1 / 67

Thresholds, Weber ’s law, Fechner’s three methods

Thresholds, Weber ’s law, Fechner’s three methods. Research Methods Fall 2010 Tamás Bőhm. Introspection. Method applied by early psychologists (Wundt) Self-observation of conscious thoughts and sensations Do I perceive this light brighter than that? Why?

Faraday
Download Presentation

Thresholds, Weber ’s law, Fechner’s three methods

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Thresholds, Weber’s law, Fechner’s three methods Research Methods Fall 2010 Tamás Bőhm

  2. Introspection • Method applied by early psychologists (Wundt) • Self-observation of conscious thoughts and sensations • Do I perceive this light brighter than that? Why? • Inherently subjective rejected as a scientific method • But it provides useful intuitions when designing experiments (helps forming hypotheses) • I realized that it is hard for me to hear very high and very low tones. Thus the frequency of a tone may be a factor in its perceived loudness/audibility. Let’s test this in an experiment!

  3. Quantifying the sensations evoked by physical stimuli Gustav Fechner (1860): established 3 formal psychophysical methods Fechner’s methods have been (and are being) widely used More reliable and accurate than introspection Results from different experiments are comparable Formal methods

  4. If a linear relationship is assumed, two values determine the function: X-intercept: minimum stimulus value that evoked any sensation;absolute threshold Slope: the rate at which sensation grows as we increase intensity;difference threshold(inversely proportional to slope) Thresholds Linear psychophysical equation Sensation magnitude X-intercept slope Stimulus intensity

  5. Thresholds General definitions (not assuming linearity): • Absolute threshold:intensity that the observer can just barely detect • Intensities below absolute threshold: undetectable • Intensities above absolute threshold: detectable • Difference threshold (aka. just noticeable difference /JND/and difference limen):minimum intensity difference that is noticeable to the observer • A change in intensity that is smaller than the difference threshold: undetectable • A change in intensity that is larger than the difference threshold: detectable

  6. Linear function difference threshold (slope) is constant An observer able to detect the difference between intensities 100 and 110 should also be able to detect the difference between 1000 and 1010. This is not the case: the observer is able to detect the difference only between 1000 and 1100 500 & 550 Hz tones 5000 & 5050 Hz tones 5000 & 5500 Hz tones Difference threshold is not constant! Linear psychophysical equation Sensation magnitude constant slope Stimulus intensity Difference thresholds

  7. Difference threshold is not constant (changes with intensity) function is nonlinear Weber’s law: difference threshold is a constant proportion of the initial stimulus value ΔI / I = c Previous examples:c=10% Weber’s lawholds only approximately! Difference thresholds Nonlinear psychophysical equation Sensation magnitude slope changes with intensity Stimulus intensity

  8. Absolute thresholds • Even in the absence of stimulation, there is some random firing on sensory nerves • This inner noise can even vary from moment to moment • Observers cannot distinguish inner noise from the effect of a weak stimulus • Even when there is no light (perfect darkness), observers may experience a dim light (dark light, intrinsic light) • Observers in an anechoic chamber often report hearing a whistling sound  Measuring truly „absolute” thresholds is problematic: observers may confuse inner noise with the real thing

  9. Psychophysical methods • Threshold measurements: detection of small intensities (absolute thr.) and discrimination of small intensity differences (difference thr.)Is it intense enough to see? How small a difference can you see? • Fechner’s 3 methods • Method of constant stimuli • Method of limits • Method of adjustment • Modifications of Fechner’s methods • Staircase method • Modifications of the method of constant stimuli (adaptive, no standard) • Forced choice, objective methods • Sensory decision theory (SDT) • Psychophysical functions from psychometric data • Direct scaling: growth of sensation with intensityHow bright do you see a light? • Magnitude estimation and the power law • Multidimensional scaling: degree to which stimuli are comparable along some dimensionsAlong which dimensions do you judge the similarity of two stimuli?

  10. Fechner’s three methods • Presenting one stimulus at a time • The stimulus is very weak • Possible responses:“Yes, I see it.” /“No, I don’t see it.” • Presenting two stimuli at a time: • Standard: fixed, easily detectable • Comparison: either more or less intense than the standard • Possible responses:“Comparison is stronger.” /“Comparison is weaker.”

  11. Weak Strong Method of constant stimuli for measuring absolute thresholds Light intensity • Select a range of light intensities from certainly invisible to certainly visible • Pick a few (4-7) points uniformly in this intensity range; this will be the constant stimulus set

  12. Method of constant stimuli for measuring absolute thresholds • Test each stimulus many times (20-25) in random order …

  13. Method of constant stimuli for measuring absolute thresholds • Present the stimuli one at a time and ask the observer if it was visible or not Visible?YES NO

  14. Click to start

  15. Could you see the spot of light?

  16. Could you see the spot of light?

  17. Could you see the spot of light?

  18. Could you see the spot of light?

  19. Could you see the spot of light?

  20. Could you see the spot of light?

  21. Could you see the spot of light?

  22. Could you see the spot of light?

  23. Could you see the spot of light?

  24. Could you see the spot of light?

  25. Could you see the spot of light?

  26. Could you see the spot of light?

  27. Could you see the spot of light?

  28. Could you see the spot of light?

  29. Method of constant stimuli for measuring absolute thresholds • Calculate the proportion of “yes” and “no” responses at each light level 0% 5% 20% 50% 80% 95% 100% + - + + + - - - + + + - - - - +

  30. Method of constant stimuli for measuring absolute thresholds • Plot the percentages against stimulus intensity psychometric function 100% 75% 50% Percentage “seen” 25% 0% Stimulus intensity

  31. Ideal FIG (Sekuler) Fixed absolute threshold Step function Actual FIG (Sekuler) Absolute threshold varies somewhat from trial to trial (due to constant fluctuations in sensitivity) Conventionally, the intensity corresponding to 50% is considered to be the threshold Psychometric functionfor absolute thresholds sigmoid function

  32. Standard stimulus: Comparison stimuli: Method of constant stimuli for measuring difference thresholds • Standard stimulus has a fixed intensity • The intensities of comparison stimuli bracket the standard Light intensity

  33. Method of constant stimuli for measuring difference thresholds • All pairs of standard and comparison stimuli are tested many times

More Related