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Lec 3.docx

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Matthias Neimier

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Lec 3 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 processing differ from that of a usual computer? - Horizontal and the vertical pathway - Horizontal: horizontal cells and amacrine cells. Functions are lateral inhibition. They have centre surround organization. - Vertical: photoreceptors, bipolar cells, ganglion cells. Photoreceptors transform physical energy into neuro signlas which passes on the info. Pass on information. Convergence of information. - Difference between this and computer, computers have about 4 central processing units. The retina has millions of the central processing units (nucleus of every cell. - Why care about gratings? Printing press: 1439, with movable types. Gutenberg. 3 types of gratins: rectangular, sine wave, gabor (combination of sine an Gaussian) Frequency, phase, amplitude Frequency is important to recognize a scence and helps us with efficient codings. Similar to basic alphabets of vision. Cnan creae a computer code with less memory effort. Info on lower frequency channels tell us that it is lincoln’s face, Higher spatial frequencies tell us that we are looking at squares. This inhibits our ability to recognize the face. The two are interfering Mona lisa: when you look at her mouth directly, is she smiling? Not really, but if you look ar the corner of face she looks like she is smiling. This happens because da vinci used many layers to superimpose the paints. The paint is transparent. Makes it seem like the objects are blurred, 3D. also applied it to the face. Amplifying lower frequencies when looking through periphery. When looking straight on it is the fovea and we are seeing higher frequency so we don’t see the smile. Bell and sejnovski independent component analysis: basic components of natural images. Independent component analysis: takes a stimulus and breaks it down into its components. Even if no knowledge of languuae, you can still do so. - Decompose photos of natura scenes. - Finds aspects that are independent of one another. - All pictures are composed of little lines and grating. Just superimpose the lines onto eachother and you can recreate almost any kind of photo - Can create an alphabet to describe all sorts of images - It is efficient - Reason that it happens in the brain is that the visual system learn, train and adapt Certain neurons are specialized for fine edges and lines Limits of cosing images in terms of oriented stripes: visual acuity and contrast. The two intereact in a non linear way Acuity: smallest sspatial detail that can be resolved (20-visual acuity, smallest detail you can see in 20 feet.) the second 20 is what normal ppl see) Limit of visual acuity: spacing of photoreceptorsi n the retina - Convergence - Sampling Aliasing: signal reconstructed fro msamples is different from the original continuous signal. As you go further away you see thigns differently, at higher frequency (smaller) Really, contrast changes uniformly, its just that our visual system is nto as good at seeing these visual lines Best tuned to see ntermediate freuqnccies reather than higher or lower. Csf: sensitvity is inversion of threshold . when high sensitivity, then low thresholds Low threshold mean high sensitivity Everything gets inverted - Not all information goes to contralateral side of the brain. This means that it is the visual fields. Separation in terms of visual fields. - Before optic chiasm, separation in terms of eyes. Right brain sees the left visual field. Left brain sees the right visual fields. Happens after optic chiasm - LGN, part of thalamus. Plaace where senses pass thorugh except for olfaction also place where inhibits senses when sleeping. - LGN-pulvinar-superior colliculus - Primary visual pathway:**eye, optic nerve, optic chiasm, optic tract, lgn, V1. - If you have a lesion at the optic nerve, before the optic chiasm, the result is that one eye is blind. - Heteronymous hemianopia: Severed the two tracts that cross over in the chiasm- see blindness on half of the visual field, always the temporal part of the opposite side. Because the nasal portion of the retina cant pass info anymore. It is all flipped so this is why. *with left eye you fail to see left visual field portion (temporal). - Homonymous hemianopia/quadrantanopia or scotoma: you cant see the left visual field when there is a lesion in the right brain. - Scotoma is smaller area blindness. You might not even notice it until yu test it ou
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