Vision Flashcards
What is the wavelength range of visible light?
390-700nm
Which variables of light are encoded?
Intensity
Wavelength
Position in space
Position in time
Contrast?
Delta I/I
Pointspread function? And what causes it to increase or decrease?
d = lambda//D. D = diameter of lens/aperture of pupil.
Increased by increasing pupil diameter, because even though diffraction decreases, chromatic and spherical aberrations increase.
Definition emmetropic?
If an object at infinity is sharply focussed
Definition ametropic?
Object at infinity not sharply focussed
Definition myopia? What does it predispose to?
Short sight, 20% pop
Retinal detachment, degeneration nd glaucoma
Definition hypermetropia
Long sight, 30% pop
Presbyopia?
Hardening of lens/loss of elasticity causing eye to focus light behind the retina when looking at close objects (loss of accommodating power)
What does two point resolution depend on?
- Pointspread function
- Diffraction
- Aberration - Refractive errors
- Receptor spacing
How are the lens and cornea supplied with metabolites?
Aqueous humour - secreted by epithelium of ciliary body and drained off through trabecular network in Canal of Schlemm
What causes glaucoma?
Reduced rate of outflow through trabecular network or raised intraocular pressure
How do you measure power of a lens?
Dioptres - 1/focal length in metres
U = distance to lens from object
V = distance from lens to image
1/u+1/v=1/f = Power
How many optical surfaces are there for light to pass through and which has the highest power?
4: outside cornea, inside cornea, outside lens, inside lens
Front of cornea! 48.7 dioptres!
How does the lens accommodate?
Radial elastic ligaments (suspensory ligaments/zonule) and a circular ciliary muscle.
When ciliary muscle is relaxed, ligaments stretch the lens.
At rest, a normal eye is focussed on an object at infinity.
Near reflex
associated with accommodation and constriction of pupil. Convergence of the two eyes to fix on a close target.
Argyll-Robertson pupil
Characteristic of neurosyphilis.
pupil doesn’t react to light, but does to accommodation.
Maybe due to bilateral pretectal damage?
What do retinal glial cells do? What are they called?
Muller Cells
Act as optical waveguides to aid transmission of light
What size are the fovea and foveola and what is the difference?
Fovea central 1.5mm, foveola central 260micrometers. foveola higher acuity. avascular. completely rod free.
Where are there most rods and where are there most cones?
Most cones in foveola
Most rods in parafoveal region - 20 degrees either side of the fovea
What is the structure of the outer segments of the rods and cones and what are they used for?
Transduction!
Rods = stacked membranous discs
Cone = continuous folds of invaginating lamellae
What is rhodopsin? What does it bind?
GPCR pigment
binds chromophore 11-cis retinal
What is the peak absorption of retinal when it is bound to rhodopsin?
500nm
Discuss the amplification cascade for phototransduction
1 R+ –> 150 transducin –> 150 PDE –> 10^5 cGMP –> closes 500 channels
What mediates photoreceptor adaptation?
Calcium ions
Define colour vision
Ability to differentiate objects based on spectral reflectance independently of intensity
What is the maximum absorption wavelength of the 3 classes of cone?
S = blue = 420 M = green = 534 L = red = 564
Explain the principle of univariance
The same photoreceptor can be excited by different combinations of wavelength and intensity, so the brain cannot know the colour of a certain point of the visual image just from one photoreceptor.
E.g. if a receptor absorbed 10x as much energy per red photon as green, couldn’t tell if incident was 10 green photons or 1 red photon.
Definition protanopia
no red cones - X linked
Definition deuteranopia
Green dichromacy. No green cones - X linked
Definition tritanopia
No blue cones, rare
Definition anomalous trichromat
Unequal intragenic recombination resulting in formation of hybrid genes with different spectral sensitivity
Definition deuteranomaly
Anomalous trichromat of hybrid green-red gene, peak shifted away from yellow (reduced sensitivity to green light, requires more green to Rayleigh match)
most common form of colour blindness
Definition protanomaly
Anomalous trichromat of hybrid red-green gene, peak shifted towards yellow (reduced sensitivity to red light, requires more red to match in Rayleigh match)
Definition tritanopia
Loss of blue pigment gene. Rare.
