Cognitive Semester 1 Week 3: Perception II - Psychophysics Flashcards
What does psychophysics explore?
Explores the relation between subjective perceptual experiences and the objective physical stimuli that give rise to that experience. Our perceptions don’t always match up to reality.
Describe an example of auditory duration perception.
Play two tones, one is the standard duration and one is the comparison which changes each time. The participant reports which was the longer tone. Repeat each comparison many times and for each participant plot the data. Fit a psychometric function (a best fit line) to the data.
What is a psychophysical function?
Represents the general relationship between physical stimulus intensity and the resulting psychological perception. Links the physical characteristics of a stimulus with the subjective experience or response it produces in an individual. Psychometric functions are more specific.
What is the just-noticeable difference (JND)?
The smallest difference in the intensity of a stimulus that a person can detect.
What is the significance of 0.75 in perception studies?
This is usually where the JND is.
0.75 falls between 50% where they’re just guessing and 100% where they know every time. It’s the point where they can just notice a difference.
What is Weber’s law?
The JND is a constant proportion of the stimulus intensity. ΔP / P = k
P: Intensity of standard - chosen by experimenter, always constant for each experiment but varies between experiments.
ΔP: JND - dependent on the individual
K: Percentage change in the comparison needed for the change to be detected (constant).
- Discriminating differences is more difficult when the standard is more intense (e.g., longer).
What is an absolute threshold?
- Minimum intensity of a stimulus that an individual can detect.
- Experiments: no standard, participants asked whether they detect a stimulus when presented.
- Threshold at 50% where stimuli are detected 50% of the time.
Three threshold finding methods
Requires psychometric function:
1) Method of constant stimuli
Does not require psychometric function:
2) Method of limits
3) Method of adjustment
Method of constant stimuli
Stimuli are pre-selected by the experimenter and run in random order, then a psychometric function is fit to results.
Most precise but slowest.
Method of limits
Increase intensity step by step, stopping when they detect the stimulus. Start higher and decrease step by step, stopping when they no longer detect it. Take the average of points where change is detected as the threshold.
Faster but affected by order effects.
Method of adjustment
Participants adjust intensity themselves. Repeat with different starting intensities. Used more in early psychophysics experiments.
Quickest but dependent on participant cooperation.
What is magnitude estimation?
Assign perceptual value to standard (e.g., 10). Present another stimulus, ask participant to rate magnitude with respect to standard. Plot a magnitude function which varies with stimuli type.
- for line length, this function is linear
- for brightness, lower intensities = greater increase in perceived magnitude, higher intensities = smaller increase in perceived magnitude (response compression)
Stevens’s power law
P = KS^n
P: Perceived magnitude
S: Stimulus intensity
K,n: constants (specific to percept under investigation)
- Response compression (n<1) - e.g. brightness, loudness
- Response expansion (n>1) - e.g. electric shock, muscle force
Adaptation - we want to perceive dangerous things as having a greater magnitude, whereas things such as brightness we encounter often.
What would be the answers to a perception test if it was only bias and no sensitivity?
All yes or all no regardless of experience.
Why might we get different results between participants in perception tests?
- One participant may be more sensitive
- Bias: E.g. “if i’m not sure i’ll just say yes” (response criterion)
Bias may be influenced by the costs and benefits of the response outcome
What is signal detection theory?
An approach to unpick the participant’s sensitivity (ability to detect signal) from their bias (tendency to say yes or no).
Describe a signal detection experiment.
Use 1 single low-intensity tone that is difficult to hear. Stimulus presented on some trials, and no stimulus on other trials. Participant responds yes/no to whether a stimulus was presented.
What are the four possible outcomes in a signal detection experiment?
Hit: Correctly says yes to a stimulus presented.
Miss: Incorrectly says no to a stimulus presented.
False alarm: Incorrectly says yes to no stimulus presented.
Correct rejection: Correctly says no to no stimulus presented.
How can the percentage of hits and false alarms be changed?
By manipulating motivation with payoffs. For example, making rewards so that participants can earn the most money by responding yes, increasing their motivation to say yes more frequently.
What is a receiver operating characteristic (ROC)?
Percentage of hits plotted against percentage of false alarms. If the ROC for two people is the same, they have the same sensitivity.
What are probability distributions in signal detection theory?
Trials are named noise or signal + noise. A probability is calculated for the likelihood that a perceived loudness is due to noise or the signal.
