1 Weber s Law: background. 1 Weber fraction (k) depends upon dimension:

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1 1 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 1 Weber s Law: background Question: Is the difference threshold constant for different intensities? Answer: No, difference threshold changes with stimulus intensity 1 Weber s Law:? I / I = k or? I = k*i? I = difference threshold I = stimulus intensity k = Weber fraction 1 Weber fraction (k) depends upon dimension: brightness:.08 loudness:.05 heaviness.02 line length.03 electric shock.01 1

2 1 Example: Brightness: Candle Example (k =.08)? I =.08 * 1 candle need increase of.08 candles to notice change? I =.08 * 500 candles need increase of 40 candles to notice change 1 Example: Heaviness Weight example (k =.02) ex: weight 100 grams? I =.02 * 100 grams = 2 grams 100 => 102 grams ex: weight 200 grams? I =.02 * 200 grams = 4 grams 200 => 204 grams 1 The Dawn of Psychophysics (cont d) 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 1 Fechner s Law S = k log R S = sensation intensity k = constant (adjusts scale) R = physical stimulus intensity Derived from Weber s law JNDs perceptually equal to one another JNDs increase with stimulus magnitude 2

3 1 Magnitude Scaling: S.S. Stevens Relation between: Stimulus magnitude magnitude of perceptual experience 1 Direct Scaling -- S. S. Stevens Tested variety of dimensions Technique: present a standard stimulus assign arbitrary value (e.g, 10) present another stimulus Subject assigns number (magnitude) if twice the perceived magnitude ==> 20 if half the perceived magnitude ==> 5 1 The Dawn of Psychophysics (cont d) Stevens Power Law:A principle describing the relationship between stimulus magnitude and resulting sensation magnitude, such that the magnitude of subjective sensation is proportional to the stimulus magnitude raised to an exponent 1 Steven s Power Law Stevens data fit by power function S = ai b S = sensation intensity a = scaling factor I = physical intensity b = exponent that differs by dimension Similar in purpose to Fechner s Law Better fit More flexible (could account for ALL dimensions) (next slide) 3

4 1 Power Law -- effect of exponent 1 Magnitude Estimation b< 1 concave down -- response compression Psychological grows slower than physical b>1 concave up -- response expansion Psychological grows faster than physical b = 1 -- direct relationship Psychological grows at same rate as physical 1 Steven s Power Law: Not necessarily a linear relationship between physical and psychological 1 The Dawn of Psychophysics (cont d) Signal Detection Theory: A psychophysical theory that quantifies the response of an observer to the presentation of a signal in the presence of noise 4

5 1 Signal Detection Theory (Part 1) 1 Signal Detection Theory (Part 2) 1 Signal Detection Theory (Part 3) 1 Signal Detection Theory (Part 4) 5

6 1 Primer on Neuronal Physiology 1 Anatomy of neuron Transduction -- stimulus energy to neural signal Receptors cause other neurons to respond Dendrites Receives input Cell Body Integrates input & decides Axon Carries signal Axon Terminals Transmits signal to other neurons 1 How do neurons carry information Neurons have a resting potential of -70 mv Intracellular and extracellular fluid Contain ions (charged particles: +/- ) More negative ions inside vs. outside (-70 mv) 1 Neurons as information processors Incoming info => changes in membrane potential Input -- two flavors EPSP - excitatory postsynaptic potentials IPSP - inhibitory postsynaptic potentials Reach threshold -- action potential occurs 6

7 1 EPSP & IPSP Figure 1 EPSPs & IPSPs: Membrane potential + ions outside neuron -- pressure to enter neuron electrostatic pressure diffusion (higher concentration outside) ion channels in neuron s membrane 1 Communication between Neurons: Synaptic Activity Action Potential reaches terminal buttons Changes membrane potential Causes terminal buttons to release neurotransmitters (NT) NT diffuse across synaptic cleft NT binds with receptor sites 1 EPSPs & IPSPs: Ion Channels EPSPs -- excitatory NT bind with specific receptors These receptors: Open ion channels -- more + ions in Membrane potential increases IPSPs -- inhibitory NT bind with specific receptors These receptors: Close ion channels -- more + ions out Membrane potential decreases Membrane potential fluctuates with EPSP and IPSPs If Threshold reached = Action Potential 7

8 1 Single Cell recordings Insert electrode into a single neuron Record action potentials Firing rate = # of action potentials / time 1 Firing Rate Spontaneous firing rate [c] More EPSP -- increases firing rate [a] More IPSP -- decreases firing rate [e] 1 Final Issue: The Coding Problem Action potentials -- all the same How does a neuron carry information? How can it code stimulus properties? Doctrine of specific nerve energies (Müller, ): A doctrine stating that the nature of a sensation depends on which sensory fibers are stimulated, not on how fibers are stimulated Info about environment: Which neurons are firing Rate of firing Pattern of firing 8