What is in the outer nuclear layer?
Cell bodies of photoreceptors
What is in the outer plexiform layer?
Synapses between photoreceptors, bipolar cells and horizontal cells
What is in the inner nuclear layer?
Cell bodies of bipolar cells, horizontal cells, and amacrine cells
What is in the inner plexiform layer?
Synapses between bipolar cells, amacrine cells and ganglion cells
What is in the ganglion cell layer?
Cell bodies of ganglion cells
Where is there divergence and where is there convergence in the retina?
Fovea = divergence 1:3
Periphery convergence 16:1
Define receptive field
Receptive field is the area of a retina (or its projection in the visual field) from within which a neuron’s activity can be influenced by light
What type of synapse is between a cone and an off centre bipolar? What is the neurotransmitter and receptor?
sign conserving
Glu –> AMPA
What type of synapse between a cone and an on centre bipolar? NT and receptor?
sign inverting
Glu –> mGluR6
Discuss what is meant by parallel streams in the context of vision
Different aspects of the image are encoded by different cells with overlapping receptive fields - so message to brain = parallel neural images:
on v off, spatial detail, temporal detail, colour
What is the purkinje shift?
In scotopic range, peak sensitivity shifts from the average of red and green cones (560nm) to the peak sensitivity of rods (500nm)
What layer of V1 do LGN projections end on?
Spiny stellate neurons layer 4c. 4Calpha = magno, 4Cbeta = parvo
Definition ambylophobia
Eye problems in early life lead to a permanent defect in cortical function.
Definition achromatopsia
Cortical colour blindness caused by a lesion in V4
Lesions where can lead to prosopagnosia?
Infratemporal cortex
Inability to recognise faces
Lesions where can lead to akinetopsia?
V5/MT
Definition colour constancy
Perceived colour of objects remains the same irrespective of illumination
Describe the method of the increment threshold test
Green test spot on an orange background with an eccentrically-placed cross. Fixate on cross, so stim (which stimulates ble cones) falls on the parafoveal region. Stimulate threshold test spot intensity as a function of steady background intensity.
Describe the results of the increment threshold test
Over a wide range of intensities, the log of the threshold intensity increases linearly with the log of the background intensity.
At very low intensities, threshold is independent of background intensity - an absolute threshold. set by dark light.
When the background becomes very bright, the rod system saturates and so there is a very steep increase in threshold.
Weber’s law?
Threshold contrast of delta I/I is constant. Io = dark light K = weber's law ratio DeltaI/(I+Io) = k if I >>> Io then Delta I/Io = k.
Limits to sensitivity in vision?
Quantal fluctuations (have to differentiate, during dim steady light, any stimulus as distinct from the random fluctuations) Need to add more
What is the critical fusion frequency?
Frequency above which a flickering light is perceived as steady.
At low freq, depends on rods, best stim by blue-green. Rod responses are slow, giving rise to a low flicker fusion frequency (<15hHz)
At high intensities, cones take over, exhibit far better temporal resolution, up to 60Hz.
As mean intensity increases, the visual system becomes progressively better at following fast changes.
Size of fovea
1-1.5mm
size of macula
6mm
optic disk size
1.5mm = 5 degrees
acuity at fovea
0.5 min = 30 s = 2.5micrometers
eyeball diameter
17mm
thickness of retina near fovea
418 micrometres
thickness of retina peripherally
200 micrometers
thickness of axon layer near fovea
30 micrometers
thickness of axon layer near optic disk
200 micrometres
blind spot size
4-6 degrees
diameter of head of typical foveal cone
2.3 micrometers
1 degree corresponds to what?
1cm object 57cm away
300micrometres on the retina
