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Lecture 3

PSYB51 Lecture 3.docx

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Matthias Neimier

PSYB51: Lecture 3: Spatial Vision Spatial Vision pinhole plus lens focus light in retina - precondition of vision achieve neurally - more than percieving things in 3d - spatial vision What does visual world look like? or designed for. Natural scenes that brain will extract and filter it our and individual neurons serve as filters Diff. version of gratings what are the bound or limitations of acuity primary visual pathway - retina - synapses - cortex to area 17 functional prop. of stripe cortex Natural Scene Statistics: Why care about gratings? gutenberg's invented printing press - impact on why it is impt. to represent something gratings in visual world is the building block e.g. Fourier's transform assoc. with natural world as well Printing press metaphor huge impact in education Cambridge own 122 books need to be copied these books by hand so printing press can mass produced books - has a big impact on education in Europe. frame and cover with ink cover paper and print takes some time to create book - but much quicker than doing it by hand invention not new at all bec. in Korea earlier why gutenberg invention - is it bec. of bias found out in western countries? culture bias? reason printing press in Europe is successful bec. the way to represent language much more efficient when you use it on alphabet. need this movable types - efficient in english than chinese. Why care about sine wave gratings? create filters that specialize and detect 1 building block at a time e.g. only detect A or Zindividual neurons do it. so visual sys. and brain can decompose it to simpler elements to have simpler definition that way. 3 types of gratings 1. white and black - luminance profile - gratings show seq. of bright and darker rectangular shape - clear boundaries sinusoidal grating - pt. smooth transition from dark and bright - sine waves up - bright down dark - little bright stripes with darkness GAbor - 3rd type of grating - instead of black blue - stripe surrounded by darkness or brightness to get idea of luminance distribution - bell shape function - gaussian shape contrast - see only inside window with blurred boundaries 3 types used in visual science as early function of visual sys. 3 characteristics of gratings freq. day and night - happening once every 24 hours freq. cycles in sound measured in hertz spatial freq - visual degree - CPD - cycle per degree how many cycle in visual degree blue sinosoidal - one cycle - one time in bright and 1 dark cycle not repeated yet - compared with blue and green curve - green twice degree frequencies blue curve Amplitude - distance from 0 - amplitudes e.g. how loud sound and light - how much contrast grating or light have red curve - similar - freq. not change and amplitude but phase change curve upward blue downward bec. little pcs. of gratings - see fuzzy edges - small pc. of gratings decompposng is frst step in visual sys. decompose sine waves helps us to encode visual world in efficient way in terms of simple mechanism works for musice - digital waves of coding music - qualitive of music on how it is stored - plate disks - analog way - cds do it diff, take sound wave of music and decompose things in sine waves speakers - operate better for diff. ranges of freq. can have one freq. bec. it is infinitive but can be broken down to simplifiy and hear better. in visual sys.transform step by step in process do it in horizontal and vertical transform and take out low freq - blurred image emphasizes lowoer freq. iage info. about transition of black and white that is slow. if pick up low freq. pattern of zebra lost but if pick up high freq. not see anymore what is black or white but emphasize picking out boundaries of transitions - get very different types of info bet. hogh or low freq.e.g. forest - cant distinguish anymore if low freq. we can distinguish hundreds of pictures in 100 sec. extracting freq. gives rise to impt. aspect of information High Spatial freq. Mask illusion - contradictory info. Lincoln illusion - higher freq. interferes with loss of info. high and low spatial freq. - spatial freq range pic. of abraham lincoln low freq. range face of lincoln - if high freq. range just pixels -grate of squares - if stand near that picture - interfering to percieve face - depends on how close you are to image - bec. high freq. more or less visible gratings - arbitrary way on perception ways best set of building blocks-build language to represent - alphabet more efficient and for visual images do stat.methods identify components of images Bell and Sjevnoski when filter many photographs of natural scenes - get line of inflated graph - every natural image is composition of this components little lights and pcs of gratings making up all kinds of images efiiciencey better thananything else in pictograms - chinese more redundancy - far more chinese types - more efficient to have independent components than analogous fines how is this relevant for biology and psy. at the end - interesting to analyze photos -pictures approximations of real life not 3d not represent complete visual reality - close enough simplifications to find out something on first place - something about brain - criteria of brain to reconstruct something - past experience - experience with natural images - scenes visual sys. adapt - we dont store every little impression we get but store in efficient way - store many images not enough space - thoretically have many images in world - shadows would fall differently - sun shine clouds 0 seasons not fit on brain - not efficient enough human brain just enough that body burns - energy across evolution not possible make use of something more efficient efficient coding - biological relevance – pictures are simplification of world - natural images have certain systematic features visual sys. adapt and learn and try to be efficient - plastic therefore - building blocks can be expected - see it somewhere - ability to pck spatial freq. mech. that fucntion as sys. pick out building blocks. system detect individual - filters -pick out building blocks - detection mechanism specialize for grating see edges and lines. individual neurons detect lines and edges Visual acuity limitation: Visual Acuity- perceive things in certain pt. spatial freq. is high not see anymore - narrowness of stripes are limited - other limitation is contrast contrast sensitivity or contrast threshold basic way to test it acuity Same or different? pick out smallest detail that can be resolved fixated x in center pt. quantify visual acuity cause hard to distinguish depends also on how much sleep you get Measuring visual acuity tamen - sharp vision - 20/20 acuity is normal distance in 20 feet normal acuity see in distance of 20 feet good visual acuity - minimum visual angel of cycle of 0.01 degrees visual angle can still be resolved less angle then can't see anymore can also convert no. in cpd if you have tiny fraction of 1 visual degree= 60 Opthalmologist measure acuity and vision science how large receptor fields are individually cones in the fovea photoreceptors pts at 1 part centers light in discreet way take in whatever light shine on entire surface - absolute limit of visual acuity Acuity is further by convergence (multiple photoreceptors projecting into single bipolar cell) - see in fovea in retina convergence not true in fovea Figure 3.3 grating- center surround in ganglion cells of receptive fields photoreceptors have receptive fields - extend of photoreceptors super impose in gratings light shines into them and gets stimulated range that there are no light from retina so not stimulated visual sys. all photoreceptors together tell me that there is grating possibility of seeing grating size of receptor fields cones is every little bubble covers grating cones not matter not care but for rods - black and white side in 2nd stripes image which part of them light shines - average of entire surface of photoreceptors that state which photorec. get stimulated or not grating freq. stimulated too high. some pt not see it as grating anymore Aliasing blurred and continues castle with brick wall - curve on right side but not what is really in there - transition to being resolve somthing or not resolve at all visual world is analogue continues but vis. sys not perceive that bec. photorecptors cares only when light is present the pt. where we see aliasing is by snellen test Hermen Snellen standardize letters with diff. orientation - pt of E to measure acuity perceive E sufficiently higher than
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