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Visual System

Vision/light numbers 1 e8 photoreceptors in each retina, only 15000 IHC in cochlea, 390 nm (violet) -> 700 nm (red)
What parts of the brain are involved in vision processing? 25% of cortex devoted to visual information analysis (entire occipital lobe, parts of parietal/temporal cortex)
What patterns of light can we recognise? Intensity -> overall outline, wavelength -> colour, variation in space -> patterns, variation in time -> proprioception
What is illuminance, luminance and albedo? Illuminance = light from source (lux), luminance = object-reflected light (cd/m2), albedo = reflectance
What intensities of light are there? Visual threshold -> saturation (environment intensity varies by 1 e15
What environmental intensities are there? Photopic -> daylight/bright light (retinal cones), mesopic -> normal vision (cones/rods), scotopic -> dim light vision (rods)
Contrast (relative intensity) equation Contrast = delta I / I Intensity increment/decrement / mean background illuminance Independent of illuminance/luminance increase
Absolute difference Difference between luminances (reflecting areas)
What do object reflectances give us? Pattern of contrasts independent of ambient illumination -> visual systems extract contrast pattern
Retinal image numbers 1 degree = 60 arc mins = 3600 arc seconds -> 300 micrometers on retina 1 radian = 57 degrees
What does diffraction limit? Optical resolution -> lightwaves spread out after passing through aperture (pupil) -> pointspread function
Diffraction equation Angular diameter d = 1.22 lambda/D (pupil aperture) Smaller aperture -> larger diffraction limit
What is the physical limit of eye resolution? Green light (500 nm), smallest pupil diamter (D) = 2mm, angular diameter (d) = 1 arc min
What lens aberrations are there? Spherical -> edge rays more refracted -> multiple foci -> larger pointspread, chromatic -> different wavelengths w/ different refractive index -> multiple foci -> larger pointspread, glare -> optical media particles scatter light -> reduce image contrast
What images would small pupils give you? Modest spherical/chromatic aberration contribution, dim image, pointspread function approaches diffraction limit
What images would dilated pupils give you? Off-axis rays contribute to image -> significant aberrations -> broaden pointspread function despite reduced diffraction
What refractive errors are there? Emmetropia -> sharply focused object at infinity (perfect), ametropia -> distant points unfocused, myopia -> short sight, hypermetropia -> long sight
What is myopia? 20% population -> lens too powerful (larger refraction)/long eyeball (retina too long for foci), corrected w/ diverging lens, correlated w/ education, predisposition for retinal detachment/degeneration/glaucoma, future requirement for bifocal (presbyopia)
What is presbyopia? Gradual loss in ability to focus on near objects (accomodate) due to loss of lens elasticity -> need extra converging lens
What is hypermetropia? 30% population -> lens too thin (smaller refraction)/short eyeball (foci behind retina), corrected w/ converging lens
Nyquist limit Distance 2x as fine as width of pointspread function -> contribute to photoreceptor packing density/size -> pointspread function = 1 arc minute -> fovea -> adj cones separated by 0.5 arc minute (2.3 micrometer)
What are the real optical structures? Cornea, lens
What are the accessory optical structures? Eyebrow, eyelids, lacrimal apparatus, conjunctiva
Cornea characteristics 650 micrometer layer of transparent collagen fibrils (stroma - prevent scatter), enclosed btwn epi/endothelium, highest refractive power (48 dioptres) -> water increases refractive index and and cancels out corneal refraction power
Lens characteristics Long ribbon-like cells packed w/ transparent crystallin protein (high refractive index), cells added from periphery (higher refractive index correcting spherical aberration), lens absorbs UV strongly protectign retina
What are cataracts? Lens clouding with age due to xcs UV exposure
What is optical power? Power/strength expressed in dioptres (1/focal length) -> high power = lower focal length = more powerful lens refraction
What contributes to eye transparency? Avascular cornea/lens (aqueous humour supplies metabolites)
Aqueous humour characteristics Secreted by ciliary body epithelium, drained by trabecular meshwork/Canal of Schlemm
What is glaucoma? Reduction in aqueous humour outflow rate due to damaged optic nerve (increased intraocular Pa) -> determined w/ puff of air at eye and measuring lens deflection
What is accommodation? Lens changes focal length -> allows focusing on objects at different distances -> simultaneous pupil constriction (improve focus depth) and eye convergence (new target fixation - reflex)
Neurophysiology of accommodation Suspensory ligaments/zonule controled by circular ciliary muscle (PNS CN III) -> contracted ciliary muscle -> slack zonules of Zinn -> lens thick -> increase refracting power -> decrease focal length -> near objects sharply focused on retina
Pupil diameter control Iris has 2 antagonistic smooth muscles under ANS -> sphincter (PNS) -> pupil constriction (blocked by atropine), dilator (SNS) -> pupil dilation
What is Argyll-Robertson pupil? Neurosyphilis -> pupil does not have pupillary light reflex but can accommodate
What retinal layer are photoreceptors found? Furthest from incident light -> remaining layers have visual interneurons -> glare, Muller cells (retinal glial cells) have optical waveguids aiding light transmission through vitreous humour
Fovea characteristics 1.5 mm/5 degree diameter -> interneurons displaced to one side reducing light scattering, cone density increases dramatically (rod expense) -> containe blue absorbing macular pigment -> absorb UV wavelengths protectign eye/reduce chromatic aberration
Foveola characteristics Central 260 micrometer (1 degree) of fovea w/ highest acuity -> avascular (minimise scattering), completely rod free -> minimum cone spacing of 0.5 arc min (2.3 micrometer) at centre
Parafoveal region characteristics Lateral 20 degrees either side of fovea -> peak rod density (same as foveal cones) -> signals summated/pooled reducing spatial acuity -> most sensitive vision under mesopic/scotopic condition
Blind spot characteristics 5 degrees at optic disc where optic nerve exits retina
Optic nerve characteristics Non-myelinated retinal fibres -> myelinated optic nerve fibres, CSF continuous w/ brain CSF -> raised intracranial Pa -> swollen optic disc (papilloedema)
Optical imaging methods Ophthalmoscope, slit lamp microscopyp w/ Volk lens, optical coherence tomography, scanning laser ophthalmoscope
Optical coherence tomography Gives vertical section in intact eye using reflected light to visualise retinal layers -> can detect central serous chorioretinopathy (fluid leakage under retina via epithelial pigment detachment)
Scanning laser ophthalmoscope Ellipsoidal mirror allows viewing of wider retinal area (200 degrees)
Created by: vykleung
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