lecture 3: psychophysics

psychophysics

-the relation between subjective perceptual experience and the objective physical stimuli that gives rise to such experience

-asks a range of questions such as:

• what is the smallest difference between two stimuli we can detect?

• how is perceived magnitude related to stimulus intensity?

• what are the absolute limits of perception?

history of psychophysics

-school of psychology that dates back to Victorian times

-early theorists look at the applied philosophy that people have different perceptual interpretations of physical stimuli

importance of psychophysics to the real life

-help us understand how our sensory systems work

-comparison of psychophysical thresholds between different groups allows us to understand how perception is different for certain populations

-relevant to new technologies like VR

-use for corporations e.g., shrinkflation

measuring perception

• describing → indicating characteristics of a stimulus

• recognising → placing a stimulus in specific category

• detecting → becoming aware of a barely detectable aspect of a stimulus

• perceiving magnitude → being aware of the size or intensity of a stimulus

• searching → looking for a specific stimulus among a number of other stimuli

the absolute threshold

-the smallest amount of stimulus energy necessary to detect a stimulus

-three basic tests for determining the absolute threshold:

• limits

• adjustment

• constant stimuli

method of constant stimuli

-auditory duration perception

-give two auditory tones and ask participants to determine which one was longer

-one tone was the standard, which is a fixed stimulus and does not change

-other tone was the comparison and would change with each trial

-aim is to see what is the smallest difference between the comparison and the standard that the participants can detect

psychometric function (method of constant stimuli)

-for each participant, plot the data on a graph

-psychometric function is a graph that shows an observer’s response as a function of the stimulus level or intensity

-like a line of best fit

just-noticeable difference (JND)

-the smallest difference between two stimuli that can be detected

-also called the difference threshold

-minimum difference in the intensity of two stimuli necessary to detect they are different

-allows psychologists to understand the relationship between the physical intensity of a stimulus and people’s perception of it

relative thresholds

-the smallest difference between to stimuli that can be consistently and accurately on 50% of trials

-amount that a stimulus of standard intensity must be changed in order for a difference to be noticed

-on a graph → use the line of best fit to find corresponding values to determine the JND

Weber’s law

-states that when the standard is more intense, then detecting the differences from another stimuli is more difficult

-so, as the magnitude of the stimulus increases, so does the size of the JND → JND is a constant proportion of the stimulus intensity

example of Weber’s law

-if you were holding a feather in each hand, and added a pea to one hand then you would be able to distinguish there had been a change in weight to one hand

-whereas, if you were holding a dumbbell in each hand, and you added a pea to one hand, then it would be much harder to distinguish the change in weight

Weber’s law formula

ΔP / P = k

breakdown of Weber’s law formula

ΔP / P = k

P = the intensity of the standard (the stimulus being presented to the participant)

ΔP = the JND

K = the percentage change in the comparison for the stimulus to be detected (should remain the same as a constant proportion, for comparing standards across people)

specific values for each precept (Weber’s law)

• pitch → 0.003 (0.3%)

• lifted weights → 0.02 (2%)

• loudness → 0.04 (4%)

• pressure on skin → 0.14 (14%)

absolute thresholds

-about the limits of perception and so there is no comparison of two stimuli

limits of perception

• the quietest sound we can hear

• faintest light we can see

• lightest touch we can feel

-our sensory system allow us to experience only a limited part of our physical environment

-we cannot take in an unlimited amount of information and there is a lower limit of what we can take in

absolute threshold tasks

-there is not two stimuli and the participant is only presented with one stimulus on each trial

-they are asked to say when they can detect it

-once the results are plotted on a graph, look at the 0.5 point of the psychometric function → 0.5 is the point at which the stimulus is detected

threshold finding methods

• method of constant stimuli → requires fitting a psychometric function

• method of limits → does not require fitting a psychometric function

• method of adjustment → does not require fitting a psychometric function

finding the threshold - method of constant stimuli

-experimenter presents 5-9 stimuli with different intensities in random order

-stimuli are pre-selected by the experimenter and run in random order

-a psychometric function is fit to the results

-focus on 0.5 point in the threshold

finding the threshold - method of limits

-presents stimuli in either ascending order or descending order

-step-up and step-down approach

-if the stimulus is not detected then the intensity is increased slightly

-once the participant has detected the stimulus, the researcher will move the stimulus intensity to a point well above what the participant can detect

→ they will then move downwards, decreasing the intensity of the stimulus, until the participant can no longer detect it

-this shows a crossover point to help pinpoint the exact stimulus intensity that the participant can and cannot detect

finding the threshold - method of adjustment

-observer or the experimenter adjusts the stimulus intensity continuously until the observer can just barely detect the stimulus

-participants are asked to adjust the stimulus themselves

-find their own perceptual threshold

evaluation of method of constant stimuli

+precise and uses a large range of stimuli

-takes a long time, experiments are boring so it is difficult to retain attention, thus it is not appropriate for all types of population

evaluation of method of limits

+faster, the stimulus that is chosen is calibrated to the individual participant’s performance, so there is no time wasted testing stimulus that is nowhere near their threshold

-performance can be impacted by order effects as participants sit through trials and adapt to the stimuli that they just experienced therefore the perception of the new stimuli is different because of the stimuli that was just experienced

evaluation of method of adjustment

+quickest

-can be difficult for participants as they may not know what they are being asked to do and it may be influenced by participant’s biases

signal detection theory

-all perceivers typically exhibit some bias

-bias is influenced by the costs and benefits of the response outcomes

-for example, different response criterion

-participant 1 has a low response criterion

-participant 2 has a high response criterion

-signal detection theory mathematically helps us to distinguish perceivers’ sensitivity from their bias

different response criterion

-when using the method of constant stimuli, participant 1 may say ‘yes’ even if there is only the slightest possibility they could see the light

-however, participant 2 may only respond ’yes’ when the light is completely visible to them

magnitude

-magnitude is measured by asking a participant to assign the standard a perceptual value e.g., 10

-they are then presented with another stimulus and asked to judge it relative to the standard that was given

-the results can be used to create magnitude functions

-the line length is the participant’s perception of the stimulus intensity

-different responses are due to how our sensory systems are adapted to our environment

response compression (magnitude)

-some lines have larger increases at smaller values

-once the stimulus becomes more intense then these increase slow down and flatten because it is harder for the participant to detect the changes

response expansion (magnitude)

-where the stimulus intensity and perceived magnitude are not proportional

-for example a participant may be given a very weak electric shock and the intensity will be gradually increased, but once the intensity is increased enough for larger electric shocks then their perception of the magnitude increases and it is no longer proportional

-the shock intensity doubles but the participant’s reaction more than doubles

Steven’s power law formula

P = KS^n

breakdown of Steven’s power law formula

P = the perceived magnitude

S = stimulus intensity

K,n = constants (specific to the sensory systems being investigated)

Steven’s power law - response compression

• n < 1

-e.g., brightness, loudness, vibration on skin

-our sensory systems are calibrated to the certain environments that we are in, and so change/adapt when we change our environment

Steven’s power law - response expansion

• n > 1

-e.g., electric shock, taste of salt, muscle force

-these are all examples of things where our body could actually be harmed