Prof. Anderson Department of Psychology Week 5 Psy280: Perception Prof. Anderson Department of Psychology Week 5

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?

Computational problems in object recognition What is it? Object constancy: Variability in sensory information Retinal position Viewing position Occlusion Lighting, colour

Computational problems in object recognition Where is it? Where’s Waldo? ?

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

Dissociation of what and where in the monkey Landmark and object discrimination task (Pohl, 1973) Parietal lobe Where Temporal lobe What Landmark Object discrimination

Neuroimaging evidence for “what” and “where: Attend to change in objects or locations Objects Occipito-temporal Locations Posterior parietal Same objects Different location

Ventral “what” pathway V4: Isoluminant color Lingual, fusiform gyrus MT (V5): Motion Middle temporal gyrus

Neuropsychological evidence: Color (V4) and Motion (MT) Fractionation of perception following cortical lesions Achromatopsia Loss of colour vision Akinetopsia Loss of motion

Higher-order “what” pathway characteristics Complex response profile Dissimilar to V1 Not simple orientation, colour, motion Selectivity Hands, faces etc

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

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

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

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?

Object recognition: Invariance Recognize object despite differences in: Size, orientation, viewpoint, lighting, colour, location Inferotemporal cortex (IT) Anterior ventral stream Invariance in neural response

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

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

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

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

Break! 10 minutes please

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

Integration (binding) across feature maps Synthesis requires attention—allows coherence across feature maps: Objects

Attention is mental glue Allows features to stick together Without which, perception falls apart No coherent perception of world

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)

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

Pay attention!

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

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

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

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

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

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!

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

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

Some of the principles/heuristics Similarity Good figure/simplicity Good continuation All have multiple interpretations but we all tend to agree on one

Visual “guessing”: Shape from shading Square raised Square recessed Sensitivity to perceptual invariance Light from above, not from below

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

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

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

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

Meaning and perceptual primitives Common fate Can perceive objects based on motion Higher-order meaning influences lower-order perceptual primitives E.g. motion

Meaning influences perceptual primitives Structure from motion Can perceive objects based on motion Higher-order meaning influences lower-order perceptual primitives E.g. motion

The End