375: Vision Pt1

1. Visual Illusions
1. Why study visual illusions?
1. They tell us about some of the assumptions that the visual system is making
2. Ill-posed Problem
1. 2D on the back of your eyeball but 3D in your brain.  How does that happen?
1. Visual shortcuts and illusions help us understand those shortcuts
2. The Eye
1. The retina
2. The cornea and the lens do all the focusing on your eye
1. Cornea does about 80% of focusing
2. Lens does about 20%
3. Fovea
3. The Retina
1. Light has to pass through several layers before it gets to photoreceptors at the back of the eye.  Kindof a backwards way of setting it up.
2. Two types of Photoreceptors
1. Cones
1. Color visions
2. Daylight vision
3. Fine detail
4. 3 types of cones
1. Short - blue
2. Medium - yellow
3. Long - red
2. Rods
1. low-light vision
2. More sensitive to Motion (but not the only thing giving you motion)
3. More sensitive but they have less acuity
4. Fovea
1. No rods in the fovea, there are only cones in the fovea
2. This means that you are not very sensitive in the fovea
1. So to see something dim you can look to the side of the object, like a dim star, and you will see it better
3. Rods are in the periphery.  Why is that a good thing?
1. So if something is going to hit us that we are not paying attention to then we can move.  that is why it's important to be sensitive in the periphery to motion.
5. Retina
1. Photoreceptors project to bipolar cells which project to ganglion cells
2. Blindspot
1. Area where there are no ganglion cells so no photoreceptors.  But you don't see a whole in your vision because your brain makes assumptions about the surroundings.
1. It assumes that what is surrounding the blind spot will continue.  Color constancy.
2. Why don't glaucoma patients have a similar brain assumption as the blindspot?
1. You have always had your blindspot so developmentally you could assume that your brain has gotten accustomed to it.  Glaucoma is new.
2. Scotoma
3. If cones see colors and rods do not, then do you see color when you are getting a glass of water at night?
1. Yes, but things will look blue because rods are the most sensitive at around 500 nm
2. You also have horrible color vision in the periphery but if you already know the color then your brain will make an assumption about the color.
4. The image on our retina is upside down and backwards.  Why don't we see the world upside down and backwards?  
1. Because the image has always been that way and our brain is used to it.
6. Retinal Ganglion Projections
1. Optic nerve
2. Optic chiasm
3. Optic tract
4. LGN
1. In the thalamus
2. projects to the primary visual cortex
3. The left part of the brain receives images from the right side of space and vice versa
7. Primary Visual Cortex
1. Located in the occipital lobe
2. What does it do?
1. The visual system must solve a number of problems
2. Chief among these is the impoverished stimulus
1. Impoverished Stimulus
1. This is the lack of information that you receive but your brain will make assumptions to make up for it.  Like seeing 2D but perceiving 3D
3. Another problem is indeterminacies in the visual input
3. Low-level vision: we assume that we can't get much information out of this array of intensity values. 
4. High-level vision: Taking the output of the low-level processes and transforming it to get objects and their properties
5. First thing your visual system does is find the edges
1. They are important because edges define boundaries
2. Edges are invariant to lighting conditions
3. How do you find edges?
1. You visual system takes individual columns and finds the average.  If the average is different then it is probably an edge.  Does the same thing for rows and diagonals.
2. Well then why don't you see something like a hat, with many edges, as a bunch of different objects?
1. It's because we also take the average of larger objects.
2. You assess these averages at more than one scale.