Vision and Neurons

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.”