1 Introduction. 1 Early Philosophy of Perception. 1 Early Philosophy of Perception (cont d)

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Introduction Sensation and perception The processes by which physical energy impinging on the senses is converted into electrochemical energy by the senses, transmitted to the brain for adaptive

Introduction Early Philosophy of Perception Nativism and Empiricism Psychophysics Biology of Perception Early Philosophy of Perception Plato s The Allegory of the Cave (380 BCE) Early Philosophy of

1 Introduction Sensation and perception The processes by which physical energy impinging on the senses is converted into electrochemical energy by the senses, transmitted to the brain for adaptive interaction with the environment. Introduction Early Philosophy of Perception Nativism and Empiricism Psychophysics Biology of Perception Early Philosophy of Perception Plato s The Allegory of the Cave (380 BCE) Early Philosophy of Perception (cont d) perceptual system as a filter What kind of filter? perception and thereby sense of reality evolved: survival individual animals rely on different types of energy sensory ability tailored to the environment we are selective perceivers Early Philosophy of Perception (cont d) some 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

Some Animals are Able to Sense Stimuli that Humans Cannot Early Philosophy of Perception (cont d) Heraclitus (540 480 BCE): You can never step into the same river twice.

2 Some Animals are Able to Sense Stimuli that Humans Cannot Early Philosophy of Perception (cont d) Heraclitus ( BCE): You can never step into the same river twice. both world and perceiver are constantly changing idea that perceiver cannot perceive the same event in exactly the same manner each time Early Philosophy of Perception (cont d) Adaptation: A reduction in response caused by prior or continuing stimulation light over time. sun to dark basement dark basement to sun Early Philosophy of Perception (cont d) Democritus ( BC): The world is made up of atoms that collide with one another, and the sensations caused by these make contact with our sense organs perception is the result of the physical interaction between the world and our bodies primary qualities and secondary qualities touch versus vision Early Philosophy of Perception (cont d) Sensory transducer: receptor that converts physical energy from the environment to neural activity Nativism and Empiricism Approaches to perception nativism the mind produces ideas that are not derived from external sources innate and created ideas empiricism experience from the senses is the only source of knowledge 2

3 Nativism and Empiricism (cont d) Descartes ( ) dualist view of the world dualism mind and body exist separably mind body dualism: originated by Descartes, the idea positing the existence of two distinct principles of being in the universe: spirit/soul and matter/body Nativism and Empiricism (cont d) monism mind and matter are formed from, or reducible to, a single ultimate substance or principle of being materialism physical matter is the only reality, and everything including the mind can be explained in terms of matter and physical phenomena. Materialism is a type of monism. Nativism and Empiricism (cont d) Hobbes ( ) believed that everything that could ever be known or even imagined had to be learned through the senses Nativism and Empiricism (cont d) Nativism and Empiricism (cont d) empiricism: experience from the senses is the only source of knowledge Nativism and Empiricism (cont d) Locke ( ) sought to explain how all thoughts, even complex ones, could be constructed from experience with a collection of sensations tabula rasa recovery from blindness What can you see? 3

Nativism and Empiricism (cont d) Berkeley (685 753) studied ways in which perception is limited by the information available to us through our eyes Fechner (80 887) invented psychophysics, founder of

limited distance of objects infer distances from cues in the image To be is to be perceived The Dawn of Psychophysics (cont d) psychophysics the science of defining quantitative relationships between

4 Nativism and Empiricism (cont d) Berkeley ( ) studied ways in which perception is limited by the information available to us through our eyes Fechner (80 887) invented psychophysics, founder of experimental psychology work relating changes in the physical world to changes in our psychological experiences all of our knowledge about the world comes from experience, even if perception is limited distance of objects infer distances from cues in the image To be is to be perceived The Dawn of Psychophysics (cont d) psychophysics the science of defining quantitative relationships between physical and psychological (subjective) events The Dawn of 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 The Dawn of Psychophysics The Dawn of Psychophysics (cont d) Weber ( ) 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 How much weight must you add to a: tea cup? 0lb bag of sugar? How much light must you add to a: flashlight in basement? flashlight in outdoor sunlight? Knock on door (with/without background) 4

The Dawn of Psychophysics (cont d) Weber Fraction k = ΔI/I k = constant I = intensity of the standard Example 00 gram weight 03 gram weight 3 grams = JND k = 3/00 =.

