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Chapter 3

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Laurie Manwell

 focusing light onto the retina o light - the stimulus for vision  vision based on visible light  band of energies within electromagnetic spectrum  electromagnetic spectrum  continuum of electromagnetic energy produced by electric charges and radiated as waves  can be described by wavelength  distance between peaks of the electromagnetic waves  measured in nanometers  visible light to humans is between 400 to 700 nanometers  also associated by the colours in a wavelength o the eye  light refracted from objects enter through the pupil and focused on cornea and lens to form sharp images on retina  retina has two receptors for vision  rods and cones  contain light-sensitive chemicals called visual pigments  reactive to light and triggers electrical signals  signals go through optic nerve and sends it to the brain o light is focused by the eye  cornea accounts for 80% of the eye's focusing power  fixed in place and can't adjust focusing power  lens provides the remaining 20% of focusing power  can change shape to adjust focus for stimuli's distance  accommodation occurs to help the ciliary muscles increase or decrease curvature to create sharp image on retina o demonstration  near point  distance where your lens can't adjust to bring close objects into focus  distance of near point increases as age increases  presbyopia  myopia / near-sightedness  inability to see distant objects clearly  can be caused by two factors  refractive myopia  cornea and/or lens bends light too much  axial myopia  eyeball is too long  faraway objects are not focused clearly so they look blurred  far point  distance which spot of light becomes focused on retina  hyperopia / far sightedness  can see distant objects clearly, but not nearby objects clearly  LASIK (laser-assisted in situkeratomileusis)  surgery that involves sculpting cornea with excimer laser  if surgery is successful, it will result in good vision  transforming light into electricity o visual receptors and transduction  transduction carried out by receptors  neurons specialized for receiving external stimuli  rods  key part is outer segment  light acts to create electricity on this  contains stacks of discs  discs contain thousands of visual pigment molecules  molecules are long strands of protein (opsin)  each molecule only contains one molecule called retinal  retinal - crucial for transduction since it's the only part that's sensitive to light  when retinal is hit by a photon of light, it changes shape  process known as isomerization  triggers transformation of light into electricity in receptors o how does transduction work?  isomerization of visual pigment is a chemical process  can be studied by looking at the chemistry of visual pigments  can also measure it by doing psychophyiscal experiments  physiology of transduction  only one photon needed to activate a rod receptor  process known as enzyme cascade  sequence of reactions triggered by activated visual pigment molecule  pigments and perception o distribution of rods and cones  rods and cones interspersed in retina, not evenly  ratio of rods and cones depends on location on retina  fovea only contains 1% of all cones in retina (about 50,000)  fovea is all cones, when we look directly at object, image falls on fovea  peripheral retina (rest of the retina outside fovea) contains rods and cones  about 120 million rods, 6 million cones  most of retina's receptors are located here  two degenerative eye diseases that affect that retina  macular degeneration  common in older people  destroys cone-rich fovea, leaving a blind spot  retinitis pigmentosa  degeneration of retina passed from one generation to the next  first attacks peripheral rod receptors then foveal cones in extreme cases  creates tunnel vision then ultimately complete blindness  there is a blind spot normally where the optic nerve leaves the eye  blind spot is hard to detect  located off to the side of our visual field  mechanism in our brain fills up the spot where the blind spot missed  creates a perception that matches surrounding pattern o dark adaptation of the rods and cones  dark adaptation  process which causes eye to increase its sensitivity in the dark  two stages of dark adaption  initial rapid stage and a later, slower stage  known as dark adaption curve  as dark adaption proceeds, light sensitivity increases  increases rapidly in first 3-4 minutes, begins increasing at about 7-10 mins and continues to do so 20-30 mins after light source is extinguished  dark-adapted sensitivity is about 100,000 times greater than light- adapted sensitivity  measuring cone adaptation  cones only help with the first stage of the dark adaption curve  cones are only sensitive to light in the beginning  only takes 3-4 mi
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