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Perception lec 3.docx

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Matthias Niemeier

Perception lec 3 Practice short answer : retina pathways neuron roles etc: 2 pathways vertical from photoreceptors to bipolar cells gang cells then projecting to brain -other pathways includes other cells, emecrene cells and horizontal cells -vertical pathway passes on info from the light that is being surfaced on the retina to other parts of the brain, there is also convergence of information onto a single bipolar cells, simplifies info and blurs it but you get higher sensitivity -horizontal and amacrene cells interconnenct neighboring bipolar cells laterally in a way that is working in an inhibitory fashion -difference between retina and computer? Records things in binary cords, there is qualitative differences, have a small number of central processing units, only have 4 mechanisms that do it (in a computer), in the retina there are hundreds of neurons that do it Sine wave gratings -one of the major libraries in the world only owned 122 books, they were written 1 by 1 by hand -with the invention of printin press, could mass produce books -gutenbergs printin press was not the first pritinng idea, it was first done in china and japan but his became the biggest invention -the way to represent language is more spefici when you use letter alphabet -visual images are building blocks for the visual world as well -you can describe the visual world by creating a picture but you could also now create filters that are specialized in one building block at a time -the elements of an image can be separately filtered out -we can decompose into simpler elements that are each specific Three types of gratings -rectangular: sequence of colored stripes, shows the sequence of bright and dark, shows the abrupt changes -sine wave grating: smooth transition in terms of luminescence -gabor: light in centre dark on outside, could be a dark stripe sourrunded by lightness, shows luminescence Characteristics of grating -frequency: has to with cycles, used in terms of sound, calculated in terms of spatial frequency- determined by cycles per degree -how many cycles fit into 1 degree -amplitude -phase: visual systems has to extract We see throughout the world little pieces of gratings -we see sharp edges fuzzy edges boundaries all the time, these are gratings -there are gratings everywhere -any image can be decomposed in to sin wave gratings -important for decomposing, is the first step for visual recognition -decomposing helps us to decode stuff in an efficient way -fourier transformer: breaks down into different frequencies ex: music from records to cds to mp3 -at first music was stored in a analog fashion vibrations in disk rings -cds and later they take the sound wave of music and decompose it into ind sine waves of diff frequency amplitude and phase -the 2 speakers on speakers operate different for different frequencies -what works for music works for visual -you did it for horizontal space and vertical Frequency components of an image -blurred picture emphasized lower frequency -with high frequency, the boundaries are preserved, you pick out the boundary of transition -you get different types of information that are consistant ex: zebra pic Linkerman illusion -in terms of high and low frequencies, on the low frequency range you can see Lincolns face -on the high frequency range, you just see pixels just squares -high frequency is closer to the picture -grating way to decompose images into building blocks -every natural image somehow is a composition of line and edges and superimposed on each other will form a picture -the more independent the building blocks are the less you will need of them -the brain needs past experience to reconstruct something -there if biological relevance to efficient coding -natural images have systematic components, certain things reoccur over and over again, it can learn and adapt to whats out there and try to be efficient -the components should be expected we should see them somewhere and be able to see in the visual systems functions that pick out the components -ind neurons detect lines and edges Visual acuity -acuity: how good are we at seeing stripes -being able to process stripes has to do with how close they are together -we can see things to a certain point, if frequency is too high can’t see anymore -visual acuity: point we cant see stripes anymore, at what frequency we can see stripes being close together -acuity: smalles spatial detail that can be resolved -acuity depends on many factors -opthamologist use distance to detect vision acuity -20/20 represnets that your acuity is normal, what you see at 20 feet is what is suppose to be seen at 20 feet -vision scientist: look at angles, good visual angle 0.017 can still be resolved, that’s the threshold, lil than that can’t see -photoreceptor spacing in retina limit resolution acuity -degree angles how large are receptor fields of cones in the fovea -the visual system samples light in a discrete way -it will take in whatever light is shinning on its entire surface, it cant take in anything beyond that -visual acuity is limited by convergence, doesn’t happen in fovea -each cones projects to bipolar cells Sine wave gratings -All photoreceptors tell us there is a grating, how they are oriented together -cycles are less than what the size of the receptor field cones are -b grating, they each fall on the dark or light area they’re all touching -all photoreceptors get stimulated equally in graph b, its not grating anymore, its becoming a uniform- aliasing -when sampling in a discrete way it is analog, continuous -snellen test: has a bunch of ‘E’ of diff orientation and you had to find the point where they foun
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