5 The Dawn of Psychophysics (cont d) Weber Fraction k = ΔI/I k = constant I = intensity of the standard Example 00 gram weight 03 gram weight 3 grams = JND k = 3/00 =.03 Fechner s Law: A principle describing the relationship between stimulus magnitude and resulting sensation magnitude such that the magnitude of subjective sensation increases proportionally to the logarithm of the stimulus intensity S = k log R S = perceived stimulus intensity (sensation) R = stimulus intensity (reality) True: yes if the standard is not too close to the threshold Fechner s Law Psychophysical Methods Overview Quantitative Classical Threshold Theory Absolute threshold (Fechner, 860) Difference Threshold Weber (834) Above Threshold Magnitude estimation (Stevens, 950 s) Signal Detection Theory Green and Swets (960 s) Physical stimulus Detection -> absolute threshold Two physical stimuli Discrimination -> difference threshold Above detection/discrimination Psychophysical function magnitude of physical magnitude of the perceptual 5

6 Detection The Dawn of Psychophysics (cont d) DETECTION absolute threshold minimum amount of stimulation necessary for a person to detect a stimulus 50% of the time e.g.,» how much intensity do you need to detect a light at 500µ Absolute Detection Threshold Assumption Questions Least intense sound I can hear? Least intense light I can see? Smallest concentration of chemical I can taste? Intensity exists for each observer, above which the observer detects the stimulus, below which, the observer does not detect the stimulus % detect 00 0 Physical intensity Methods Methods for Determining Detection Threshold Method of limits Methods of constant stimuli Method of adjustment 6

Method of Limits Method of Limits Method of Limits 5-9 stimuli varying in intensity Least never detected Most always detected present in ascending/descending order observer responds when he/she

7 Method of Limits Method of Limits Method of Limits 5-9 stimuli varying in intensity Least never detected Most always detected present in ascending/descending order observer responds when he/she detects data - function Percent detection as f(stimulus intensity) threshold - arbitrary but often 50% or 75% Method of Common Stimuli Method of Limits Method of Constant Stimuli Example use??? Eye doctor. Potential problems? 5-9 stimuli varying in intensity Least never detected Most always detected present in random order observer reports which he/she detects data - function Percent detection as f(stimulus intensity) Method of Constant Stimuli Method of Adjustment Method of Adjustment observer adjusts an analog device that controls stimulus intensity until he/she just detects stimulus Multiple trials can be used Example Von Bekesy audiometer 7

8 Accuracy constant stimuli Speed adjustment Evaluation Discrimination The Dawn of Psychophysics (cont d) DISCRIMINATION Methods minimum difference at which two stimuli can be distinguished examples two-point threshold two different frequencies two different colors Discrimination threshold Reliably discriminate between two stimuli % different 00 0 Physical intensity change Weber s Weight Study Smallest difference in weight that can be reliably discriminated Just noticeable difference (JND) Weight difference that was discriminable depends on the reference weight Above Threshold 8

Above Threshold Magnitude Estimation Directly relating the physical and the psychological Magnitude Estimation Method Steven s Power Law to describe findings Present a standard stimulus Rate it as

9 Above Threshold Magnitude Estimation Directly relating the physical and the psychological Magnitude Estimation Method Steven s Power Law to describe findings Present a standard stimulus Rate it as having an intensity of x Present a comparison stimulus How intense is it? Possible results. Linear response Response Compression Psychological Intensity Psychological Intensity Physical Intensity Physical Intensity Magnitude Estimation Response Expansion Psychological Intensity Physical Intensity 9

10 Steven s Power Law ψ = kµ n ψ = perception k = constant µ = Physical stimulus intensity Signal Detection Theory n = linear n > response expansion n < response compression What is wrong with Classical Absolute Threshold Theory? Who is the better detector? % detect 00 0 Subject A Subject B detects stimulus intensities smaller than those detected by Subject A But.. % detect 00 Physical intensity Subject B The problem of guessing????? 0 Physical intensity Decision Bias Decision Bias Factors Can you trust a subject s response? Not really.. individual differences (in personality) liberal responders conservative responders task demands conservative liberal 0

11 Method -> Practical Response = f(detection ability and decision making) We want to know detection ability independent of decision making factors (response bias) Trick subject include null ( dummy trials) measure bias directly the number of dummy trials the S falls for Things to remember yes response stimulus hit reality no stimulus false alarm =bias yes reality adds up to 00% accuracy is on the diagonal bias is in the rows no miss correct rejectio n =bias no =00% =00% d = z(hit rate) - z(false alarm rate) Signal Detection Theory signal + noise noise Model of the detector and/or discriminator yes hit false alarm Green and Swets (966) response no miss correct rejection

