Anthony J Greene 1

THE STUDY OF PERCEPTION

Anthony J Greene 2

Chapter 1 OutlineI. Why study perception?

1. Perception is reality

2. How we percieve.

3. Historical Approaches

II. Scientific Study of Perception:The Scientific Revolution: Hypothesis, Data & Theory

1. FunctionalismThe problem of Perception:

• Psychophysics

• Evolution

2. Structuralism

• Neuroscience

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Why Study Perception?

• What we get from perception

• Perception is our only source of information: we have no knowledge, or experience except through perception

• Perception allows survival

• The utility of perceptual systems informs us about why they evolved

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The way we perceive

• Perceptual systems are incredible–Nothing man-made is even close

• The mechanisms of perceptual systems inform us about how they evolved

• Sensory enhancement (glasses, hearing aids),

• Sensory substitution

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A Song of Ourselves

• Perception is not always veridical

• What we are able to perceive

• We are fundamentally perceptual beings

• Thought, memory and experience are perceptual (either directly or indirectly)

• Art, Music, Food, Physical Sensations etc.

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LemonA man makes a pictureA moving picture.Through the light projectedHe can see himself up close.Man captures color,Man likes to stare,He turns his money into lightTo look for her;She is the dreamer,She's imagination.--U2

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Philosophical & Historical Approaches to Perception

Realism

Subjectivism-e.g. Democritus, Plato

Dualism-e.g. Descartes

Materialism-e.g. Bacon

Nativism-e.g. Plato

Empiricism-e.g. Socrates

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Science

Derived from the Philosophies of Empiricism & Materialism–

Fact & Theory

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Fact & Theory • Facts must be observable (data)

• Theory = understanding

• Theory is not hypothetical

• Theory is broad, fact and hypothesis are narrow

• Theories must be consistent with all available (relevant) facts

• Theory guides the search for fact

• Facts are only important if they inform theory

• Theory is more important than fact

• The progress of theory is the purpose of science

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The Advancement

of Theory

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Scientific Approaches to perception

Functionalism (purposes of perception)1. Evolution

2. Psychopohysics

Structuralism (mechanisms of perception)1. Neuroscience

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Perceptual SystemsVision• Object Identification/recognition• Spatial Vision: Navigation & Motion PerceptionAudition• Object Identification/recognition• Object LocalizationTouch• Object Identification/recognition• Pain (detection of tissue damage)• ProprioceptionGustation & Olefaction• Chemical detection and identification• Nutrition & and poison avoidance

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Problem of Perception

1 Cornea

2 Lens

3 Retina

4 Optic Nerve

5 Brain

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Proximal vs. Distal StimulationProximal Stimulus is upside down• The brain is not looking at retinal picturesProximal stimulus is 2 dimensional (Depth

Perception)• 3rd dimension is lost from distal to proximal,

however we perceive in 3 dimensions• How does then do we experience a 3rd

dimension?

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Perceptual Experience Mirrors Distal Stimulation

1 Size Constancy

2 Shape Constancy

3 Position Constancy

4 Brightness Constancy

5 Color Constancy

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Size Constancy

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Size ConstancyIf object moves 2x further away, the retinal image decreases by a factor of 2, but we do not perceive it to shrink

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Shape Constancy

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Position Constancy

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Brightness Constancy

A light meter would read that the right side of the panel is

white and that the left side is gray

Visual systems interpret them both as white

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Color Constancy

• Under different ambient lighting conditions, the mondrian will reflect different frequencies (as measured by a light meter). e.g. green light reflected off a red surface would be read by a light meter as orange or yellow

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Brightness Constancy

• Visual systems compensate for ambient lighting, so that under almost all conditions the colors appear stable

• There are instances when there does not exist a correspondence between distal image and perception (illusions and ambiguity)

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Illusions• Cases where our perceptual experience is inaccurate

• How does the brain get tricked?

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Illusions

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One proximal stimulus produces many perceptual experiences

Perceptual experience is not just a function of what hits the eye

The man bent over his guitar

Perceptual Ambiguity

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The Man With The Blue Guitar

A shearsman of sorts. The day was green.They said, "You have a blue guitar,You do not play things as they are."The man replied, "Things as they areAre changed upon the blue guitar."And they said then, "But play, you must,A tune beyond us, yet ourselves,A tune upon the blue guitarOf things exactly as they are."--Wallace Stevens.

