• eyes sit in the frontal orbital cortex in the skull
  • information form the eye is primarily processed in the back end of the cortex
  • some information going past the optical chiasm
  • which is then processed in two path ways going along to the front of the brain
    • top path (dorsal) for “where” processing
    • bottom path (ventral) of “what” processing

Distance Vision

Anatomy of the Human Eye

refractive indices

  • determines the amount of bending of the light rays
    • vacuum (theoretical) = 1.00
    • air = 1.00
    • water = 1.33 ≈ cornea
    • glass = 1.42 ≈ lens

light in the eye

  • light enters the eye through the cornea
  • then a small fluid filled chamber
  • then the lens
  • then a larger fluid filled chamber called vitreous humor
  • then on the retina
  • a vertical mirror image forms on the retina
  • cornea is the coarse focus
  • lens is the fine focus

object distance

  • different mechanisms are used for near and far objects
  • objects at lens infinity send parallel rays to the cornea
  • objects in the near vision range send light rays that are at an angle
  • the lens changes shape to accommodate the different types of light

near vision and the near triad

  • convergence (voluntary motor)
  • pupillary constriction (parasympathetic)
  • rounded lens (parasympathetic)

pupil size

  • to change focus from something far to something near
    • the eyes converge and
    • pupils constricts
  • if pupil is narrow, cone of light is small
    • sharp focus for near objects
  • if pupil is wide open, cone of light is large
    • blurred focus for near objects

lens shape

  • a rounded lens has a higher refractive index than a flatter lens

  • results in light from near objects being focussed onto photoreceptors

  • flat shape - for far vision

  • rounded shape - for near vision

  • in 40s, the lens stiffens
    • it’s ability to change shape reduces
    • the change in shape takes a longer time
    • this is why people need reading glasses when they age

cranial nerve III

  • the near triad depends on this nerve
  • it is called the ocular motor nerve
  • functions
    • moves the eyes toward the nose (eye adduction)
    • constricts the pupil
    • rounds up the eyes
  • sits in a place that is particularly vulnerable to pressure build up in the brain
    • the uncus part of the side lobes herniates
    • pushes down on the motor nerve
    • the near triad function is lost as a result
    • considered a medical emergency

length of the eye

  • the image has to focus accurately and precisely at the back of the retina
  • myopia:
    • the eyes are long,
    • the image focus is in front of the retina, in the eye ball
    • short sightedness, can’t see far
  • hyperyopia:
    • the eyes are too small, and the image is focus is in the back of retina, out of the eyeball
    • can’t see close,

correction

  • in both myopia and hyperopia, the focus is not on the back of the retina
  • the process of making the eye just the right length is called emmetropization

  • the person with the perfect length of eye is said to have emmetropia

  • in myopia, the near object is focussed fine
    • it exists in epidemic proportions in asia
    • vision is corrected by glasses, but there is still risk for retinal detachment

  • in hyperopia, the far object is focussed fine

  • emmetropization: is a developmental process that ensures that the length of the eye is appropriate for far objects
    • eyes lengthen as we grow
  • emmetropia, myopia, hyperopia, and astigmatism

retina

  • the retina has several layers of cells

  • retinal ganglion cells (the outermost cells)

  • eventually reach the photoreceptor cells
    • these turn photons into neural energy i.e. electricity
    • they have a cell body and a ball like outer segment
  • the light has to be focussed on these outer segment
    • photo-transduction occurs here, i.e. light energy is converted into electrical energy
    • rhodopsin molecules absorb a photon
    • change their membrane potential based on the received photon
  • a critical piece of rhodopsin is vitamin A
    • there are a lot of people in the world that don’t get vitamin A to fuel their eyes
  • one of the peculiarities of the light pathway in the retinal cell layers is the rhodopsin layer works only with the retinal pigmented epithelium (RPE)
    • RPEs are dedicated group of glial cells for photoreceptors
    • without the RPEs, the photo-receptors does not work
    • this happens in the case of retinal detachment
  • photo receptors get consumed in the process of photo-transduction
  • the RPEs play a vital role in renewing rhodopsin molecules after they are consumed
    • there is a marriage between the RPEs and photoreceptors that enables vision

