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Prof. Anderson Department of Psychology Week 5
Psy280: Perception Prof. Anderson Department of Psychology Week 5
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Visual philosophers What’s the meaning of life? Not exactly
Computational approach to vision David Marr (MIT) What is the purpose of vision? What are the problems it must solve?
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Computational problems in object recognition
What is it? Object constancy: Variability in sensory information Retinal position Viewing position Occlusion Lighting, colour
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Computational problems in object recognition
Where is it? Where’s Waldo? ?
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Two visual cortical pathways
These problems are reflected in the organization of the visual system Ventral “What” pathway Inferior longitudinal fasciculus Dorsal “Where” pathway Superior longitudinal fasciculus
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Dissociation of what and where in the monkey
Landmark and object discrimination task (Pohl, 1973) Parietal lobe Where Temporal lobe What Landmark Object discrimination
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Neuroimaging evidence for “what” and “where:
Attend to change in objects or locations Objects Occipito-temporal Locations Posterior parietal Same objects Different location
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Ventral “what” pathway
V4: Isoluminant color Lingual, fusiform gyrus MT (V5): Motion Middle temporal gyrus
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Neuropsychological evidence: Color (V4) and Motion (MT)
Fractionation of perception following cortical lesions Achromatopsia Loss of colour vision Akinetopsia Loss of motion
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Higher-order “what” pathway characteristics
Complex response profile Dissimilar to V1 Not simple orientation, colour, motion Selectivity Hands, faces etc
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Disorders of higher-order ventral visual pathway
Agnosia: “without knowledge” Visual agnosia: vision w/out knowledge Modality specific: Restricted to vision Not a memory disorder Item can be recognized through other modalities Touch, sound, smell
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Higher-order cortex is highly specialized: Prosopagnosia
Largely specific to faces Can distinguish between faces and objects Difficulty in distinguishing between faces Facial identification Across category Within category
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Is there a region of the brain devoted to faces?
Fusiform face area (FFA) Right middle fusiform gyrus especially responsive to faces relative to other objects FFA
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Neural selectivity: Evolution vs experience
FFA and other objects “Greebles” Train to recognize individuals Experts but not novices activate FFA Potentially not face specific Reflects both evolution and experience?
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Object recognition: Invariance
Recognize object despite differences in: Size, orientation, viewpoint, lighting, colour, location Inferotemporal cortex (IT) Anterior ventral stream Invariance in neural response
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Evidence for constancy: Lateral occipital complex (LOC)
Likely locus of object constancy Reduction in fMRI response w/ repetition Invariance Size, location,viewpoint, illumination, occlusion No effect of occlusion
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How does cortex represent all objects?
Specificity coding Grandmother cells Every orientation Every color Distributed coding “Coarse” coding across neurons Combinatorial Color 3 cone types Form Shape columns in IT Geons Geons
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Ventral stream neurons and consciousness
Binocular rivalry fMRI evidence: FFA and awareness FFA turns on when aware of faces FFA turns off when unaware of faces Ventral stream representations support consciousness FFA PPA
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Dorsal pathway: Action
Double dissociation Agnosia vs. optic ataxia Apperceptive Agnosia Ventral stream damage Impaired perception Intact action Appropriate reaching grasping Optic ataxia Dorsal stream damage Intact perception Impaired action
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Break! 10 minutes please
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How does it all come together? The binding problem
Division of labour: Parallel processing Colour, shape, motion, depth, location All in separate regions How bound together? Unified perception Not separate features
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Integration (binding) across feature maps
Synthesis requires attention—allows coherence across feature maps: Objects
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Attention is mental glue
Allows features to stick together Without which, perception falls apart No coherent perception of world
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What is attention? Everybody knows what attention is. It is taking possession by the mind, in clear and vivid form, of one out of what seem several simultaneously possible objects or trains of thought. Focalization, concentration of consciousness are of its essence. It implies withdrawal from somethings in order to deal effectively with others, and is a condition which has a real opposite in the confused, dazed scatterbrain state …. William James (1890)
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What is attention? “taking possession of the mind”
Control of the focus of attention “one out of what seem several simultaneously possible objects” Inability attending to multiple things at once “It implies withdrawal from somethings in order to deal effectively with others” “Paying” attention comes with a cost It has limited capacities that must be shared “has a real opposite in the confused, dazed scatterbrain state” Attention is the glue that keeps perception together
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Pay attention!
