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PSY 280 L11.07

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University of Toronto St. George
Matthias Niemeier

PSY 280 - P ERCEPTION M. N EIMEIER 11/07/12 SA: What is a heteronymous hemianopia? Please describe the symptoms. Which brain structure is likely damaged? What is the contrast sensitivity function? Can you sketch what it usually looks like? What does that reflect in terms of perception? - Visual field defect – blindness in one part of the visual field (half of each eye’s VF, on opposing sides); optic chiasm damaged o Left VF of left eye, right VF of right eye blind o Indicative of brain tumour (hypothalamus) - Ability to see spatial frequency - Relationship between contrast and spatial frequency o Inverted u-shape (bell-curve-ish) - Limit to how you can see spatial frequencies - Inability to see lower spatial frequencies - Object perception – ability to piece together something - Object recognition – associating visual input with personal knowledge – memory component o Object identification – recall of object recognition - Object naming – language function of object ID Summary: - Middle vision combines features into objects – result is obj. perception - Obj. recognition means we can match a perceived obj. representation to a representation encoded in memory - These memory traces can contain information about obj. categories or information about that particular obj. – obj. identification - Recognized obj’s usually have semantic labels and names assigned to them - Edge detection o Differentiate pixels according to luminance so to form lines and fill gaps - Mechanism to fill in gaps and join pixels to lines needed – not a bottom-up process - Not everything is straight lines – discontinuity, curved contours o Might join contour lines that were really different obj’s - Structuralism fails – piecing stuff together not enough - Gestalt psychology (school) – “whole” “Whole is greater than the sum of its parts” - Reaction to structuralist school of psych - Gestalt laws – grouping rules o Help us see objects as a whole - Good continuation: 2 elements tend to join if they seem to lie on same smooth contour o Co-linearity required - V1, RFs, simple cells with preferred orientation o 2 neighbouring cells that prefer similar orientation can activate one another, inhibit others that have different orientation – connectivity according to contour continuity – chain reaction - Good continuation – indicates line contours belong to same obj. only if they have smooth continuity - Without colour, smoothness paramount for identifying continuity - Aligned end stopping leads to perception of a brighter occluding contour (image) o Ability to fill in information for something not there (if alignment of end stops are good) o Co-linearity - Visual system always reduces coincidences - Don’t even have to be straight – curved contour - High level areas recognize illusion first before feeding information back to V1 where gaps filled in, lines joined o Recognize arrow first before piecing edges - Similarity: texture segmentation – image into regions of common texture properties - Grouping together elements if they are similar size, colour, etc. - Proximity: grouping together if the distance is small o E.g., 2 squares, 4 rows , stars and octagons o Similarity plays a role - 2 things close together more likely to belong to the same surface/obj. - Don’t work across 2 different dimensions o E.g., shape, colour - Parallel and symmetric elements – weaker grouping principles - Common fate: elements moving in the same direction together o Shape knowledge and good continuation employed - If lines moving at the same direction, gestalt can still be recognized through ‘noise’ (unless static) o E.g., lamp - Powerful cue, this gestalt law - Synchrony: group elements changing at same time together – evolution in time - Common region: elements perceived to be a part of a larger region grouped together - Connectedness: elements that are connected to each other group together Pandemonium - Good continuation and similarity with line/figure example - Parallel processing o Perceptual committee models  Collection of specialists for certain features - Selfridge pandemonium model – demons = (sets of ) neurons, level = brain area – for letter recognition o Feature and cognitive demons under decision demon - Cells with tuning functions that are slightly or fully stimulated because of close similarity of features recognized to what it is attuned to - Limitations to model – fall apart when things are not standardized - Committee rules: o Honour laws of physics (biology and stats)  Grouping of elements, recognition according to what’s physically, biologically and statistically possible  Make assumptions (e.g., light orientation makes circles seem concave or convex)  Based on what is most likely o Resolve ambiguity – flip between 2 interpretations instead of thinking it is something weird o Reject accidental viewpoints – sees one certain image according - Visual system reduces coincidences - Figure-ground assignment o Determining image region as a foreground obj., other regions part of background o Surroundedness, size, symmetry, parallelism – Gestalt figure assignment principles - Extremal edges – horizons o Non-transparent objects have horizons – point where you can't see anything anymore - Projections of viewpoint specific horizons of self-occlusion on smooth, convex surfaces o Powerful figure-ground cue - Shading helps figure out contour o Sphere, annulus, hole - Rubin figure – vase-face image o Object recognition starts before figure-ground assignment finishes o Switch back and forth between interpretations - Back/foreground perception (depending on colour o Similarity also involved (parallel) - Circle hole example – similarity, foreground, background, linear orientation - Heuristics – mental shortcuts that usually works o For partially occluded features - Completing edges behind occluders when edges are relatable by an elbow curve - Relatability – degree to which 2 line segments appear to be part of same conto
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