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Matthias Niemeier

PSYB51 (from lec 3) The Primary Visual Pathway - Both eyes see the same thing except for the periphery - Both visual fields get projected in both eyes - See a partial crossover at nasal part of retina that crosses over at the optic chiasm Lec 4 Perceiving and Recognizing Objects SAQ: What is a heteronymous hemianopia? Please describe the symptoms. Which brain structure is like damaged? - Hemianopia = hemi blindness - Left eye blind from left part of visual field, right part of right eye - Comes from damage to the optic chiasm What is the contrast sensitivity function? Can you sketch what it usually looks like? What does that reflect in terms of perception? - Describes the relationship between contrast and spatial frequencies - Upside down U shaped curve - Horizontal axis = spatial frequency , measured by cpd - Vertical axis = contrast sensitivity, luminance contrast – black and white o The more sensitive you are to these contrasts, the higher your sensitivity is - At a certain range of (60 cpd), at some point we are not able to resolve spatial frequencies anymore High – level vision = identifying things Object perception: Process that pieces things together into a cohesive 3D structure Object recognition: associates what you perceive with what you have seen before. Memory plays a role. Associating things in memory o Object identification  Connection to a specific example of a category Object naming: language function. Ex. a house is a called a house Mechanisms of middle vision (image with a house/arrow) - Looks like there is a figure in front of a background. - Implicitly the networks in our brains and eyes - In second picture of the arrow, the lines are individual dots - Problem: get gaps that have to do with the original image; in those areas, luminance contrast is not as great. Our visual systems fill in that area What V1 sees (different pictures of house) - Consider only the neurons with the proper orientation sensitivity - Problem: what happens with edges and curves? o The tip of the roof has a discontinuity  Fix by inventing a type of neuron that is good at detecting these kind of edges  Same neuron would detect other edges  Ex. in c) the neuron would detect the snowman - Bottom-up rules – computer algorithm will always fail and will never work - Perception is the sum of atoms of sensation - Structuralism and its early versions will always fail – can’t tell the difference between an incomplete object and 2 different objects - When we can’t use retina to piece pictures together, we can use certain rules that have to do with the world not being random Gestalt: German for “whole” - The whole is greater than the sum of its parts - Reaction to earlier structuralist school of psychology - Identified Gestalt laws (grouping rules) o A set of rules that describe elements in an image in a way that predicts how they will get grouped together Gestalt law of “good continuation” – two elements tend to group together if they seem to lie on the same smooth contour - 2 lines that are kind of collinear and lie on a straight line are more likely to be thought to go together - If orientation sensitive neurons have a positive connection with other orientation sensitive with close by receptive fields, they enhance each other - If they inhibit neurons within their receptive fields, that is enough to establish a connection - The neurons that are sensitive to the line segments of the loop will be firing more strongly – are more salient - All has to do with smooth contours Smoothness - The break point where you don’t see things as continuous is the threshold for smoothness - Geisler et al took natural scenes and quantified smoothness o What people perceive with being smooth actually goes together in the real world o Our visual system may have evolved in this way Gaps with contours - Illusory contours: Kanisza figure o Image that looks like there is a picture in front of other pictures  Ex. the arrow in front of circles - Has to do with aligned end stoppings; are not coincidences; certain things are more likely than others Some people argue that V1 causes these illusory contours - Differences occur when you try to figure out where the signal is coming from o Lateral occipital cortex  Don’t see objects anymore - With feedback, can perceive objects Texture Segmentation - Important visual task - Being able to piece together the contrast of things and the surface of the object - Carving out an image in certain properties Gestalt Law of Similarity - Things are grouped together if they are similar Gestalt Law of Proximity - 2 or more elements are grouped together if they have a small distance apart - Lines may also group together if they are parallel or symmetric  weaker grouping principles Common fate: group elements moving in the same direction together - Things that move together probably belong together Gestalt Law of Synchrony - Group elements changing at the same time together - Changing in synchrony Common region - When elements are perceived to be part of a larger region get grouped together Connectedness: elements that are connected to each other group together Gestalt laws work in PARALLEL PROCESSING - Letters are special types of objects - Area LOC is an object area; the left hemisphere is specialized in letters and words - Pandemonium model o Perceptual committee models  Middle vision are similar to a collection of “specialists” for certain features and vote on what is visible - 3 levels of “demons” o Feature demons: specialized for certain lines/features o Cognitive demons: specifically talk to feature demons that pick out the proper features of the letter o Decision demon: decides what the letter is depending on the demons that are “awake” - Things work in parallel, extracting features at the same time Committee rules - 1 : honour the laws of physics and biology o Ex. the balls with different shading  Take into account where the light is coming from - 2 : Resolves ambiguity o Ex. necker cube  Both orientations are
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