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Visual experience: A grand perceptual illusion
Rich and complex? Sorry folks, its an illusion We don’t “see” as much as we believe Don’t notice big changes in our environment Change blindness We fill-in our experience to make it coherent Little persistence of experience from one moment to the next Seems coherent
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Selective attention Definition Why do we need selective attention?
Process relevant and ignore irrelevant Why do we need selective attention? Can’t remember/processes everything Can be independent from eye movements Helmholtz (1894) Fixate eyes Brief flash Can pick what to perceive Perception is not fixed Not just what your eyes do Perceptual free will Can choose what to perceive
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Attention: Feature Integration Theory
Parallel Cereal Attention needed to glue features together Feature vs. conjunction visual search (Treisman) Parallel (“Pop out”) E.g., Color, orientation Requires little attention Serial Color and orientation Requires focal attention Need to move attention around
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Attention is required for binding across feature maps
Need attentional beam Without attention? Illusory conjunctions Unbound features Recombined Illusory perception Need attention to glue features together Otherwise fall apart Report seeing Yellow Square and purple triangle
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Perceptual primitives: What makes a “feature”?
Building blocks of perception Cortical feature maps Luminance, orientation, color, Motion, depth Higher-order objects Synthesis of primitives Objects defined by conjunctions of primitives Need attention to bond into compounds Molecules of perception? Pop-out No pop-out Unique primitives Share primitives
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Constructivism vs Gestalt approaches to perception
Structuralism Perception is created by adding elements together Perception from the bottom up Like building a house Gestalt approaches Top-down perceptual organization Perceptual inferences: Best Guesses “Different than sum of its parts” Not like building a house Don’t want best guess, want correct answer Don’t want illusory foundation or beams!
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Constructivism vs Gestalt approaches to perception
Perception is much more than what is projected on retina Structuralism inputs determine perception Gestalt Active role of perceiver Same input, many different kinds of perception
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Gestalt principles of perceptual organization
Best guesses about world Heuristics “Rules of thumb” Visual system is problem solving for us Visual intelligence Takes into account probability of occurrence What is likely vs not Likely Unlikely
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Some of the principles/heuristics
Similarity Good figure/simplicity Good continuation All have multiple interpretations but we all tend to agree on one
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Visual “guessing”: Shape from shading
Square raised Square recessed Sensitivity to perceptual invariance Light from above, not from below
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Figure-ground segregation
What’s an object? What’s background? Mental imposition of depth Occlusion Infer continuation of background underneath object Best guess Unlikely share same contours
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Figure-ground segregation
Factors that influence it Symmetry Development Beauty Meaning See arrows? Emotion Perceptual autism Pair faces w/ shock Bias perception toward vase Black White
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Chicken or the egg? Meaning processed before or after figure-ground segregation? Top-down perceptual organization over-rides initial processing Only aware of the final products of this process
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Substantial feedback onto early visual processing
Higher-order cortex influences lower-order cortex More feedback than feedforward connections Has delayed influence Neural processing is dynamic Need to revise classic definition of receptive field Greater # of feedback connections applies to early cortex as well
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Meaning and perceptual primitives
Common fate Can perceive objects based on motion Higher-order meaning influences lower-order perceptual primitives E.g. motion
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Meaning influences perceptual primitives
Structure from motion Can perceive objects based on motion Higher-order meaning influences lower-order perceptual primitives E.g. motion
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The End
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