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Haptics and Virtual Reality

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  1. Haptics and Virtual Reality

    Lecture 2: Haptic Perception M. Zareinejad
  2. Psychophysics Methodology for investigating relationships between sensations in the psychological domain and stimuli in the physical domain Central to experimental psychology
  3. Why Psychophysics? – HapticsResearch Procedure Determine specifications for a haptics application based on the human capability of perception and action Design and construct an appropriate haptic interface Develop adequate software library including haptic rendering algorithms
  4. MeasurementThresholds Sensorythreshold”isacentralidea Absolutethreshold -Sensitivity -Smallest amount of stimulus energy required to produce a sensation Differencethreshold -Resolvingpower -The smallest amount of stimulus energy change to produce a sensation or that can be reliably discriminated The smallest amount of stimulus energy change to produce a sensation or that can be reliably discriminated
  5. Differencethreshold -Amountofchanceinthestimulusrequiredto produceajustnoticeabledifference(JND)in thesensation
  6. JustNoticeableDifference:JND Theamountofchangeinastimulusthat createsaperceptibleincrementinsensation Example -Stimulusintensity=10units -Goesupto12unitsbeforeobservernoticesa change -Therefore,JND=2unitsatthatstimuluslevel
  7. Psychometric Function A function from stimulus intensity to probability to perceive the stimulus Usually a S-shaped ogive (cumulative normal distribution)
  8. 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.”
  9. MethodofConstantStimuli Absolutethreshold location Repeatsame 5-9stimuli Randomlypresent ~100x Askfeelit?yes/no Proportionof“yes’s”computedfor eachstimuluslevel Fitcurvetogetpsychometricfunction(usuallys-shaped) Threshold=0.5
  10. Weak Strong Method of constant stimuli for measuring absolute thresholds 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 Light intensity
  11. Method of constant stimuli for measuring absolute thresholds Test each stimulus many times (20-25) in random order …
  12. 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
  13. 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% + - + + + - - - + + + - - - - +
  14. 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
  15. 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
  16. 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
  17. Method of constant stimuli for measuring difference thresholds All pairs of standard and comparison stimuli are tested many times
  18. Method of constant stimuli for measuring difference thresholds For each pair, the observer judges if the comparison stimulus was stronger or weaker than the standard http://www.yorku.ca/psycho STRONGERWEAKER
  19. Method of constant stimuli for measuring difference thresholds For each comparison level, the percentage of “stronger” responses is calculated and results are plotted as a psychometric function 100% 75% 50% Percentage “stronger” 25% 0% Light intensity of comparison stimuli
  20. Psychometric function for difference thresholds When the observer cannot see a difference, he/she chooses randomly between “stronger” and “weaker”; this corresponds to 50% on the psychometric function point of subjective equivalence (PSE) 100% 75% 50% Percentage “stronger” 25% 0% PSE Light intensity of comparison stimuli
  21. Psychometric function for difference thresholds By convention, the intensity at 75% is considered to be just noticeably stronger than the standard DS A comparison intensity at 25% is just noticeably weaker than the standard DW Difference threshold = the average of DS and DW 100% 75% 50% Percentage “stronger” 25% 0% DS DW Light intensity of comparison stimuli
  22. On each trial, the observer reports if she/he could see the light or not. Start with presenting a light intensity well above the expected threshold (the observer can certainly see it) Decrease the intensity until the observer cannot see it Threshold estimate: the intensity at which the response changes + Light intensity - - - - Trials Method of limits for measuring absolute thresholds + + + + + - threshold estimate Descending series: start from above the expected threshold and decrease intensity Ascending series: start from below the expected threshold and increase intensity
  23. Ascending and descending series may yield different results use both Even in the same direction, there is variability in the threshold (inner noise, etc) average many measurements Measured threshold corresponds to 50% point in a psychometric function (method of constant stimuli) + + + + + + + + + + Light intensity + - - - - - - - - - - Trials Method of limits for measuring absolute thresholds pure-tone audiometry
  24. Weber’sFraction,1834GermanPhysiologistE.H.Weber Linearrelationship betweendifferential thresholdand stimulusintensity Forexample:tofeel different,2heavy weightsmustdiffer morethantwolight weights
  25. Fechner’s Law
  26. Fechner’s Law– Graphical Illustration
  27. Steven’s Power Law
  28. Steven’s Power Law -Examples
  29. Power law Continuum Exponent (a) Stimulus condition Loudness 0.67 Sound pressure of 3000 Hz tone Vibration 0.95 Amplitude of 60 Hz on finger Brightness 0.33 5° target in dark Lightness 1.2 Reflectance of gray papers Visual length 1 Projected line Visual area 0.7 Projected square Taste 1.4 Salt Smell 0.6 Heptane Cold 1 Metal contact on arm Tactual roughness 1.5 Rubbing emery cloths Tactual hardness 0.8 Squeezing rubber Heaviness 1.45 Lifted weights Viscosity 0.42 Stirring silicone fluids Electric shock 3.5 Current through fingers Vocal effort 1.1 Vocal sound pressure Duration 1.1 White noise stimuli