SNS)
What is the control pathway from the retinal to the pupillary sphincter (light reflex)?
Retina → Pretectum (midbrain) → <b>Bilateral</b> Edinger-Westphal nucleus → CN III → Ciliary ganglion → Pupillary sphincter
What is Argyll-Roberston pupil?
Characteristic of neurosyphilis<br></br>Pupil does not react to light but does react to accomodation
What are Muller cells?
Retinal glial cells that act as optical wave guides.
How wide is the fovea?
1.5mm<br></br>5 degrees
Features of foveola:
Middle (260um/1 degree) of fovea<br></br>Avascular<br></br>No rods<br></br>Centre of foveola has a cone spacing of 0.5min arc
Features of parafoveal region:
20 degrees either side of fovea<br></br>Peak rod density<br></br>Most sensitive to mesopic + scoptic vision
Where is the blind spot? (how big is it?)
At the optic disk<br></br>(5 degrees)
Describe papilloedema and why it occurs:
Papilloedema = swollen optic disk<br></br><br></br>Seen with ↑ intercranial pressure, because CSF around the optic disk is continuous with brain
Describe the general anatomy of rods and cones
1) Outer segment (transduction)<br></br>2) Inner segment (cellular machinery)<br></br>Both joined with cilium
Describe the visual pigments in rods
<b>Rhodopsin</b><br></br>GPCR embedded in disk membrane<br></br>7 transmembrane domains<br></br>Binds to <b>11-cis retinal</b> (chromophore)<br></br>Wavelength peak = 500nm
Describe the visual pigments in cones
Cone opsins <br></br>Bind to <b>11-cis retinal</b><br></br>Different interactions tune the absorption to different wavelengths (compared to rodopsin)
Describe photoisomerisation of the photopigment
- With absorption of single photon the chromophore isomerises from 11-cis to all-trans retinal.<br></br>- Photoisomerisation leads to catalytically active form of rhodopsin = metarhodopsin II (R<b>)<br></br>- After R</b> has played its role, all-trans retinal dissociates slowly (100-1000s) from the opsin<br></br>- Rhodopsin is now in its bleached form and must be regenerated before it can be used again
Where does rhodopsin regeneration occur?
Retinal pigment epithelium (RPE)
What are the steps for regeneration?
1) Retinal is reduced to all-trans-retinol <br></br>2) Transported out of the photoreceptor into the RPE where it is converted back to 11-cis retinal<br></br>3) Transported back to the photoreceptors, rejoins the bleached opsin to form rhodopsin<br></br><br></br>An alternative pathway via Müller cells allows rapid pigment regeneration in cones.
How long can regeneration take after bright light?
Up to 30 mins for full regeneration
How do cones regenerate?
Via an alternative pathway via Müller cells <br></br>Allows rapid pigment regeneration in cones
What causes night blindness?
Deficiency of vitamin A (11-cis-retinol)<br></br>Retinal is a derivative of Vit A
What are the steps of phototransduction?
1) R* activates transducin<br></br>2) Transducin activates PDE<br></br>3) PDE hydolyses cGMP → 5’GMP<br></br>4) ↓cGMP → closes cyclic nucleotide gated cation channels → hyperpolarisation<br></br>5) Guanylyl cyclase (GC) resynthesises cGMP to terminate response
What 5 proteins are needed for phototransduction?
1) R* (Rhodopsin)<br></br>2) Transducin <br></br>3) PDE<br></br>4) cyclic nucleotide gated cation channels <br></br>5) Guanylyl cyclase (GC)
What is retinitis pigementosa?
- Progressive hereditary retinal degeneration <br></br>- 1 in 3000 people <br></br>- There is a gradual onset of night blindness in adolescence, leading to loss of all peripheral vision by adulthood, and in extreme cases total blindness<br></br>- 5-10% of cases are caused by mutations in the gene for rhodopsin. <br></br>- Other molecules of the transduction cascade are affected in other cases including PDE and the light-sensitive channel
What is photoreceptor light adaptation?
