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Matthias Niemeier

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Lec 3 SAQ: the retina processes information similar to a computer. Which neurons in which pathways play a role? Describe some of the functions of the pathways. In which important way does retinal information differ from that of a usual computer? - ( The ganglion cells differentiate borders to emphasize the difference of shapes, depending on which part of the cell is being stimulated) - Lateral pathways and vertical pathways o Vertical starts at photoreceptors to ganglion to bipolar cells to other parts of the cell o Lateral pathways include amacrine and horizontal cells - Functions: - Vertical pathway passes on the information from the retina to other parts of the cell. Also convergence of information which simplifies information - Lateral pathway involved in lateral inhibition; interconnect neighbouring bipolar cells laterally in a way that inhibits - Ex// one photoreceptor stimulated by one photon of light will inhibit the periphery which gives rise to the center-surround receptive fields of ganglion cells - Lateral inhibition is what lateral pathway is for, which leads to the center-surrounds - How does retinal information differ from that of a usual computer: o Major difference between a retina and a computer: o Computers have a cpu : one mechanism that does the computations ; computational power of the retina is much stronger than that of computers o Main difference between brain and computer: computers are a pea brain compared to humans Spatial Vision - Having an eye with a pinhole mechanism and a lens can focus light through spatial vision Natural scene statistics: why care about gratings? - Gutenbergs printing press metaphor o Gutenberg invented a printing press with movable types (1439) Put blocks in a frame and cover with ink then cover with paper Within a few minutes, can produce many pages o Before this, Camrbidge owned only 122 books (1422) o greatest invention in 1000 years o With the invention of the printing press, books could be mass produced o Similar technology was made in ancient China and Korea but they had no comparable impact In Western language, only need 26 letters + caps, punctuation but there is a much smaller amount of characters needed Chinese characters are around 10 000 This is the reason the printing type in Europe was much more important The way to represent language/ text is more efficient o Is similar to visual world; visual matters can be represented as simple, visual elements are basic building blocks for the visual world as well o These are GRATINGS or LINES o Not only can we describe the visual world this way, but we could also create filters that specialize in detecting one building block at a time. Eg, making a filter to find the letter A or Z o The visual system is capable of representing many aspects of the visual world Can decompose into simpler elements - Gratings are the building blocks in our visual world Three types of gratings - Rectangular grating o A sequence of white and black stripes, for example o Abrupt changes between light and dark; clear boundaries o Dont see this all the time - Sine wave (sinusoidal) grating o Bright = up, dark = low (on sine wave) o Can cut out a piece of it and get a stripe of light with dark around it - Get the third grating: gabor from the sine wave o Like a white stripe surrounded by darkness, can be the other way around o Is a small piece of a grating o Becoming shorter and with smooth transitions o 2D Gaussian distribution bell shaped function - These three gratings are used throughout visual science Three characteristics of gratings - Frequency o Has to do with cycles Eg. Day and night happening every 24 hrs Cycles per second = Hz (sound Cycles oer visual degree = cpd - Amplitude o Distance upwards or downwards from 0 o In terms of light: shows how much contrast the gratings have - Phaseo When blue curve runs upward, red curve runs downward Sine wave gratings - We see fuzzy/sharp edges everywhere small pieces of gratings - Any image can be decomposed into sine wave gratings o Decomposition is the first step in visual recognition o Also helps us code things in an efficient way Fournier transform: an operation that breaks down an image (for ex) into sine waves of different frequencies - Music gets broken down into temporal frequencies - Records used vibrations to get sounds - CDs decompose into sine waves - Speakers are an example showing frequencies can be decomposed into different waves Images can be decomposed into sine waves with different spatial frequencies - Eg. Have a picture of zebras o Taking the lower frequencies makes the picture look blurry emphasizes overall picture o With high frequencies there is an emphasis on the picture outlines
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