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Psychophysics

• Psychophysics: The science of defining quantitative relationships between physical and psychological (subjective) events

• Fechner (1801–1887) invented psychophysics, thought to be the true founder of experimental psychology

– Pioneering work relating changes in the physical world to changes in our psychological experiences

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Psychophysics (cont’d)

• Weber (1795–1878) discovered that the smallest change in a stimulus, such as the weight of an object, that can be detected is a constant proportion of the stimulus level: “Weber’s Law”

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Psychophysics (cont’d)• JND (Just Noticeable Difference): The

smallest detectable difference between two stimuli, or the minimum change in a stimulus that can be correctly judged as different from a reference stimulus. Also known as difference threshold

• Two-point threshold: The minimum distance at which two stimuli (e.g., two simultaneous touches) can be distinguished

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Psychophysics (cont’d)Fechner’s Law: Relationship between stimulus magnitude and resulting sensation magnitude is exponential.

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Stevens’ Power Law: Relationship between stimulus magnitude and resulting sensation magnitude. Exponent can be positive, zero, or negative.

Psychophysics (cont’d)

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EvolutionSome species sense energies that humans cannot:

– Bees see ultraviolet lights

– Rattlesnakes sense infrared energy

– Dogs and cats can sense sounds with higher frequencies

– Birds, turtles, and amphibians use magnetic fields to navigate

– Elephants can hear very low-frequency sounds, which are used to communicate

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Darwinian Evolution

Variation• Every species has

enormous diversity• Sexual reproduction

insures diversity by recombining genes into new combinations

• Variability allows a species (not an individual) to survive

Conch

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Darwinian Evolution

Selection - reproduction of the fittest

• Differential survival advantage

• Differential reproduction advantage

• No selection pressure after the age of reproduction

Tarsier

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Evolution of Accuracy and Acuity in Perception

• Strong Selection Pressure for accurate perception.• At every stage of evolution, organisms with better

perception gained a differential survival advantage• Better acuity• Larger range of detectable stimuli• Consistent representation of distal stimulus• Illusions Don't Occur in Natural Scenes -- Selection

pressure for perceptual systems not to be tricked (e.g. black light)

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Evolutionary Tree of Life

• We can think of variation as branching • And selection as pruning

• There is no distinction between micro- and macro-evolution.

• Species alive today are the tip of the branch, not the top of a ladder.

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Neuroscience:Review of Physiology

Central Nervous System (CNS)

• Consists of the brain and spinal cord

• Communicates with the Periphery (anything other than the brain and spinal cord)

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Nerves

•Efferent- outflow (CNS to Periphery)•Afferent- inflow (Periphery to CNS) Neurons•Motor - Associated with muscles (efferent)•Sensory - Associated with sense receptors (afferent)

•Interneuron - Rest of the CNS - Makes up pathways between motor and sensory neurons and the CNS. Most of brain.

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Cerebral CortexWhite Matter Vs. Gray MatterFissures & Sulci1 Central Sulcus2 Lateral (Sylvian) Fissure3 Longitudinal FissureLobes1 Temporal Lobe - Auditory Cortex - Language Processing - Object

Identification (Visual-Auditory-Tactile)2 Occipital Lobe - Visual Cortex3 Parietal Lobe - Somatosensory Cortex - Spatial Perception (Visual-

Auditory-Tactile)5 Frontal Lobe - Motor Cortex - Behavior Control - Planning/Strategy

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Cerebral Cortex

White Matter Vs. Gray Matter

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Corpus Collosum

A P

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Corpus Collosum

A P

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Cerebral Cortex

Central Sulcus

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Cerebral Cortex

Lateral Fissure

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Cerebral Cortex

Longitudinal Fissure

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Cerebral Cortex

Temporal Lobe

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Cerebral Cortex

Occipital Lobe

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Cerebral Cortex

Parietal Lobe

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Cerebral Cortex

Frontal Lobe

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Neurons

The Basic Neuron

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Neurons

The Synapse

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NeuronsThe Action Potential

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Perceptual Processes

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Perceptual Processes