Lecture 15 Experiment 3

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Lecture 15Experiment 3

• Martin Giese

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Programming the Experiments
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DrawPoints
High-level function that plays a movie with
dots DrawDots(f, A, t) plays sinusoidal tone
with frequency with frequency f
and amplitude A for t seconds Playsound(f, A, t,
An, fn, Bn) plays sinusoidal tone with band pass
background noise with mid frequency fn and band
width /-Bn
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Reading In Key Presses
Simple example function
function key_demo()   initialization ret_
strg wait for a valid key
press while strcmp(ret_strg, 'a', 'l') 0,
disp('Enter String.) ret_strg
getchar end actions for admissible
letters a and l if strcmp(ret_strg, 'a') 1,
disp('A') elseif strcmp(ret_strg, 'l')
1, disp('L') end
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Data Analysis
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Data Analysis

• Thresholds from probability data
• 50 threshold
• Cool (optional) fit logistic threshold function
• Thresholds directly from adjustment methods /
  stair case
• Statistics for comparing thresholds
• t-test (pairwise comparisons) if enough items
  watch for (de)pendent measures !
• non-parametric test for few items
• ANOVA (next lecture)
• Extremely cool (idea for final project)run 2AFC
  experiment and estimate d

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Data Analysis

• Correction of the a level
• Problem test difference between 2 AFs
• Given N threshold values for K values of the
  frequency ?? Akn and Akn
• Possibilities
• t-test (pairwise comparisons) ? K independent
  tests
• Better ANOVA (next lecture !)
• How can we combine the results from K independent
  tests?

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Data Analysis

• P(single test significant even though H0 true) lt
  a
• P(single test non-significant if H0 true) gt 1-
  a
• Tests independent
• P(all K tests non-significant if H0 true) gt (1-
  a)K
• P(at least one out of the K tests significant if
  H0 true) lt 1 - (1- a)K aeq

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Data Analysis
Equivalent significance level for K independent
tests aeq 1 - (1- a)K ?? a 1 - (1-
aeq)1/K For large K a ? aeq/K (Bonferoni
correction)
Example K 4 aeq 5 a 1.27
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Experiments
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Measuring Percept Probability Dependent on Aspect
Ratio

• Aspect Ratios 0.5, 0.75, 1, 1.2, 1.5, 2
• 3 display cycles
• Register if subject saw horizontal or vertical
  motion or the whole trial
• Neglect trials with a perceptual switch
• Calculate percept probabilities
• 30 repetitions per condition
• 3 subjects

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Measure Switching Probability Dependent on Aspect
Ratio

• Aspect Ratios 0.5, 0.75, 1, 1.2, 1.5, 2
• 6 display cycles
• Register if subject saw at least one perceptual
  switch during the trial
• Calculate switching probabilities
• 20 repetitions per condition
• 3 subjects

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Measure Hysteresis Loop

• Aspect Ratios 0.5, 0.75, 1, 1.2, 1.5, 2
• Ascending and descending series

ascending 0.5-0.75 0.5-0.75-1 0.5-0.75-1-1.2
0.5-0.75-1-1.2-1.5-2
descending 2-1.5 2-1.5-1.2 2-1.5-1.2-1
2-1.5-1.2-1-0.75-0.5
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Measure Hysteresis Loop

• Register if subject saw at least one perceptual
  switch during the trial
• Plot P(horizontal was seen in ascending trials)
  and 1-P(vertical was seen in descending trials)
• Quantify area of the loop
• 20 repetitions per condition
• 3 subjects

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Quantify Visual Inertia

• Prime quartet with motion that forms angles a
  and 90o- a with the sides of the quartet
• Angles 0, 15, 30, 45, 60, 75, 90o
• Vary aspect ratio of the quartet in a staircase
• 5 reversals of change of the AR
• titrate the influence of the priming stimulus
  against the change of the aspect ratio (AR) of
  the quartet

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Quantify Visual Inertia

• Use deviation from aspect ratio without priming
  as measure for priming effect
• 1 repeat per subject
• 3 subjects