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Lectures 2 Vision I.
(all overhead numbers converted to slides 8/31/10 groh/teaching/psy182/slides_to_replace_overheads)
(also Lect_eye_retina.ppt)
We'll spend more time on vision than any other sensory system. Why?
-- humans are good at vision
we use this sense a lot. probably have a better visual system than
any other species. The same cannot be said for our other senses.
-- lots of research on vision=> large body of knowledge => model for
information processing in general
Begin with:
I. Light
II. Eye
(quick review of neurons & action potentials)
III. Neural pathways
Retina
LGN
Primary visual cortex
I. Light = electromagnetic radiation
Physics:
both wavelike & particle-like properties
wave => has frequency/wavelength
particle = photon = quantal unit
intensity = # of photons = amount of energy
Wavelength (Overhead figure 3.2)
EMR extends way beyond what is visible
we (& most vertebrates) can see EMR in range of sunlight that is not
filtered out by atmosphere or water - i.e. roughly tuned for this range
exception:
sun is not only source of EMR
IR = heat = emitted by warm blooded animals
= visible to snakes and armed forces
II. Eye
Light is only informative when it impinges upon a receptor for light => eye
Light sensitivity comes in multiple varieties
- light gathering
- image-forming = more advanced
arthropods
molluscs
vertebrates
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Eye types:
pinhole
- e.g. molluscs
small aperture => image on surface behind
always in focus but small amount of light
compound eye (3.4)
- arthropods
- multiple tubular units = ommatidia
=> image synthesized across eyes
= highly pixilated & coarse
- fixed focus on short distance
vertebrate eye
- adjustable focus, though not very close
Anatomy of vertebrate eye (Lab)
(Figure 3.5 overhead)
Retina where the action is
= photosensitive layer of cells + other neurons
= part of brain, developmentally-speaking
Lens focuses light on retina
Sclera
- outer coat, 1 mm thick, tough & white
Cornea
- extension of sclera at front of eye
- clear
Choroid
- inner layer
0.2 mm thick
contains blood vessels
heavily pigmented & dark => blocks out extraneous light
Iris
- extension of choroid in front
- disklike & pigmented
- contains muscles for adjusting the pupil
Pupil
- aperture that lets light through
- constricts & dilates based on ambient light levels
[Aside: Red eye in photography: pupil dilated, flash of light reflects off choroid,
solution: preliminary flash to dilate eye or move flash away from camera]
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Vitreous humor
- inside eye
- jelly like
doesn't turn over => debris accumulates => floaters
Aqueous humor
fluid between cornea and lens
Muscles
Intraocular muscles
muscles of the iris => adjust pupil
muscles of the lens => adjust lens
= ciliary muscle
Extraocular muscles (3.15)
Attach to outside of eye (sclera) & move the whole eyeball
So what happens?
Light refracts when it leaves air and enters fluid
refracted by cornea & lens to focus on retina
bulk of refraction = air => cornea,
lens refraction = smaller, but adjustable
Process of adjusting lens = Accommodation (3.17)
- modification of lens shape (mammals)
flatter => less refraction => longer focal length
rounder => more refraction => shorter focal length
Fish: more like cameras. Lens shape = fixed, move lens closer to or
farther away from retina.
Refraction errors: (3.19)
Myopia - nearsighted
axial - eye too long (or lens too round)
refreactive - lens/cornea = abnormal
Hyperopia - farsighted
Presbyopia
reduced accommodation
age related
lens gets hard & stiff
ciliary muscle can't adjust it as well
Astigmatism
cornea & lens not spherical & symmetrical
Fig 3.22, 3.23
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3.22: Astigmatism chart. To an astigmatic viewer, some sets of lines
appear blurred relative to others.