P rcep e t p i t on n a s a s u n u c n ons n c s ious u s i nf n e f renc n e L ctur u e 4 : Recogni n t i io i n

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1 Lecture 4: Recognition and Identification Dr. Tony Lambert Reading: UoA text, Chapter 5, Sensation and Perception (especially pp ) 151) Perception as unconscious inference Hermann von Helmholtz (19C) Richard Gregory (1970 s) Perception often goes beyond the immediate sensory information Influence of background knowledge and context The problem of perceiving depth Richard Gregory 1966 We are given tiny distorted upside-down down images in the eyes, and we see separate solid objects in surrounding space. From the patterns of stimulation on the retinas we perceive the world of objects, and this is nothing short of a miracle! Perception as unconscious inference Analogy with scientific discovery evidence & hypothesis PERCEPTION 15 HYPOTHESIS Visual Illusions Illusions involving paradox: local perceptual hypotheses vs. overall interpretation 1

2 The problem of perceiving depth Image on retina is 2D But the world is in 3D!! The problem of 3D perception Perceptual system (i.e. the brain) uses cues in the 2D image to infer 3D layout of a scene Perception as a process of unconscious inference Hermann von Helmholtz Binocular depth cues Monocular depth cues Binocular convergence Binocular disparity Each eye gets a slightly different view of the world This is called binocular disparity Only comes into play when viewing relatively close objects (less than 3 m) The brain uses this disparity to compute relative distance Binocular disparity Stereoscope (Viewmaster) 2

3 Random Dot Stereograms (Julesz) Monocular cues You can judge depth reasonably well with only one eye There are actually more monocular than binocular cues Also known as pictorial depth cues Seeing depth with one eye Interposition or occlusion Wesley Walker blind in one eye from birth Played wide receiver for New York Jets for 13 years We see the green in front of the red. Linear Perspective Linear perspective Discovered by 15C Italian architect More realistic paintings Enthusiastically adopted by Renaissance artists 3

4 5/6/2008 Shading and lighting Texture Gradients Perceptual system assumes that light is from above Effect of changes in texture gradients Depth perception & motion The Ames Room Linear perspective cue vs. familiar size cue Motion parallax Motion parallax If you move your head from side to side, near objects move more than far objects Distant object hardly move at all This gives an important clue to depth 4

5 Binocular depth cues Binocular convergence Binocular disparity The story so far Monocular (pictorial) depth cues Interposition Linear perspective Texture gradients Shading & lighting Motion parallax Perception of Motion Movement of image across the retina can be seen as motion This is detected by motion detectors single cells in the visual cortex that respond to moving edges or moving lines Apparent motion When an object appears in different locations in succession The brain fills in the motion, even though nothing has moved Visual Pathways The basis for movies & television Primary visual cortex (aka V1) What & Where Pathways What & where pathways: evidence Brain lesions in what pathway Visual agnosia Inability to recognise common objects But can point & reach accurately for objects Brain lesions in where pathway Optic ataxia Can recognise common objects Unable to reach or point accurately 5

6 Recognising Faces The fusiform face area (FFA) A special brain system for faces? Prosopangnosia The man who mistook his wife for a hat Brain imaging & the fusiform face area Kanwisher et al. Face recognition vs. houses & hands Identifying gaze direction Friesen & Kingstone 1998 If someone is gazing in one direction do we automatically look there too? The answer seems to be Yes! Gaze direction & reflexive attention Fast responses Summing Up Perception as unconscious inference Perceiving depth & motion Slow responses Perception of faces & automatic response to gaze direction 6