rods and cones

  • photoreceptors are of two types
    • rods
    • cones
  • human vision can see over a large range of brightness magnitude
    • from very dark conditions of a new moon (scotopic)
    • to the bright sunlight of the noon (photopic)
    • and everything in between (mesopic)
  • this range spans 12 log magnitudes of brightness
    • this is achieved by using rods and cones
    • rods are for dim light conditions (scotopic)
    • cones are for bright conditions (photopic)
    • there is an overlap for mesopic conditions in both rods and cones

structure

  • rods and cones have different structures, which is reflected by their names
  • they both have an outer segment
    • which contains rhodopsin molecules on the inner surface
    • rods have a paddle like outer segment
    • cones have a bisected chirstmas tree like outer segment
  • the light is converted into neural-electrical energy in the rhodopsin molecules
    • they line the membranes within the cell
    • they are metabotropic receptors, i.e. no channel for molecule transfer
    • they change an enzyme which travels to the edge of the cell to open or close an ion channel
    • this is slower than directly opening a channel
  • vision is a slow process
    • in the physical world what we look at doesn’t change on less than 2ms time scale
    • this is not particularly bad

difference

  • rods are extremely sensitive to light
    • there is one (1) type of rod
  • cones are less sensitive compared to rods
    • in humans there are three (3) types of cones
    • each has a preferred wavelength of light that it responds maximally to
    • this enables colors to be perceived
  • cones enable color perception, rods do not
    • under photopic conditions, we see vibrant colors
    • under mesopic conditions, we see muted colors
  • rods gather light from a larger field of the visual domain
    • rods are for low light and broad picture vision
  • cones gather light from a small field
    • cones are for focussed, bright light vision with fine details

central vision

  • the point of fixation in the field of vision
  • the center of the eye ball is called fovea
    • cross section of this looks like a pit
    • this is the point of fixation (foveation)
  • macula (area centralis) is wider
    • contains the fovea
    • along with the fovea, macula is the for high acuity vision
    • this is densely populated with cones
  • in the pit area, there are only cones
    • no rods at all
    • density of cones peaks at the pit of fovea
  • the concentration of the rods peaks at 15º from the point of fixation
    • this implies that for star gazing at night,
    • it helps to better see the stars 15º off the actual star for clear sighting
  • the act of fixation (foveation) is voluntary
    • the mechanism of focussing in involuntary

optic disc

  • all the ganglial cells send their axons to the optic disc
  • then the optic nerve takes it to the thalamus
  • the optic disc creates a blind spot in the vision
  • when both eyes are open, the other eye supplies the information for the blind spot of a given eye
  • even with one eye open, the blind spot is not visible
    • because the spot is very small
    • it gets filled in

macular degeneration

  • age related condition
  • wet/dry type
  • central vision gets distorted
  • peripheral vision remains the same

color vision

  • does light have color?
    • light has no color
    • color is constructed by the human brain
    • light only has wavelength
    • the color is perceived
  • color is entirely a brain constructed concept
    • each human perceives colors a little differently
    • higher wavelengths are interpreted as reds
    • lower wavelengths are interpreted as violets and blues
    • in between is green, yellow and orange
  • three types of cones
    • type 1 - short wavelength cone (S)
      • peaks at blues
    • type 2 - medium wavelength cone (M)
      • peaks at green
    • type 3 - long wavelength cone (L)
      • peaks at yellow-green
  • retina itself makes three calculations from the information from the three types of cones
    • channel #1 mixes M + L channels: called the luminance channel
    • channel #2 (S-(M+L)):
    • channel #3 (L-M):
  • 5%-10% of men have color blindness
    • the gene for the M and L cones sit side by side on the X chromosome
  • four types of color blindness:
    • no L cones
    • no M cones
    • L shift to M
    • M shift to L

readings