What is difference thresholds?
- the smallest change in a stimulus that can be detected
Just noticeable difference
What is absolute threshold?
The minimum intensity of a stimulus that can be detected
The absolute threshold is the quietest (lowest intensity) sound that a person can just barely hear
What is a threshold in psychology?
The stimulus intensity value at which a stimulus is just detected or stops being detected
What is webbers law
In regards to difference threshold
The size of the JND depends on the size of the original stimulus, not the absolute difference.
For example, If a weight has to be 41 g before it can be discriminated from a 40 g reference weight (JND = 1 g), then the JND would be 10 g for a 400 g reference weight
Your senses respond to proportional differences, not absolute ones.
So as the stimulus gets larger, you need a larger change to notice the difference.
What is Fechners law?
If Webers fraction is a constant for a given stimulus, then maybe our brains might use Weber’s fraction as a unit for perceiving that stimulus dimension
What does Psychophysics mean
Something that can help us relate our internal experience (like how bright a light is) and physical objective things in the external environment
(how i feel the brightness of the light related with how bright the light actually is)
Implications of Fechner’s law
Focuses on absolute intensity, not just differences between stimuli.
Shifted research from difference thresholds (Weber) to absolute thresholds (minimum detectable stimulus).
Shows that our perception doesn’t increase linearly with physical changes in stimulus intensity.
How do you measure threshold?
- Method of constant stimuli
- Method of limits
- staircase procudures
What is the method of constant stimuli
- create an approximate threshold value
- construct a set of stimuli with differing magnitudes above and below the threshold value
- present these stimuli to the participant in random order
- each trial, participants respond with whether they detect the stimulus
- plot the proportion of stimulus detection at each magnitude (e.g. 50% detected sound at 500HZ)
In the method of constant stimuli, is it normally clear when a stimulus becomes detected/not detected
It is not always clear
normally a curved graph rather than a straight one
Advantages of method of constant
- it allows the shape of the psychometric function to be established
- provides an accurate estimate of threshold
Disadvantages of the method of constant
- requires pre-testing to develop an indication of where the threshold may be
- waste a lot of trials which lie far from the threshold (time consuming) - many trials are wasted at levels that are too easy (always “yes”) or too hard (always “no”).
- Not suitable for tracking quick changes in threshold (e.g., during fatigue or adaptation)
What is method of limits
- measures threshold without developing the shape of the psychographic function
Stimuli are presented in order (either ascending or descending). When the person can detect or no longer detect the stimulus is the point of threshold
threshold measure when there is a reversible point - eg. a person says yes to stimulus when they have previously said no
Advantages of the method of limits
- more efficient (quicker) than the method of stimuli
- still reasonable accurate in determing threshold
Disadvantages of the method of limits
- many trials are still wasted as they are presented far away from the threshold value
- participants may habituate and get used to saying yes or no and thus overshoot the true threshold
- overall shape of the psychometric function cannot be determined
What is the staircase procedure
staircase procedure is designed to overcome the inefficiency (wasted trials) of the other two threshold mesures
- Start with a sound or light that’s easy to detect (or too weak).
- If the person detects it, make it harder (lower intensity).
- If they don’t detect it, make it easier (higher intensity).
- Every time their answer changes (from “yes” to “no” or “no” to “yes”), it’s called a reversal.
- Keep going until you get several reversals.
- The threshold is the average of those reversal points.
Advantages of staircase procedure
- more efficient than the method of limits
- can be modified in a number of different ways to overcome limitations
For the staircase procedure what does it normally estimate?
Normally estimates the 50% threshold → the point where a participant detects the stimulus about half the time.
You can change how the staircase behaves to find different thresholds:
Example: Requiring two “yes” responses before lowering intensity makes the test harder,
→ finds the 70% threshold (where stimulus is detected 70% of the time).
Disadvantages of staircase method
estimation of the threshold seems to be a bit more complex
The concept of ‘noise’ in psychophysics
We can never perceive stimuli under the perfect condition
There is always some noise, even when there are no
stimuli in the environment
we can never know whether we are perceiving the true stimuli (signal) or the noise
What is signal detection theory
Signal Detection Theory explains how we decide whether a signal is present amid noise.
It separates what you can detect (sensitivity) from how you decide (bias).
The separation between the signal+noise and noise
distributions tells us how sensitive an observer is to that
stimulus
What is the measure of sensitivity called? (in signal detection)
is called d-prime (d’)
What is sesnativity d
d′ (d-prime) measures how well you can tell the difference between signal+noise and noise alone.
Larger d′ → better detection (signal and noise more clearly separated).
Smaller d′ → harder to distinguish signal from noise.
What is the hit rate and false alarm rate
The Hit rate tells how often you detect real signals
False Alarm rate tells how often you get tricked by noise.
Comparing these two shows how sensitive you are and where your decision line (criterion) is set.
