Unit 2 Study Guide Low Level Vision Lecture 1The eye is very similar to a CAMERA because…● Both have lenses that focus light● Both can adapt to overall light levels ● Both invert the original light source and compensate afterwards BUT… ● The eye does NOT passively receive light like a camera● The eye ACTIVELY processes like before it leaves the eye THE RETINA…>This is where all the magic happens (transduction)>Light comes in from left to right >But it is processed from right to left >First light travels through rods and cones, then bipolar cells, then the ganglion cells. ON/OFF Bipolar cells● ON Bipolar Cells- if the rods/cones sense light these bipolar cells increase firing rate (They like light)● OFF Bipolar Cells- if the rods/cones sense light these bipolar cells decrease firing rate (They dislike light) Ganglion Cells ● ON/OFF Bipolar cells connect to the ganglion cells ● ON Center Ganglion Cells- The center is excitatory (ON) ON bipolar cells are in the center and the surround is inhibitory (OFF) OFF bipolar cells are in the surround ● OFF Center Ganglion Cell- The center is inhibitory (OFF) OFF bipolar cells are in the center and the surround is excitatory (ON) ON bipolar cells are in the surround● ON/OFF Center Ganglion Cells respond differently to different light patterns, pretty much light is (+) and no light is (-) so when light is hitting a spot in the ganglion cell that is (+) it gets excited (Firing rate increases). When light hits the (-) spots in the ganglion cell it gets inhibited (Firing rate decreases). When there is no light the firing rate stays thesame.● If it was a 3D distribution it would look like a sombrero, positive is high and negative is low. ● Ganglion Cells are tuned to different spatial frequencies (The black and white barcodes) ● The number Grating Cycles in a given unit of space (one set of black and white)● They are commonly studied with Garber patches● Low Spatial Frequency (not many bars) ● High Spatial Frequency (a lot of bars)● They are tuned to different spatial frequencies, so if the (+) part of the sombrero is in a light part of the bar it becomes excited (fires at a faster rate). If the (-) part of the sombrero is in the dark part of the bar it becomes excited (fires at a faster rate). But if the (+) or (-) parts of the sombrero are in the opposite color bar they become inhibited and their rate of firing decreases. Contrast ● Contrast is how different the bars are from each other in color or luminance ● Contrast is independent of spatial frequency● High contrast is like black and white, Low contrast is like grey and darker grey ● There is High, Medium and Low Contrast ● The higher the contrast the more the ganglion cell will fire (the more excited the faster the rate of firing)● The retina is covered in these ganglion cellsAfter the eye…● The left side of your brain represents the right side of your visual field● The right side of your brain represents the left side of your visual field● Meaning… its Contralateral (Opposite sides of the brain) ● Ipsilateral means same side Lateral Geniculate Nucleus (LGN)● It is organized very specifically with an underlying organization structure● It receives input from 2 eyes and the signals are separated ● It has 6 layers that alternate between Ipsilateral and Contralateral● The left and right LGN are opposite in their order of alternating 6 layers ● It then moves to the Primary Visual Cortex● The whole visual field is not represented equally -> Cortical magnification● Fovea (center of the eye) has a lot of cortical representation Primary Visual Cortex ● V1 Neurons do not have the same tuning functions as bipolar ganglion cells ● The primary visual cortex detects simple visual features, like line and edge orientation ● With the line and edge orientation correlated with the firing rate, vertical lines excite the V1 neurons so the rate of firing increases. Horizontal lines inhibit V1 neuronal activity, so the firing rate decreases and everywhere in between like diagonal lines is pretty self explanatory. Diagonal lines don’t do much for V1 neurons, there is a medium firing rate. (When thinking about firing rate vs. orientation of line think of the bell curve graphwhere the vertical line is the median, so the highest point and the rest just radiates out likea normal bell curve ending with the horizontal line.)Object Recognition I Lecture 2Inverse optics Problem- given the retinal image (A), what is the state of the world (B). I think this pertains to us having to make some guesses when it comes to our surroundings because not everything is clear. ● Humans are very good at object recognition, it is very hard to mimic even with artificial intelligence. That's why when theres some type of security check online they ask you to identify certain things because it cannot be done by other beings or things. What happens after V1… Extrastriate Cortex!● After V1 there are regions that border V1, they are called the Extrastriate Cortex.● The examples of the more specific areas in this region are V2, V3, ,V4, and MT. ● As you progress through the areas they become more and more specifically tuned to complex features.● The wiring of the post V1 areas are pretty complicated. ● Macaque monkeys have regions of the visual cortex that are comparable/similar to humans. That's the gist, i don't remember the exact word that he used but the point is theyare similar and that's why we use them to further research. (rewarding them with food like skittles) ● The reason why we use Macaques is because other animals that we commonly use for research do not have homologous brain structures. For example a rats visual cortex is laidout much differently. Humans Visual cortex has 6 layers and their eyes face forward. Ratsare very different. Neurons in V2 Type of Stimuli● V1- SIMPLE FEATURES only caring about line orientation, but not…● V2- EDGE OWNERSHIP● If an edge is clearly owned by a color then the neurons in V2 get excited and their firing rate increases. But when it is no longer clear which color own the edge the V2 neurons donot respond. The stimulus can be altered to change and improve the line ownership to excite the V2 neuron and increase the firing rate. ● When thinking of good vs. bad ownership think about closed vs. open