”"”A reduction in sensitivity as the steady light intensity increases”“<br></br>Used to avoid saturation and allow operation over a wide range of background intensities”
What is the mechanism of photoreceptor adaption?
• Mechanism of photoreceptor adaption:<br></br> 1. Ca2+ influx inhibits guanylyl cyclase (GC)<br></br> 2. Less cGMP is remade<br></br> 3. ∴ photoreceptors saturate more quickly<br></br> 4. This leads to less light response<br></br>• This sets a regulatory negative feedback loop<br></br> o Ca2+ enters via cyclic nucleotide-gated channels<br></br> o Ca2+ then inhibits GC ⟶ ↓ cGMP ⟶ less Ca2+ entering the cell<br></br> o Ca2+ however continues to be extruded via Na+/Ca2+/K+ (NCX) exchanger<br></br> o ∴ Ca2+ concentration ↓ ⟶ relieving the inhibition of GC ⟶ more cGMP is synthesized
Define saturation
↑ stimulus intensity does not cause any more increase in receptor firing because the transduction pathways are limited
What is photoptic vision?
Vision in well lit conditions (uses cones)
What is scotopic vision?
Vision in poorly lit conditions (uses rods)
Where is scotopic sensitivity the highest?
In the parafoveal region (highest rod density)
What is mesopic vision?
Combination of photoptic and scotoptic (uses cones and rods)
Give two differences between rods and cones in terms of transduction and adaptation?
- Cones are ~50x less sensitive, than rods, and cannot detect single photons.<br></br>2. Cone responses are much faster than those of rods<br></br>3. Rods saturate at lower intensities therefore are not used in photoptic conditions
Define colour vision:
Ability to distinguish objects based on their spectral reflectance, independent of their intensity
Define the principle of spectral univariance
Individual cone cell absorbs optimally at a particular wavelength, but <b>cannot distinguish between two colours on its own</b><br></br><br></br>For example, a green cone might be activated by 10 green photons, but 100 red photons may also activate it.
What type of colour system do humans have?
<b>Trichromic</b><br></br>3 cone classes (Blue, green, red)<br></br>Colour = ratio of excitation of these cones<br></br>Can distinguish > 2 million colours
Define colour opponency:
<b>Colour perception is based on the comparison of responses of the different types of cone receptors</b><br></br><br></br>Humans use double opponent cells
Define colour constancy:
Perceived colours of an object remain the same irrespective of spectral composition of illuminating light
What is ratio of cone class excitation for white light?
Equal excitation of all classes
What cones absorb short/medium/long wavelengths preferentially?
Blue = short<br></br>Green = medium<br></br>Red = long
Define protanopia
Red-dichromacy (no red cones)
Define deuteranopia
Green-dichromacy (no green cones)
Define tritanopia
No blue cones
Define deuteranomaly
<b>Shifted spectral sensitivity of green cone</b> →<br></br>green cone pigment is shifted towards yellow
Define protanomaly
<b>Shifted spectral sensitivity of red cone</b> (red cone pigment is shifted towards yellow)
Define tritanomaly
<b>Congenital abnormality of the blue cones/mechanism</b>. Very rare compared to red-green abnormalities
Define anomalous trichromacy
<b>Inherited color vision deficiency - one of the three cone pigments is altered in its spectral sensitivity</b>
Define natural polymorphism
Slightly shifted red cone
Define rod monochromat
Rods only
What are the 6 main types of cells of the retina?
