Psychophysics

1. Psychophysics علی یونسی

2. What is psychophysics • Methods • Applications

4. Cones in Retina

8. 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

9. Linear psychophysical equation Sensation magnitude constant slope Stimulus intensity Difference thresholds • 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!

10. Difference thresholds • 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! Nonlinear psychophysical equation Sensation magnitude slope changes with intensity Stimulus intensity

11. 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

12. 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?

13. 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.”

14. 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

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

16. 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

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46. 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% + - + + + - - - + + + - - - - +

47. 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

48. Psychometric functionfor absolute thresholds sigmoid function 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

49. 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

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