2012-02-01
Stanford
Vision
Fundamentals
Organization of visual system
eye is first part, retina
layer of neurons at the back of the eye
very complicated structure, extremely complicated
layer of photo receptors
output cells ganglion cells, go out to the optic nerve
interesting things: different types, rods and cones
retina is inside out: the light passes through the output cells
inverted
probably not evolutionarily caused, blood flow at the back, because they need more energy
macular degeneration
visual acuity
touch people on the back they can’t tell the different places
density of the photoreceptors shows a strange pattern
fovea is an indentation with more photoreceptors
fovea all cones
when we want to focus on something we look with the fovea
rods, low levels of light
cones, greater levels
density of rods in the fovea is terrible, which is why you can’t see well in the dark when looking specifically at it
eyes are very different across species
physically different shapes, rods and cones
rods, single photopigment, no color
cones, three types, different photopigments, color
you need three different dials of colors to allow you to see all the colors
hue (color)
saturation (amount)
brightness (light)
we have there different cones
perception the same as long as cones are equally activated
all other things being equal
you can change the spectrum in the same amount, the same total level of activity across the three
sensitivity curves
600nm wavelength, near yellow
retinal ganglion cells, first step of processing, first to thalamus then to cortex
center neuron architechure, spot of light in the middle makes it go
long reactions back to the thalamus
cross at the optic chasm
half the axons cross
ipso-lateral, same side
different sides of the vidual fields
information of each eye goes to the other hemisphere but only half the field
“I don’t think there’s a good answer.”
“Very well studied.”
layered organization
different sized neurons with very different functions
lateral geniculate nucleus (LGN)
magnocellular layers (2 of them) – motion, big neurons
parvocellular layers (4) – objects, small precise neurons
processing takes place, not just a relay
modulation by attention
information projects out to the cortex
many different regions of cortex
“we are very visual animals.”
each has a complete topographic map of visual field
primary (vi, or striate) “beautiful striae”
v2-v2, extra-striate
unfold the visual cortex
“brain inflated”
organization within v1 is the most well understood
foveal is at the very back, periphery spreads out
simple cells best to lines and edges, summed activity of a bunch of ganglion cells
complex cells, preferred orientiation, not location dependent, some movement dependent
hyper complex cells, lines of specific length, pictures/shapes of cars
neurons invade space with phantom limbs
competition for neural real estate
“It’s all in the name of science.”
critical period where if your eye is sewn closed it will never have the column dominated
contralateral crossing
ocular dominance columns
maybe because of depth perception, but other animals can perceive and they don’t have these columns
“Don’t memorize this, this is just true.”
cortical connections can deduce organization of visual cortex
“This takes a long time to do.”
“You end up with this beautiful hierarchy of the visual field.”
“How you perceive things in neuronal terms is really complicated.”
for perception, you do psychophysics
“This historical note is really important.”
veridical representation like a camera
“We see what is evolutionarily advantageous to see.”
“There are lots of consequences to this.”
“We have a lot of theories about how we perceive light, none really hold up.”
opponency
no reddish green or bluish yellow
the V4 critical for color, high order visual cortex
cerebral achromotopsia, hemorrhage
“Complicated, we don’t understand, no good theories.”
“Exactly how we perceive color is not known.”
brightness is a subjective percept
we know because of simple visual illusions
“Perhaps they are subtracting off the background”, brightness is not just the amount of light
“We can make visual illusions that makes this theory appear false.”
“The current theory is probably learning from experience. How, where, is not really known.”
“We intrepret reality as far as what is advantageous for us.”
“You can’t help it, it’s very automatic and developed through lots and lots of experience.”