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Perception

Perception. Max Wertheimer. Credited with the founding of the Gestalt school of thought based on his description of the phi phenomena – the illusion that a light is moving from one location to another

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Perception

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  1. Perception

  2. Max Wertheimer • Credited with the founding of the Gestalt school of thought based on his description of the phi phenomena – the illusion that a light is moving from one location to another • The major implication of his research was the the phi phenomena cannot be reduced to the stimulus elements presented to the subjects – the subjective experience of motion is the result of a dynamic interaction between an observer and the stimuli

  3. Wolfgang Kohler • Considered the most systematic of the early Gestaltists, his many publications gave Gestalt psychology definitive form • He applied a Gestalt interpretation to the acquisition of relationships between stimuli, as opposed to learning the absolute value of stimulus dimensions • His work with chimpanzees demonstrated that they used insightful strategies in solving puzzles rather than relying only on trial-and-error learning

  4. Kurt Koffka • The most prolific of the three, he introduced Gestalt psychology to a wide American audience through his publication in 1922 of “Perception” • They initially focused on perception because that is where Wundt’s attention was given and they were out to attack his theory

  5. The Problem of Perception • How might perception by studied? • Marr’s computational approach: • Different levels of attacking the problem • What is the goal of the process? • How is the process carried out from input to output? • How is it realized physically? • But before all that…

  6. Sensation • The nervous system’s job is to allow us interact appropriately with our environment. • Electrical/chemical process • We are exposed to an enormous amount of stimuli. • To deal with this, our perceptions can be biased toward certain interpretations relative to others

  7. What is sensation? • How does physical energy from the environment get encoded as neural signals? • Sensation: • Process by which our sensory receptors and nervous system receive and represent stimulus energies from our environment.

  8. What is sensation? • Sensation vs. Perception • Sensation is not all we require to make sense of world (“to see the bear”) • Sensation: detecting physical energy.... • Perception: How we select, organize, and interpret the information we sense. • Active process, involves imposing order on stimuli • Sensation provides “raw” information (stimuli) that is selected, organized, etc.

  9. Basics of Sensation: • Sensation involves converting one type of energy into another. • Energy from environment – to neural impulses. • External Stimulus (energy) – big, furry, smelly bear • Stimulus takes different energy forms... • see bear: light waves... • That energy interpreted by receptors. • see bear: light waves received by photoreceptors in retina • Convert that energy into form brain can understand. • Transduction: Stimulus is converted into neural impulses

  10. What do we sense/detect from the environment? • We do not detect all of the stimuli that are present. • Senses are limited or restricted. • Absolute Threshold • Based on stimulus properties • The minimum stimulation necessary to detect a particular stimulus (about 50% of time) • But is there really an absolute threshold?

  11. What do we sense from the environment? • Signal Detection Theory (SDT) is used to predict how & when we will detect a stimulus. • Considers: • Strength of signal • Absolute thresholds vary – not inherent to the stimulus. • Situational differences (expectations, motivation, fatigue) • Individual differences (experience) • Based on Information Science • Outcome Matrix

  12. Outcome Matrix True State Signal Noise False Alarm “Signal” HIT Decision Correct Rejection “Noise” Miss • Compute parameters based on cell proportions that describe the decision process

  13. Components of SDT • Information quality lies on a continuum • “Information” can represent any type of cognitive representation • It can be seen as perceptual effect or perceived quality of the stimulus • Stimulus brightness, memory strength, etc. • There are distributions along the continuum • Noise • Information in the background of the signal • Signal • The true information

  14. Components of SDT • Criterion • Decision point set by the system • Based on the context and processing objectives • Decision • Probabilistic “yes/no” decision • Based on: • Criterion • Relative position of signal and noise distributions (discriminative ability)

  15. SDT model “NO” “YES” Criterion Noise Probability effect of that perceptual intensity will occur Signal Perceptual Effect (e.g. loudness)

  16. SDT model Criterion Noise Signal Correct Rejection HIT False Alarm Miss

  17. Parameters of SDT • Variables that affect proportions of responses: • Sensitivity • d’ • Criterion • Liberal or conservative

  18. SDT model d’

  19. SDT model • When d’ decreases… the person is less discriminating

  20. SDT model • When d’ increases…the person is more discriminating

  21. SDT model • When criterion shifts right… the cutoff for a decision now reflects a more conservative position • Results in more misses/correct rejections, fewer false positives

  22. SDT model • When criterion shifts left… the cutoff for a decision now reflects a more liberal position • More false positives/hits, but fewer misses

  23. Receiver Operator Characteristic (ROC) Curve L N C

  24. So • What will eventually be perceived is dependent upon factors not just related to the stimulus alone (including payoff), but by the cognitive system interpreting that information • Other considerations • Sensory adaptation • Subliminal perception

  25. The visual system • Humans and other primates devote much in the way of neural resources to processing visual information • Information flows from the eyes through the subcortical structures to the primary visual area at the back of the brain • From there, it flows along cortical paths to more anterior portions of the brain for further processing

  26. The visual system • Light enters in the eyes and is detected by photoreceptors (rods and cones) • The receptor cells synapse onto bipolar cells and these onto ganglion cells, whose axons leave the eye and form the optic nerve, which goes to the brain • Information from the left visual field travels to the right side of the brain and vice versa

