Psychophysics

Lecture 1

What is Weber’s Law?

Weber’s Law states that the smallest change in a stimulus that can be perceived, the just noticeable difference (JND), is proportional to the intial stimulus.

What are aspects associated with classical psychophysics?

Absolute thresholds (is there anything there?)

Difference thresholds (is there any difference?)

Scaling (Estimation of stimulus intensity - how much)

Multidimensional scaling

What is classical psychophysics theory?

Supra-threshold stimuli (strong stimuli) the relation between stimulus strength and experienced magitude. eg. what’s the relation between experienced loudness and actual dB

Near-threshold stimuli (weak stimuli) sensory limits (hearing tests)

The discrimination threshold and Weber’s law

the idea that to obtain 75% correct discriminations, stmuli must differ by a constant %, not a constant amount.

Fechner’s law

if a stmulus is multiplied by a fixed factor, the corresponding perception is altered in additive constant amounts.

Why is there no ideal thresholds?

• Threshold varies from trial to trial

• Random neural firing (noise) - if there is inner noise then participants can’t distinguish it from a weak stimulus

• Stim-magnitude varies between trials

Name 4 types of scaling methods

Magnitude estimation - estimate the sensory magnitude with a number

Ratio estimation - estimate a number by how much stronger a test stimulus is compared to a standard.

Ratio production - the inverse of ratio estimation

Magnitude matching/adjustment - with a number: adjust a stimulus to match a number

What is the YES-NO paradigm?

• respond yes if you perceive something and no if you perceive nothing

• results in bias (how liberal or conservative a person is answering)

• proportion false alarms (FA) at catch trials gives an estimation of guessing frequency

What is the two-alternative-forced-choice (2AFC) paradigm?

2 stimuli is presented, one has to be chosen and guessing if necessary.

liberated by the bias from the YES-NO paradigm

could result in systematic bias though or time-order effects

Method for threshold measurements - Method of constant stimuli

• A range of stimuli from very hard to relatively easy are selected.

• pick them uniformly and this will be the constant stimuli set.

• Decide answering format yes/no etc

• Test each stimuli many times (20-25) in random order

• Calculate the % of yes and no responses at each level

• Fit the psychometric function

What is the high threshold theory (HTT)

assumes that all events above threshold are detected

but threshold is influenced by trial-to-trial variability

all catch trials are below threshold but occasionally lead to false alarms due to guessing

What are the problems associated with HTT

correction for guessing is far from perfect

predicts linear relations between hits and false alarms (ROC-curve) but the actual relations are often curves!!!!!! not straight lines.

Assumptions of the Signal Detection Model

• assumes that there is no absolute threshold

• assumes that there is always noise present in the environment

• assumes that the noise can be internal or external and varies randomly

Procedure for SDT

• Participants are repeatedly presented to same S+N (weak signal + Noise) and N (no signal only noise) stimuli in random order.

• For each observaation (x) a decision has to be made whether x is a signal + noise (SN) or just noise (N)

The ROC curve

• receiver operating characterisitics

• plot hits against false alarms for all criterions C

what is d-prime

is useful when a researcher wishes to find out whether sensitivity is actually changed or whether a person is simply more willing to say Yes under some conditions than others

SDT and the criterion