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Lecture 3

Lecture 3 PSYB51.docx

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Matthias Neimier

Lecture 3- Spatial Vision 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?  One bipolar cell connects to a photoreceptor  Convergence: Inhibitation and facilitation: Spatial vision refers to our ability to resolve or discriminate spatially defined features. • Natural Scene Statistics: Why Care About Gratings? • What Is Visual Acuity?
 • The Primary Visual Pathway
 • Functional Properties of the Striate Cortex • The Girl Who Almost Couldn’t See Stripes  Natural scene statistics: why care about gratings? – Gutenberg’s printing press
 – Gratings and their characteristics – What is a Fourier transform? – The independent components of the visual world Printing press metaphor – 1439 Johannes Gutenberg invented a printing press with movable types. th – Huge impact on 15 century Europe (1424 Cambridge 122 books!) 
 – “Greatest invention in the last 1000 years”. – Similar technology in ancient China & Korea had no comparable impact. Why?  not much cultural exchange. In China and Korea, movable types not as important as Europe  The simpler the alphabet, the easier to recreate text (China has THOUSANDS of characters) A page of text can be pieced together from a small set of simple elements. – Similarly, visual images can be described/ represented as compositions of simple elements. – Each of these elements can be separately filtered out. Word document is much smaller than BITMAP (bitmap: notice or memorize what each pixel value is) Three types of gratings
 – Rectangular grating
 – Sine wave (sinusoidal) grating • Gabor (sine wave viewed through a circular aperture defined by a 2D gaussian) Three characteristics of gratings – Frequency • Cycles per second = Hz • Cycles per visual degree = cpd – Amplitude – Phase Sine wave gratings – who cares? – Patterns of stripes with fuzzy boundaries are quite 
 common – The edge of any object produces a single stripe, often blurred by a shadow, in the retinal image – Any image can be decomposed into sine wave gratings and other simple forms. 
 • Important cues for visual recognition 
 • Efficient coding.
 – (This is what the visual system appears to do.) Fourier transform: 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 (get rid of extra frequencies)  you still see the animals but have difficulty seeing the details  image of zebra: low frequency tells body of the zeb
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