1) Photoreceptors<br></br>2) Bipolar cells<br></br>3) Horizontal cells<br></br>4) Amacrine cells<br></br>5) Ganglion cells<br></br>6) Muller cells
Define the five layers of the retina
<b>1) Outer nuclear layer</b><br></br>Photoreceptor cell bodies<br></br><b>2) Outer plexiform layer</b><br></br>Synapses between photoreceptors, bipolars and horizontal cell<br></br><b>3) Inner nuclear layer</b><br></br>Bipolar, horizontal and amacrine cell bodies<br></br><b>4) Inner plexiform layer</b><br></br>Synapses between bipolar, amacrine and ganglion cells<br></br><b>5) Ganglion cell layer</b><br></br>Cell bodies of ganglion cells.
Fill in the blanks:<br></br><br></br>The direct pathway of information flow is from photoreceptors to ___________ to _______________. <br></br>All of these cells use ____________ as neurotransmitter.<br></br>_________ is mediated by horizontal cells (H) which are _________. <br></br>Amacrine cells (A) mediate a diversity of interactions in the inner retina, and use many different transmitters. <br></br>Photoreceptors synapse only with _________cells and __________ cells; ganglion cells receive input from _________and ___________cells. <br></br>The output of the retina is carried by the axons of _________cells, which together form the _________
The direct pathway of information flow is from photoreceptors to bipolar cells to ganglion cells (G). <br></br><br></br>All of these cells use glutamate as neurotransmitter.<br></br><br></br>Lateral inhibition is mediated by horizontal cells (H) which are GABA-ergic. <br></br><br></br>Amacrine cells (A) mediate a diversity of interactions in the inner retina, and use many different transmitters. <br></br><br></br>Photoreceptors synapse only with bipolar cells and horizontal cells; ganglion cells receive input from bipolar and amacrine cells. <br></br><br></br>The output of the retina is carried by the axons of ganglion cells, which together form the optic nerve.
How many cones, rods + ganglionic cells are there in the retina?
Cones = 6 million <br></br>Rods = 120 million<br></br>Ganglionic cells = 1.5 million<br></br><br></br><b>There is massive convergence</b><br></br><br></br>This is the opposite in the fovea where there are more ganglionic cells cf. cones
Define presynaptic ribbon
Modified presynaptic density characteristic of synapses that transmit graded signals
Define cone pedicle
- Large synaptic swelling end of cone terminals <br></br>- Allow divergence of the cone signal to numerous bipolar cells (up to 30)<br></br>- Form both invaginating and flat synaptic contacts
Define rod spherule
- Rod synaptic terminals <br></br>- Smaller (than cones)<br></br>- No divergence<br></br>- Lots of convergence
Define receptive field
<b>A visual cell’s receptive field can be defined as the area on the retina from which its activity can be influenced by light </b><br></br>- Serves to define the position of a stimulus within the retina<br></br>- Size corresponds to the degree of convergence
Define synaptic triad
Photoreceptor synapses with both horizontal and bipolar cells
Describe the structure of the receptive field
”- Retinal neurons exhibit centre-surround receptive fields, with a circular receptive field centre and a concentric surrounding annulus<br></br>- <b>On-centre</b> receptive field is excited by light in the central region and inhibited by illumination of the surrounding annulus<br></br>- <b>Off-centre</b> receptive field is inhibited by illumination of the centre and excited by the surround.<div><br></br></div><div><img></img></div>”
Describe the structure of a receptive field in bipolar cells
”- Synaptic interactions at the cone pedicle establish an antagonistic, centre-surround receptive field structure in bipolar cells.<br></br>- Each cone contacts both ““on-centre”” and ““off-centre”” cells<br></br><br></br><b>OFF-CENTRE</b><br></br>- Off-centre bipolar cells hyperpolarize to a ““centre”” stimulus, but depolarize to a ““surround””. <br></br>- In the dark the cone continually releases glutamate which depolarizes the bipolar cell by opening cation channels (ionotropic glutamate receptors). <br></br>- When the cone is illuminated, it hyperpolarizes, transmitter release is reduced and the bipolar cell hyperpolarizes.<br></br>- The antagonistic surround is generated by lateral inhibition mediated by horizontal cells, which have broad dendritic fields, collecting inputs from a large number of cones, and which form inhibitory feedback synapses back onto the photoreceptors.<br></br>- Horizontal cells hyperpolarize in response to light and therefore when only the surround is stimulated the central photoreceptor (now not illuminated) will depolarise (as it receives less inhibition from horizontal cell feedback).<br></br><br></br><b>ON-CENTRE</b><br></br>- On-centre bipolar cells depolarise to centre stimulus via metabotropic glutamate receptor (mGluR6)<br></br>- This activates a G protein resulting in the closure of cation channels in response to photoreceptor transmitter release in darkness (a sign-inverting synapse). <br></br>- Cascade may operate via direct inhibition of the channel by the G protein α subunit. <br></br>- When light hyperpolarises the photoreceptor, thereby decreasing transmitter release, this inhibition is relieved and the channels open, so the on-centre bipolar cell depolarizes”
What are the two classes of bipolar cell that synapse with cones?
