Differential sensitivity difference threshold Absolute threshold yes no
Differential sensitivity (difference threshold) • Absolute threshold (“yes” / “no”) • Difference Threshold, for detection of differences (“stronger” / “weaker”) • Io=standard και It=comparison (test) stimuli • detection of ΔI=It-Io at the level of Io • Just-Noticeable-Difference (JND): το ελάχιστο ΔI που μπορώ να ανιχνεύσω
Light intensity of comparison stimulus Method of limits for measuring difference thresholds + + + + threshold + estimate - + + - - Trials + comparison brighter - comparison weaker • Intensity of the comparison stimulus is decreased (descending) or increased (ascending) until the response changes • Threshold estimate: intensity difference between the standard and comparison stimuli where the response changes • Average results from multiple series in both directions
Percentage “stronger” Psychometric function for difference thresholds 100% 75% 50% 25% 0% PSE Light intensity of comparison stimuli • 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 (vs. POE => accuracy) slope = precision (does the threshold represent accuracy or precision? )
Percentage “stronger” Psychometric function for difference thresholds 100% 75% 50% 25% 0% DW DS Light intensity of comparison stimuli • • • 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
(from Kusunoki, Moutoussis & Zeki 2006)
TOJ task
Ernst Weber discovered that the smallest detectable change in a stimulus is a constant http: //highered. mheducation. com/sites/007 3382736/student_view 0/perception/weber_ s_law__/index. html
DETECTABILITY-THRESHOLDADAPTATION Sensitivity changes so that a more intense stimulus is required to induce a perceptual response after adaptation than before adaptation. The psychometric function shifts rightward along the intensity axis as background intensity increases. Psychometric function plotting the probability of a “yes” response to the presentation of a small light increment against a uniform background. The horizontal axis represents the magnitude of the light increment and the three curves represent data obtained at three different background intensities. Re-plotted from Mueller (1951). Source: Mather, G. (2006). Foundations of Perception. New York, NY: Taylor and Francis.
Difference Thresholds: Quantification Difference thresholds depend on the reference value ! Stimulus strength Snodgrass et al. (1985)
this is a 2 AFC task…
Psychophysical (vs. psychometric) function
spatial modulation
Sinusoidal luminance gratings
temporal modulation time
equiluminance (M-system ‘blind’? ) equiluminant grating
A psychophysical function we have already seen:
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