  27. The visual system • Ganglion cells initially encode the visual field, and this information is combined through higher visual processing to form various features • The visual system can process vast amounts and very detailed information • E.g. Individual neurons in the visual cortex may fire most to stimuli of a particular orientation • The hemispheres themselves may be more attuned to processing certain types of information

  28. Basic Principles of Low-level Visual Perception • Visual perception can be divided into an early phase, in which shapes and objects are extracted from the visual scene, and a later phase, in which the shapes and objects are recognized • Certain aspects of our environment are distinguished prior to other properties of stimuli • Low-level vs. High-Level Perception

  29. Properties of Low-level Visual Perception • Segregation • Figure-Ground Relation: how do we choose the main object in a scene • Not just determined by the stimulus alone (e.g. reversible figure below) • Contrast • Spatial Frequency

  30. X X X X X X X X X XXXXXXXXX Properties of Low-level Visual Perception • Proximity and Similarity • We tend to group objects that are similar or near each other X X X O O O X X X O O O

  31. Properties of Low-level Visual Perception • Good Continuation

  32. Properties of Low-level Visual Perception • Distance/Depth • Perceptual constancy • E.g. Size constancy • Objects are perceived as the same size regardless of retinal size • Texture gradients • Elements of a surface appear more tightly spaced together at further distances • Occlusion

  33. Properties of Low-level Visual Perception • Motion Parallax • Relative motion • Nearby objects appear to move more quickly • Retinal Disparity • Difference in size of the retinal image between eyes provides information about position • More disparity suggests a closer object

  34. Cyclopean vision • Bela Julesz (1971) • Depth from retinal disparity of spatial cues • Depth perception can take place very early on, before object recognition • Computer stereograms

  35. Higher-Order Visual Perception • Object/Pattern recognition • Featural Theories • Viewpoint dependent • Structural Theories • Viewpoint invariant (object-centered)

  36. Featural Theories • What is a FEATURE? • Elementary units used in the representation of objects • All objects are composed of (perceptually meaningful) features • Cognitive system is set-up to understand and construct features • Decompose objects into features and match existing inventory of features to representation (decomposed object)

  37. Template Matching • Template Matching • Features for a given object are held in a template (in memory). • Input is compared at the element level • Everyday example: check-sorting machines matching numbers on bottom

  38. Feature Analysis • Treisman • Items with “features” will be seen as separate from background • Pop-Out • Basic Features • Color, Curve, Orientation, Angles, etc.

  39. Feature Analysis • Features are processed first, then their combination • Nonconscious process • What is perceived are the patterns • Advantages over template matching • Reduces the number of ‘templates’ that would be required for processing • Physiological correlations • Neurons fire specifically to such simple features • Behavioral evidence • Objects with common features are more slowly distinguished (e.g. C and G) • In studies where images are fixated on the retina (eye movement not a factor), features drop out over time while whole patterns may remain until image vanishes

  40. Featural Theories • Problems with Template/Feature Analysis Theories • Computationally complex • How many templates? Even slight changes in view, luminance etc. would change object representation • Do not describe spatial relations of features • How distinguish T from + ? • How are the features combined to what is actually perceived (i.e. the object) • Structural Theories • Describe “features” • Explain how features are organized spatially

  41. Structural Theories • Biederman • Recognition by Components (RBC) • Extension of Marr & Nishihara (1978) • Set of fundamental shapes called Geons (geometric icon) • Spatial alphabet • Extract geons from the scene • Compare spatial relations of geons to existing representations (in memory) • Geons defined by their vertices (points of intersection) • Geons defined by properties of true structure • Viewpoint invariant relations • Properties of true structure (non-accidental) • Smooth continuation • Cotermination • Parallelism • Symmetry

  42. Recognition by Components • Three stages: • The object is segmented into a basic set of subobjects • Early visual processing • The subobjects are then classified as to which geons they correspond to • The arrangement of the geons specifies the object to be recognized based on past experience • Think of it like word recognition

  43. Problems with structural theories • Underlying physiology not necessarily compatible • Neurons respond to smaller differences in stimuli than those between different geons (e.g. bird beaks) • Objects could be so complex as to be difficult to process efficiently in this manner • Evidence shows that different orientations are less readily recognized • What of non-shape features?

  44. Summary • Structural vs. Featural/template theories • Structural • Object-centered: based on set of objects and inter-relations of the parts of the object • Featural • Viewer-centered: based on the stimulus array appearing to the view at that time, and transforming the information to match previous experience • Neither accounts for the entirety of research findings • Perhaps both? Neither?

  45. Global Precedence • Featural and Structural theories assume that perception is based on features of some sort, but do we process features necessarily? • Global Precedence • Gestalt principles • The “structure” of elements depends on the “meaning” of the whole. • Navon letters

  46. Navon (1977) • When people process a visual display, do they recognize the features or the overall pattern more quickly? • Featural/structural theories predict features should be recognized more quickly.

  47. Navon (1977) • Two viewing orientations • Local • Global • Comparisons in RT to correct identification • Local vs. Global orientations • Interactions between orientation and stimulus type • Stroop-like interfence Inconsistent Consistent Neutral

  48. Navon (1977)

  49. Face recognition • Processed in global fashion • Different from typical object recognition?

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