<b>1) Midget bipolar cell</b><br></br>Majority of bipolar cells in primates <br></br>- Receive input from just <b>one cone</b><br></br>- Small receptive fields. <br></br>- Signal the finest spatial detail<br></br>- Signals are also colour-specific<br></br><br></br><b>1) Diffuse bipolar cell</b><br></br>- Receive input from <b>10-15 cones</b><br></br>- Larger receptive fields<br></br>- Response is more sensitive<br></br>- Spatial detail = lost<br></br>- Colour detail = lost
What is the nature of bipolar to ganglionic synapses?
Excitatory<br></br>(therefore ganglionic cells have same receptive field as bipolar cells)
What cells modify bipolar to ganglionic synapses via lateral interactions?
Amacrine cells
What are the two types of ganglionic cells?
<b>1) Parvocellular</b><br></br>- a.k.a midget <br></br>-80% of ganglionic<br></br>- Sustained response<br></br>- Slow response<br></br>- High acuity<br></br>- Small fields<br></br>- Colour + form<br></br><br></br><b>2) Magnocellular</b><br></br>- a.k.a parasol<br></br>- 10% of ganglionic<br></br>- Transient response<br></br>- Fast response<br></br>- More sensitive<br></br>- Low acuity<br></br>- Larger field<br></br>- Speclialised for motion
Fill in blanks:<br></br>The extra sensitivity of the rod pathway sacrifices __________ due to convergence of rod signals (_____________). Colour vision is also lost in the scotopic intensity range, and the overall spectral sensitivity is shifted from ___________ (the average peak sensitivity of the red and green cones) to the rod peak sensitivity of ___________ (______________).
The extra sensitivity of the rod pathway sacrifices <b>spatial resolution</b> due to convergence of rod signals (<b>rod pooling</b>). Colour vision is also lost in the scotopic intensity range, and the overall spectral sensitivity is shifted from <b>~560 nm</b> (the average peak sensitivity of the red and green cones) to the rod peak sensitivity of <b>~500 nm</b> (<b>Purkinje shift</b>).
What is the rod pathway?
Rods → Rod bipolar cells (all ‘on’) → AII amacrine cells → ‘on’ cone bipolar cells → ‘on’ ganglion cells
Describe rod processing under twilight
- At slightly higher intensities (twilight) an alternative simpler route involves electrical synapses from rod spherules directly to cone pedicles. <br></br>- Rod signals hijack cone pathway at mesopic intensities when both rods and cones function.
What are the projections from the retina?
1) Lateral geniculate nucleus (main one)<br></br>2) Pretectum (pupillary reflexes)<br></br>3) Suprachiasmatic nucleus (circadian rhythm)<br></br>4) Superior colliculus (eye movement)
What are the layers of the lateral geniculate nucleus?
