Introduction to psychophysics

Tasks, sampling methods and measures• Measures: (how to characterise behaviour?)• Reaction times (how long to judge?). Atheoretical, but popular (e.g. IAT)• Stimulus(letter)Example I: Acuity• Task (letteridentification;10 alternatives)• Percent correct (what level of performance at a fixed stimulus magnitude?): e.g.observers memorise 10 objects & are presented with a new set containing 5 they saw and 5they hadn’t. Observer #1 recognises them all, observer #2 none; both score 50% correct...• Method (adjustment)• Point of subjective equality (stimulus mag. producing a perceptual match?)Appearance• Thresholds (minimum stimulus mag. producing some level of performance?).Absolute and relative...• Principled (signal detection theory). • Reliable/replicable• Efficient • VersatilePerformanceLetter sizeIssues:criterion,speed[1 2 3Trial #• Performancemeasure(average setting = sizethreshold)Example I: AcuityExample I: Acuity• Stimulus(letter)• Method (method of constant stimuli)Letter sizeCorrectIncorrect5 10 15 20Trial #Issues: efficiency/speed• Task(reading, 10AFC forced choice)TrialRun“B” ! “N” ! “O” " ...• Performance measure(acuity threshold)1.00.550.1PsychometricfunctionAcuity/sizethreshold[Acuity threshold:Size leading to79.2% correctidentification• Method (method of limits, adaptive, “3-down-1-up” staircase)Letter sizeIssues: efficient butdemanding(chart based)CorrectIncorrect5 10 15 20 25 30 35 40 45• Performancemeasure (threshold)Trial #Clinical visual acuity: 20/20means we can read letters20ft away, with line thicknessof 1.75mm (1 arc min.)