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Perception and Cognition Lec 3.docx

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Matthias Niemeier

Perception and Cognition Lec 3  retina pathways o lateral  horizontal o vertical  photoreceptors  bipolar cells  ends at ganglion cells o functions  vert  photoreceptors transform light into neural signals o passes on information  many photoreceptors on one bipolar cell = convergeance o very low levels of light, has greater sensitivity o lose information about spatial distribution  lateral pathway  horizontal cells o connect laterally o inhibit laterally o enhances contrast locally o every cell in the retina is a processing unit, has hundreds of millions of processing units, while a computer only has a few processors at most.  Spatial vision refers to our ability to resolve or discriminate spatially defined features  Natural scene statistics: why care about gratings? o Gutenberg’s printing press metaphor  Invention from 15 century  “Greatest invention in the last 1000 years”  China and Korea had similar technology but no impact o This was less significant because in China and Korea they write in characters, not letters  The simpler the alphabet, the simpler it is to recreate text  Visual images can be described/represented as compositions of simple elements  The brain can then create filters to represent what you are seeing  The “alphabet of vision” is gratings o Gratings and their characteristics  Rectangular grating  Sine wave sinusoidal grating  Gabor (sine wave viewd through a circular aperture defined by a 2D Gaussian)  Three characteristics of gratings  Frequency o Sound: Cycles per second = Hz o Vision: Cycles per visual degree = cpd o Cycle = 0 to max to min back to 0  Amplitude o The peak measure from some baseline is the amplitude o Usually has to do with contrast of grating  Phase o When the sinewave is set  Sine wave gratings  Patterns of stripes with fuzzy boundaries are quite common  Any image in real life can be decomposed into sine wave gratings and other simple forms o Important cues for visual recognition o Efficient coding o What is a Fourier transform?  Definition: an operation that breaks down a function/an image into sine waves of different frequencies  Music can be decomposed into sine waves with different temporal frequencies.  Images can be decomposed into sine waves with different spatial frequencies  Low frequency image -> looks blurred, you still see the animals but you have difficulty seeing their outlines  High frequency image -> you only see the outlines of the animals. You dont see black and white stripes, you only see the transitions between black and white  We have different information on low and high frequency channels  Low frequency tells you that it is a photo of Lincolns face  The high frequency system tells you you’re just looking at squares. o The independent components of the visual world  Independent component analysis  Statistical procedure to take something and break it down to its original components.  To identify the basic components of natural images  ICA found the components to be edge and line filters  The ICA is a good way of guessing which way the brain is working o Given the statistics of natural images and given the plasticity of the visual system we should expect  A certain specialization of the visual system
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