<b>1) 4 parvocellular layers</b><br></br>- Ventral<br></br>- 6,5,4+3<br></br><br></br><b>2) 2 magnocellular layers</b><br></br>- Dorsal<br></br>- 2+1<br></br><br></br><b>3) Koniocellular layers</b><br></br>- In between above layers (for colour)
How thick is the grey matter of the visual cortex?
2mm
How many layers does the visual cortex have?
6
What happens at each layer?
1 -<br></br>2 - Output to higher visual centres<br></br>3 - Output to higher visual centres<br></br>4 - Fibres from LGN (magno → 4Cα / parvo → 4Cβ)<br></br>5 - Output to deep brain structures (superior colliculus)<br></br>6 - Projections back to thalamus
Define orientation tuned:
Receptive fields respond optimally to bars/edges of specific orientation
Define the receptive field of simple cells:
- Respond to edges of specific orientation <br></br>- Specific well defined position<br></br>- Layers 4+6<br></br>- May have inhibitory flanks
Define the receptive field of complex cells:
- Respond to bars/edges of specific orientation <br></br>- Position is not as critical<br></br>- Layers 2,3+5<br></br>- Directionally sensitive (better for movement)
What is end-inhibition (end-stopping)?
Exhibited by some simple and complex cells in V1.<br></br>Respond to specific bar length and output is inhibited if bar exceeds a critical length.
What are orientation columns?
Cells of same orientation are aligned in one column
What are ocular dominance columns?
Alternating slabs in layer 4 of V1. Receive info one eye then the other.
What are disparity detectors?
Neurons that detect objects in a different depth to the plane of fixation. Used for stereopsis
Define stereopsis:
Binocular judgement of depth
Define the receptive field of cytochrome oxidase blobs:
No orientation tuning<br></br>Centre-surround<br></br>Process colour
What are hypercolumns?
Represents a region of the visual field.<br></br>Contains:<br></br>1) All orientation columns<br></br>2) Left and right ocular dominance columns<br></br>3) Colour analysis blobs
What is ambylopia?
Also known as lazy eye. Eye appears normal however vision is impaired due to a wiring defect occurring in early life. Needs to be corrected <2yrs old
What causes ambylopia?
1) Strabismus<br></br>2) Astigmatism
Define astigmatism:
Refractive error due to abnormally curved cornea
“Draw the ventral ““what”” pathway:”
1) <b>COLOUR</b> <br></br>Parvocellular → blob → thin stripe → V4 → Infratemporal cortex<br></br><br></br>2) <b>FINE FORM AND EDGES</b><br></br>Parvocellular (some magno)→ interblob → interstripe → V4 → Infratemporal cortex
Define acromatopsia:
Cortical colour blindness
Define prosopagnosia:
Inability to recognise faces
“Draw the ventral ““where”” pathway:”
Magnocellular → 4Cα → 4B → thick stripe → V5 (MT) + V5a (MST) → Posterior parietal cortex
What can occur with a lesion in the dorsal ‘where’ pathway?
Inability to see movements (akinetopsia)
What is the formula for webbers law?
∆I/I = constant<br></br><br></br>(with brighter light rod response is less sensitive and faster
Define light adaptation:
As steady light intensity increases, the sensitivity of photoreceptors decreases to avoid saturation and to allow operation over a wide range of intensities
Define bleaching adaptation:
Profound reduced sensitivity induced by bright light. Recovers slowly with dark adaptation. Due to rhodopsin bleaching
Define bleaching sensitization:
Prolonged depression of phototransduction sensitivity exhibited by rods after their exposure to bright light
Define dark adaptation:
Dark adaptation is the process by which sensitivity is recovered following bleaching adaptation. Occurs in two stages:<br></br>1) Cones recover first<br></br>2) Rod cells take longer
What is the rod-cone break?
During dark adaptation, cones recover before rods
Define critical fusion frequency:
At frequencies above this, a flickering light is perceived as steady (critical fusion frequency rises as light intensity rises)
Define post-bleach noise:
Increase in spontaneous quantal events following bleaching
