pattern and object recognition - wofford college...e.g orientation: horizontal and vertical lines...
TRANSCRIPT
Chapter 3
Perception:
Pattern or object recognition
Perception
Sensation vs. perception
What are the mechanisms responsible?
What is the process?
Q: How do we interpret lines and patterns as objects?
Q: How do we program a computer to perceive objects and scenes?
Start simple: How do we recognize these letters as A’s?
Template approach Stimulus is compared to stored pattern
Examples? Bar code, bank check, scantron, etc.
Problems:
There are an infinite number of templates to remember
Have to learn a template first
Any change in stimuli will not be recognized
Receptors in retina -> optic nerve -> occipital lobe (visual cortex)
Specialized receptors in visual cortex
Simple cells: feature detectors e.g. Orientation specific
Complex cells Combination of 2 simple features
Perception due to pattern of neural firing (neural code)
Bottom-up processing
Stimulus
Cell’s
responses
McClelland & Rummelhart (1981)
Interactive Activation Model
Pandemonium (Selfridge, 1959)
Visual perception by neurons
Respond to things that occur most often in environment
e.g orientation: horizontal and vertical lines vs. oblique
Experience-dependent plasticity
Animal reared in certain environment – brain changes to more strongly respond to those cues (Blakemore & Cooper, 1970)
Gauthier et al. (1999): “Greebles” study
Measure FFA (fusiform area)
IV: experience with Greebles
Recognition by components
Biederman’s RBC (recognition by component) theory
36 geons (3D)
Basic building blocks
Emphasis on
intersections
Recognition with missing information possible
Geons:
Identify objects
Principle of
componential recovery
Resistance to visual
“noise”
‘View invariant’
properties
Discriminability
Biederman’s Geons
Intersections are important to recognition
Beyond bottom-up processing
Pattern or object recognition
Bottom-up processing
Information from sensory receptors
Processing driven by stimulus
Data-driven
Top-down processing
Information from knowledge and expectations
Processing driven by higher level knowledge
Conceptually-driven
Problems with pure bottom-up theories:
How does brain pull all the feature information together?
How do theories deal with complex objects?
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Context and knowledge fills in the rest!
The redundancy of stimuli provide more features than required
Oliva & Torralba (2007)
Q: Does perception depend on more
than just stimulation of receptors?
Method:
Use same “blob” in multiple contexts
Result:
Perceived as different objects due to top-down processing
Conclusion:
Signal from object
Signal from context
Feedback signal: influence of knowledge
Theory of perception
Bottom-up AND top-down
Bi-directional or connectionist model
Depth perception
Relative size
Size constancy
Odor intensity
Controlled sniff intensity
Perception of language
Speech segmentation
Treisman & Schmidt (1982)
Q: Does knowledge change perception?
Method
Flash display of #s & objects 200 ms
Ask Ss to report #s then objects
IV: Give description of objects (“carrot, lake, tire”) or not
Results
Info significantly improved accuracy
Conclusion
Top-down knowledge changes perception
Able to “bind” features (group information) together more rapidly
Orange & triangle = carrot
1 3
Hollingworth (2005)
Question
How does knowledge of what objects
belong in a scene influence perception?
Semantic regularities (knowledge of function
of objects)
Method
Study scene 20s
IV: w/ or w/o target object
Test: Place target object in scene
By memory or expectation
Result
Accurate position in both conditions
Prediction based on experience
Palmer (1975)
Method Present scene
Ss ID flashed pics (a) or (b) or (c)
IV: type of picture
DV: accuracy
Results Appropriate pictures: 83%
Inappropriate pictures: 50%
Misleading pictures 40%
Conclusion Bottom-up perception interacts with prior knowledge (top-down) to
influence response
Chapter 3
Perception:
Gestalt laws and Heuristics
More perception questions
How does our top-down knowledge system change the way
we perceive the world?
How do bottom-up and top-down information get combined
to provide complete perceptual experience?
How do we integrate all the information (signals)?
Perceptual problem solving
• How do we integrate info into
meaningful whole?
• Helmholtz (late 1800’s): Theory
of unconscious inference and
likelihood principle
• Perception like problem solving
Principles of organization
Gestalt psychology (Koffka, 1935)
Gestalt laws of “perceptual organization”
Pragnanz: Good figure or simplicity
Good continuation
Similarity
Proximity
Common fate
Familiarity
Laws or heuristics (“best guess”)?
Occlusion heuristic
Light-from-above heuristic
Apparent motion: Wertheimer (1912) Method: 2 lights turned on/off in dark room, with right timing,
looks like it is moving (becomes a single light)
Expected result: For larger separations, the stimulus must "move" a
farther distance, which presumably requires a greater length of time.
CogLab
Data from Spring ‘09 (N = 8)
Gestalt law (or heuristics!) examples
Gestalt principles
Group objects together to find the 13 faces
Gestalt law (or heuristics!) examples
Optical illusions and visual phenomenon
Watercolor illusion: bright inside color spreads into enclosed area (b/c center-surround receptive fields?)
Optical illusions and visual phenomenon
http://michaelbach.de/ot/index.html
Motion aftereffect: http://michaelbach.de/ot/mot_adapt/index.html
Lilac chaser: http://michaelbach.de/ot/col_lilacChaser/index.html
Perception problems for computers
Stimulus on receptors is ambiguous
Need top-down knowledge
Segmentation
Visual separation
Speech segmentation
How to deal with visual or verbal noise
Occlusions or obscured
Blurred or degraded
Changes in shadowing (lightness/darkness)
Human perception is different due to bottom-up AND top-down processing
Mitroff & Scholl (2005) http://michaelbach.de/ot/mot_mib/index.html
Method:
Motion-induced blindness (MIB)
Objects superimposed on global motion pattern
Change target objects during MIB
MIB effect
Objects fade from awareness while looking
Mitroff & Scholl (2005) http://michaelbach.de/ot/mot_mib/index.html
Questions:
What happens if change unseen objects?
