light, eye and brain readings

• Cornea


  Main lens; focuses light on to retina (75% of eye's focusing power from cornea)
• Lens


  • At least one curved surface
  • Made of material through which light travels slower than air (for refraction)
• Refraction


  1. Light hits a different medium (air ⇒ water)

  2. It bends at the edge between the two media

  3. Travels slower in water
• Fish eyes


  Bulge more for refraction (eyes made of water)
• Iris and pupil


  1. In too much light, iris contracts ⇒ pupil shrinks

  2. Dim conditions relax the iris to make the pupil bigger
• Pupil size


  • Relaxed pupil x16 bigger than constricted
  • Capable range of luminance (blinding light - barely visible): 10,000,000,000 : 1
• Pupils also change size when excited (eg: sexual attraction)
• Constricted pupils


  Have more depth of focus
• Lens


  • Less focusing power than cornea
  • Adjusts according to luminance
  • Held between zonules of Zinn
• Accommodation


  1. Cornea must be appropriately curved/eyeball must be of right length for light to fall on retina

  2. Contraction (push; more activity) of ciliary muscles relaxes zoZ ⇒ fatter lens (nearby; this light is diverged + needs bending for focus)

  3. Relaxation of ciliary muscles ⇒ stretched/flatter lens (far away; this light doesn't need to be bent for focus)
• Emmetropic vision


  Normal vision where light is focused properly on the retina
• Myopia


  • Short-sightedness
  • Distant objects are blurry bc light is bent/focused too much (in front of retina)
  • Eyeball too long/focal length too short/strong lens
  • Needs diverging/concave/weakener lens to stretch light
• Hypermetropia


  • Far-sightedness
  • Focal/light length too long
  • Close objects fall behind the retina bc eyeball too short/weak lens
  • Needs converging/convex lens/strengthener to strongly focus light
• Presbyopia→lens loses elasticity with age ⇒ ciliary muscle relaxation does not contract the lens ⇒ near-point moves away
• Near-point


  The closest point we can focus on
• Retina


  • Where visual processing starts
  • Part of CNS
• Retinal cell order


  • Ganglion cell ⇒ amacrine cell ⇒ bipolar cell ⇒ horizontal cell ⇒ photoreceptor cell
• Photopigments on outer part of photoreceptors
• Inverse retina has blind spot where nerves exit
• Brain fills in blind spot with surrounding info ⇒ bad news for glaucoma bc it obscures diagnosis until too late
• Scotoma


  Extra blind spot from damage/retinal displacement; person is unaware of scotoma
• Rods


  • Contain rhodopsin (aka visual purple; it absorbs green and reflects red and blue)
  • Sensitive to dim conditions (eg: night)
  • Going from bright to dark conditions makes it harder to see bc rods are bleached
• Scotopic vision


  Rod only vision
• Rods are most sensitive to green light
• Purkinje shift


  Phenomenon where blue/green appears brighter than red in low light conditions (eg:WWII nighttime pilots briefed in red rooms to not bleach their rods)
• Mesopic vision


  Rod and cone vision (medium light conditions)
• Photopic vision


  Cone only vision (bc too bright for rods)
• Cones concentrated in central fovea with no rods (they are most populous 12 - 15 degrees into the periphery (why faint stars disappear on the fovea and visible in the periphery)
• 0 blue cones in central fovea; people have different red and green cone populations
• No rods or blue cones in fovea
• Blind spot/optic disc (where ganglions leave eye) is located 12 - 15 degrees into nasal retina
• Congenitally colour-blind people


  Have a problem with red or green cones (missing or weaker)
• Magnocellular (M) cells


  Care about movement and flicker
• Parvocellular (P) cells


  Distinguish red (excitatory) and green (inhibitory) cone signals
• Both M and P cells are hard to study as intermingled in retina (studied in LGN) + send signals to specific parts of LGN
• Optic nerve


  1. Optic chiasm is convergence point for both optic nerves where the nasal retinal fibers decussate

  2. Nerves past chiasm is optic tract
• Hemianopia


  Deficit in optic tract makes you blind to half the visual field (one eye's temporal, other eye's nasal)
• LGN (lateral geniculate nucleus)


  • Optic tract's first meeting point
  • Relay point to VC
  • Knee-like nucleus (genus = knee ⇒ geniculate)
  • Primate LGNs are six-layered; ganglion cells per eye terminate in 3 layers ⇒ info about each eye is separated
  • Layer 1 - 2 has big cells (magnocellular layers for magnocellular ganglions) and the rest have smaller cells (parvocellular layers for parvocellular ganglions)
  • There's 2 M layers for decussation ⇒ layer 1 receives contralateral input; layer 2 receives ipsilateral input
  • Parvocellular: ipsilateral L3/5 + contralateral L4/6
  • Retinotopic mapping→Adjacent retinal ganglions will project to adjacent LGN cells
  • M and P LGN and retinal cells are similar
• • Schiller et al., 1990 - Animals trained to fixate on a central point ⇒ move eyes to 'odd one out'
  • Tiny lesions in either P or M LGN layer
  • P LGN lesions destroy ability to detect targets of colour, fine texture, subtle shape changes
  • M LGN lesions can't detect moving/flickering targets
  • Targets with noticeable shape change or stereopsis (depth from two eyes) were unaffected by lesions ⇒ both divisions handle this?
• Koniocellular (K) cells


  • Means 'sand-like'
  • As populous as M cells (1 lakh) in LGN
  • Input from special ganglion which receives blue cone signals ⇒ K pathway about blue-yellow; P pathway is green-red