12 Model Components Amorphous perceptual continuum Model Components Noise model - normally distributed Instantaneously varying. physiological? N Add constant strength signal to the noise Add constant strength BIG signal (gunshot) N N+S N N+S Add constant strength TINY signal (pin drop) Decision threshold β, or criterion C N N+S N N+S Say no Say yes 2

13 Model Components Decision threshold β Decision threshold β Liberal responder Conservative responder N N+S N N+S Say no Say yes Say no Say yes hits - yes to a stimulus event (S+N) false alarms - yes to non-event (N) hits False alarms correct rejection - no to a non-event (N) misses - no to a stimulus event (S+N) Correct rejections Misses 3

14 Model Measures d = z(hit rate) - z(false alarm rate) d Say no Say yes Model Measures C = -.5(z(hit rate) + z(false alarm rate)) d d is the perceptual distance between the mean of the noise and the signal+noise distributions. Perceptual magnitude of the signal for the detector Say no Say yes Most important thing How response data changes with β shift A response bias free measure of the accuracy of detection A hit and false alarm? Because you measure bias and parcel it out d Response = f(detection ability and decision making) Response = f(d and C) Say no Say yes 4

B hit and false alarm? 90 40 hit and false alarm?

15 B hit and false alarm? hit and false alarm? C 80 0 d d Say no Say yes Say no Say yes Receiver Operating Characteristic (ROC) Curve hit and false alarm? D 60 0 B A d p(false alarm) C D Say no Say yes p(hit) Receiver Operating Characteristics (ROC) Model Why this works.. Assumptions about normality. 5

16 Concrete Example Detection Example Air port metal detectors.. Noise - non-gun-carrying passenger Signal - passengers with guns Detect Theory Threshold... Ice cream and low fat ice cream Experiment = 200 trials 00 with fat 00 without fat d = z(hit rate) - z(false alarm rate) Model Measures yes response no signal + noise hit 60 miss 40 noise false alarm 0 correct rejection 90 d = z(high fat rate) - z(low fat rate) d Say low fat Say high fat high fat response low fat high fat hit 60 miss 40 low fat false alarm 0 correct rejection 90 d = z(hit rate) - z (false alarm rate) d = z (.60) - z(.0).53 = d is in z-score units High d? Low d? Negative d? 6

17 Final Considerations Conclusions. Cautions Normality assumptions? Non-parametric versions (A ) Non-linearity in the extreme values corrections for these Signal Detection Theory (Part ) Signal Detection Theory (Part 2) Signal Detection Theory (Part 3) Signal Detection Theory (Part 4) 7

Doctrine of specific nerve energies (Müller, 80 858): nature of a sensation depends on which sensory fibers are

Perception The Twelve Cranial Nerves Cranial nerves: Twelve pairs of nerves that originate in the brain stem and

(vesitbular-cochlear) (VIII) nerves Eye movements Oculomotor (III) Trochlear (IV) nerves Abducens (VI) nerves

18 Doctrine of specific nerve energies (Müller, ): nature of a sensation depends on which sensory fibers are stimulated, not on how fibers are stimulated Biology of Perception Biology of Perception (cont d) Biology of Perception The Twelve Cranial Nerves Cranial nerves: Twelve pairs of nerves that originate in the brain stem and reach the periphery through openings in the skull Pure sensory olfactory (I) nerves optic (II) nerves Auditory (vesitbular-cochlear) (VIII) nerves Eye movements Oculomotor (III) Trochlear (IV) nerves Abducens (VI) nerves Biology of Perception (cont d) Helmholtz (82 894): Studied activity of neurons; how fast they transmit signals Resonators, ophthalmoscope 8

19 Biology of Perception (cont d) Synapse: the junction between neurons that permits information transfer Biology of Perception (cont d) Neurotransmitter: A chemical substance used in neuronal communication at synapses The Action Potential 9

Chapter 1 Introduction. Welcome to Our World Thresholds and the Dawn of Psychophysics Sensory Neuroscience and the Biology of Perception

Chapter 1 Introduction. Welcome to Our World Thresholds and the Dawn of Psychophysics Sensory Neuroscience and the Biology of Perception 1 Introduction Chapter 1 Introduction Welcome to Our World Thresholds and the Dawn of Psychophysics Sensory Neuroscience and the Biology of Perception Introduction What do we mean by Sensation & Perception?