What is the role of conscious visual awareness in formation
of object representations?
Can object representations not only be formed but also
updated in the face of changing dynamic scenes without
awareness?
Are representations re-formed and updated in response to
unseen visual changes?
Mitroff & Scholl (2005) Method
Ss press a key when experience MIB (dots disappear) and when reappear Choice: simultaneous, one disk, not sure
IV: line changes (50% chance that connecting line changes)
Mitroff & Scholl (2005) Results
Report reappeared simultaneously if dots connected by bar, even if connection occurred during MIB
Object representations can be formed and updated without awareness (conscious perception)
MIB with
Grouping Cues
Result:
More likely to
reenter awareness
together if grouped
together by gestalt
principle!
Current research in perception Presentations: Psychonomic society conference 2011
Metacontrast masking is processed before color-grapheme synesthesia
Action-specific effects are immune to spiders Perception is influenced by ability to act; examined perceptual processing
of spiders (Conclusion: spiders were faster than balls)
Your first organization influences your second: Hysteresis in figure-ground assignment Manipulate how people perceive figures
Pitch height independently modulates perceived time durations
Blindness enhances tactile acuity and haptic 3-D shape discrimination
Relative velocity and relative strength of illusory line motion
Heuristic spatial updating across abrupt perspective changes in dynamic scenes
Color, music and emotion
Auditory perceptual learning through multimodal training
Chapter 3: Perception Research questions
What are the processes responsible for perception?
How do we recognize objects?
Why is perception difficult for computers?
Methods
Identify objects in pictures or read words With or without “noise”
With or without prior information (context)
Indicate what you see with an illusion
Results
Requires combination of bottom-up and top-down processing
Use (gestalt/heuristic) rules of perceptual organization
Experience-dependent plasticity: depends on experiences
Future directions
Chapter 3
Perception & Action
Perception and action
Question: Does interacting with an object change perception?
Examine perception and movement
Example from textbook: how to pick up cup of coffee on table
What are the steps of the process?
What are the mechanisms responsible?
Dissociations Single dissociation
1 function absent while other present
Double dissociation
Opposite pattern in 2 or more Ss
Examples
Broca’s (can’t produce language) vs. Wernicke’s (can’t understand language)
Oliver Sachs’ case study stories
Use: case studies (lesions), imaging, experimental methods
Why is it useful?
Examine if functions are independent
Determine localization of function
Ungerleider & Mishkin (1983)
Method with monkeys:
Object discrimination task
Landmark discrimination problem
IV: lesion site (temporal lobe vs. parietal lobe)
Result:
Object discrimination deficit with temporal lesion
Landmark discrimination deficit with parietal lesion
Milner & Goodale (1995)
D.F.: temporal damage
Visual agnosia: can’t give size, shape or orientation of objects
Tasks (IV: orientation)
Match orientation of card to slot
Put card into slot
Conclusion:
Action intact; perception deficit
James et al. (2003): fMRI study of D.F.
Task 1: ID 2-D objects vs scrambled
Task 2: Reach and grasp 3d objects
(shapes) w/ or w/o pinch
Conclusion: damage to ventral stream;
lateral occipital complex
“What” and “Where” pathways
Conclusion: 2 independent processing streams
Temporal lobe - ventral stream
Perception of objects – “what”
Construct perceptual representation of visual world
Parietal lobe - dorsal stream
Location and action on objects – “where”
Visual control of actions directed at objects
New model by Goodale:
Both streams process structure and spatial attributes of object
Processes are for different purposes: Ventral: object parameters for perception
Dorsal: parameters for control of action
Goodale’s research http://psychology.uwo.ca/faculty/goodale/research/ Two visual systems (dorsal/ventral streams)
Visuomotor psychophysics How are sensory inputs transformed into motor acts
Grasping illusions Deceive eyes but not hand
Virtual grasping Judgments of object size dependent on presence of other objects but
grasping is not (absolute measurement)
Active exploration of objects Actively rotating 3d objects on computer led to faster object
recognition vs passive view group
The effects of retinal motion on reaching
And much more…
Whitney, Westwood, & Goodale (2003). The
influence of visual motion on fast reaching movements
to a stationary object. Nature, 423, 869-873.
Task: Ss focused on bulls-eye; touches screen as quickly as possible where flashing object presented
IV: vertical drifting pattern; upwards or downwards; with unpredictable switch in direction
Conclusion: motion changes reach trajectory
Examine time-course of influence of motion
Perception & action Presentations: Psychonomics Conference 2011
Discrimination of slowed rhythms mimics beat perception impairments in Parkinson’s disease
Action alters object identification: Wielding a gun creates a bias to see guns
Improved visual cognition through stroboscopic training (Mitroff)
Planning for object displacements required walking and reaching (Rosenbaum) How do we decide which paths to take to pick up and carry objects?
What factors are taken into account?
Spatial perception during joint action
Space is perceived from an expanded perspective
Reaching for the star: Rapid reaching influenced by learning reward and probability (Enns) Stimuli that are highly rewarding in a context are associated with
privileged perceptual processing