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Visual System
| Term | Definition |
|---|---|
| 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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