Week 4 - Psychophysics & signal detection theory (sensation in general) Flashcards
Briefly describe definition of sensation, perception & cognition
Sensation = eyes, ears, organs which convert physical/ chemical info into signals that our NS can understand
Perception = we acquire information from the environment. How we process that info to form internal representations of the environment. We are making sense of the information we are given and interpreting it.
Cognition = how we use the internal representations to do more “complex” things.
Describe the (2) different theories of threshold
1) Difference threshold = smallest change in stimulus that can be detected. E.g. two weights (40grams vs 41grams) can you tell the difference? Also referred to as JND (just noticeable difference)
2) Absolute threshold = absolute values sensory system can. have. The minimum intensity of a stimulus that can be detected. E.g. what is the lightest/ smallest weight you could possibly detect?
What is the Law’s associated with Difference Threshold?
Weber’s Law = the JND (just noticeable difference) value can be function of the difference stimulus. JND is function of the magnitude of a reference stimulus. You make the ratio of the two weights, which will be constant (e.g. 1g - 40g: 10g - 400g).
Weber fraction = constant ratio of JND & Intensity of a reference stimulus
What is Fechner’s Law related to?
Fechner’s Law = if a Weber fraction is constant for a given stimulus dimension, then, the mind might use the Weber fraction as a unit for perceiving that stimulus dimension. E.g. on a ruler = having a constant 1cm is needed for measurement. We need a constant unit.
So, Fechner’s Law: built on Weber’s findings. If a Weber fraction is constant for a given stimulus dimension, then the mind might use the Weber fraction as a unit for perceiving that stimulus dimension,
What are the psychophysic implications of Fechner’s Law?
Fechner’s law asserts that our psychological experience of the intensity of a stimulus tends to change less quickly than the actual change in stimulus intensity.
E.g. measuring light bulb on a scale VS our subjective experience of when lightbulb goes from 1-3. Our mind is slower than reality. Or, complete darkness to tiny bit of light = the change is massive and we can immediately feel this. However, if you keep increasing the dial of brightness, we don’t feel the change is much.
Physical stimulus intensity goes up = our mind lags behind. Underestimate change towards the higher end
How do we measure thresholds?
For each magnitude of the stimulus, you can get proportion of yes responses. Plot proportion of detections occurring at EACH STIMULUS magnitude.
The threshold is taken as the magnitude at which the stimulus is detected a criterion proportion of the time. E.g. 50%
What does it mean by defining the threshold?
Defining the threshold at 50/50 level - where the threshold is, is where the responses are evenly split so we decide this must be the threshold (Method of Constant Stimuli)
Disadvantage of method of constant stimuli = requires pre-testing to get quick rough comparison threshold.
Method of Constant Stimuli?
Entire profile of people’s sensory response on a graph
50/50 split
Method of Limits
- Uses ascending & descending series of trials. E.g., presented at a supra-threshold or sub-threshold level.
- Increase/ decrease intensity until participants can no longer detect the stimulus.
- Quickly & reasonably accurate
- Disadvantage = response habituation, might overshoot yes/ no
Staircase Procedures
- Ascending/ descending series. Keep going up and down until you get mixed responses.
- When you get the reversal, you start the next ascending run at that point of previous reversal.
- Pick the number of reversals you need, then calculate the average.
- You can tweak the staircase procedure to estimate values along psychometric function. E.g. you might require two ‘Yes’ before moving down.
- Can switch stimuli order - responses change between yes and no to avoid response habituation/ overshooting
