Perception: Pattern or object recognition. Chapter 3

Transcription

1 Perception: Pattern or object recognition Chapter 3

2 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?

3 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

4 Bottom-up processing Cell s responses Stimulus 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)

5 McClelland & Rummelhart (1981) Interactive Activation Model

6 Pandemonium (Selfridge, 1959)

7 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

8 Recognition by components Biederman s RBC (recognition by component) theory 36 geons (3D) Basic building blocks Emphasis on intersections Recognition with missing information possible

9 Geons: Identify objects Principle of componential recovery Resistance to visual noise View invariant properties Discriminability

10 Biederman s Geons Intersections are important to recognition

11 Beyond bottom-up processing

12 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?

13 Tox-Doxn Pxocxssxng To xllxstxatx, I cxn rxplxce xvexy txirx lextex of x sextexce xitx an x, anx yox stxll xan xanxge xo rxad xt ix wixh sxme xifxicxltx Context and knowledge fills in the rest! The redundancy of stimuli provide more features than required

14 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

15 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

16 Treisman & Schmidt (1982) 1 3 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

17 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 Result By memory or expectation Accurate position in both conditions Prediction based on experience

18 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

19 Perception: Gestalt laws and Heuristics Chapter 3

20 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)?

21 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

22 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

23 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) CogLab Data from Spring 09 (N = 8) Expected result: For larger separations, the stimulus must "move" a farther distance, which presumably requires a greater length of time.

24 Gestalt law (or heuristics!) examples

25 Gestalt principles Group objects together to find the 13 faces

26 Gestalt law (or heuristics!) examples

27 Optical illusions and visual phenomenon Watercolor illusion: bright inside color spreads into enclosed area (b/c center-surround receptive fields?)

28 Optical illusions and visual phenomenon Motion aftereffect: Lilac chaser:

29 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 topdown processing

30 Mitroff & Scholl (2005) Method: Motion-induced blindness (MIB) Objects superimposed on global motion pattern Change target objects during MIB MIB effect Objects fade from awareness while looking

31 Mitroff & Scholl (2005) 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?

32 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)

33 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)

34 MIB with Grouping Cues Result: More likely to reenter awareness together if grouped together by gestalt principle!

35 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 figureground 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

36 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

37 Perception & Action Chapter 3

38 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?

39 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

40 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

41 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

42 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

43 Goodale s 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

44 Whitney, Westwood, & Goodale (2003). The influence of visual motion on fast reaching movements to a stationary object. Nature, 423, 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

45 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