Object Perception Perceiving and Recognizing Objects

Transcription

1 Object Perception Perceiving and Recognizing Objects Extrastriate cortex Outside V1 in occipital lobe Dorsal pathway Ventral pathway Modular organization of visual areas associated with object recognition 1

2 As processing moves to higher cortical areas: Receptive field sizes get bigger Processing becomes more complex Three levels of vision Low level vision Middle (midlevel) vision High level vision Marr s Model of Object Recognition Raw Primal Sketch 2 ½-D Sketch 3-D Representation Object s Image on Retina Identify Edges and Primitives Group primitives and process Perceive 3D Object Level Raw Primal Sketch Information Processing Description achieved Identifying light/dark contrasts leads to a descriptionof the edges and borders of an object, including location and orientation. The Full PrimalSketch By grouping primitivefeatures together, this determines where larger structures, such as boundaries and regions, occur and are represented. The2½ -Dimensional Sketch The Three Dimensional Sketch By processing surface features, a full representation of objects in viewercentered co-ordinates iscreated. This is achieved by an analysis of depth, motion, and shading as well as from thestructures assembled in the primal sketch. A representation centered upon the object rather than on the viewer 2

3 Inverse projection problem An image can be created by any of an infinite number of features combinations in the environment Accidental viewpoint What you Perceive What s really out there? = or 3

4 Anamorphic art Anamorphic art Finding Contours Finding edges (contours) is first and critical step Contours define the object 4

5 Finding Contours Hartline & Ratliff s (1957) Work with Horseshoe Crabs Ommatidia are similar to retinal ganglion cells in humans Lateral inhibitionoccurs when neurons suppress activity of neighboring neurons Finding Contours Illusory contoursare created when features appear to have a common edge Kanizsa figures Finding Contours 5

6 Finding Contours May even affect brightness Finding Contours Modal completion occurs when we form illusory contours Amodalcompletionoccurs when we perceive a complete, but occluded, object Gestalt Grouping Rules : The whole is greater than the sum of all parts. 6

7 Law of Good Continuation Law of Closure Law of Proximity 7

8 Law of Similarity Law of Parallelism Law of Symmetry Law of Common Region Law of Connectedness 8

9 Law of Pragnänz (Good Figure) Yates Thesis and Perceptual Ambiguity Priming helps Overcome Ambiguity Prime Target Usual Response Rat Vs. Man 9

10 Young Old Figure-Ground Differentiationis the visual system determining what in the visual field is an object and what is the background Rubin Vase 10

11 Figure-Ground Differentiation is affected by: Surroundedness Figure-Ground Differentiation is affected by: Orientation and Size Figure-Ground Differentiation is affected by: Symmetry and Inclusion 11

12 Incongruent Congruent Processing Parts vs. Wholes Global Superiority Effect S S S S SSSSS S S S S H H H H HHHHH H H H H HHHHH H HHHHH H HHHHH SSSSS S SSSSS S SSSSS Name Global Letter Name Local Letters Incongruent 500 ms 650 ms Congruent 490 ms 550 ms GSE 10 ms 100 ms RT (ms) Name Global Letter Name Local Letters 0 Congruent Incongruent Processing Parts vs. Wholes Orientation Matters in Global Processing! Pattern/Object Recognition Naïve Template Theories Mental blueprints (templates) stored in memory Recognition occurs by input to matching templates 12

13 Pattern/Object Recognition Problems with Template Theories? Pattern/Object Recognition Structural Theories All object have a structural description Objects recognized by processing structure Pattern/Object Recognition Recognition based assemblage of parts David Marr: Basic components are cylinders 13

14 Pattern/Object Recognition Recognition by Components(RBC) Theory Geonsare the basic units of recognition Non-accidental properties Viewpoint invariant We recognize objects by detecting geons Pattern/Object Recognition How important is structure? Principle of componential recovery Pattern/Object Recognition Problems with Structural Theories? Viewpoint-dependent neural responding to abstract objects (Greebles) 14