Methods in Brain Research: psychophysics Shaul Hochstein

What is psychophysics? term coined by Gustav Fechner in 1860 relationship between: physical stimuli and subjective percepts; world events and personal experiences; physics and mind.

What is psychophysics? relationship between: physical stimuli and subjective percepts; world events and personal experiences; physics and mind. We know how to measure physical stimuli,, world events, physics. How do we measure subjective percepts, personal experiences and mind?

What is psychophysics? How do we measure subjective percepts, personal experiences and mind? Just ask! Experimenter=observer introspection. Measure behavior. - 3 ways leading to 3 different results

Psychophysics stimulus  task  method  analysis  measure

Example 1: Determination of contrast detection threshold. What is the minimum amount of contrast needed for a stimulus to be just detectable?

Psychophysics stimulus  task  detect circle method  analysis  measure

Psychophysics Methods: Method of limits: start with very invisible contrast, gradually increase until observer says, “yes”. OR: start with very visible contrast, gradually decrease until observer says, “no”.

Psychophysics Methods: Method of limits: start with very invisible contrast, gradually increase until observer says, “yes”. OR: start with very visible contrast, gradually decrease until observer says, “no”. Analysis: average point of switch from No to Yes and from Yes to No over many repetitions. Average is Measure of Detection Threshold

Method of Limits Adaptive Testing - Staircase Procedure Determination of Absolute Threshold. Response (Stimulus Perceived): yes (Y), no (N). Stimulus Alternating Ascending and Descending Series Intensity 0 N N N 1 N N N 2 N N N N 3 N N N Y N N 4 N Y N Y N Y 5 N Y Y Y Y Y 6 Y Y Y Y 7 Y Y Y Transition 5.5 3.5 4.5 2.5 4.5 3.5 Points Threshold = Average Transition Points = (5.5.+3.5+4.5+2.5+4.5+3.5)/6 = 24/6 = 4 Adaptive Testing - Staircase Procedure Staircase – increase each error, decrease each 2 correct

Method of constant stimuli: Psychophysics Methods: Method of constant stimuli: Used fixed set of contrasts in random order, plot % “yes” responses as function of contrast. Psychometric function % yes responses Stimulus intensity

Psychophysics Forced Choice: respond yes/no. Two Alternative Forced Choice: Present two alternative stimulation periods, one of which contains the stimulus, the other not. Observer responds which period contains the stimulus: the first or the second. Measure fraction of correct responses, assuming fraction will be 0.5 far below threshold, define 0.75 as threshold.

Psychophysics Methods: Method of adjustment: Observer adjusts stimulus intensity to point where he or she feels this is “just” detected, determining his or her subjective threshold.

Psychophysics Threshold Methods: Besides determining threshold itself, useful to compare thresholds for different conditions (e.g. wavelengths of light) to determine absorption/action spectra, compare spectra with/without drugs to determine effect, etc.

Psychophysics Methods: “detection” versus “discrimination” Is there a face in this picture? versus Is the face in this picture of a man or a woman? “threshold” versus “supra-threshold” j.n.d. or “just noticeable difference”

Psychophysics Methods 1AFC //// left/right oblique 2AFC \\\\ - //// 1st/2nd left? Same/Different \\\\ - //// same/different? 3AFC oddity \\\\ - \\\\ - //// which is odd? 2AFC sample: //// match to sample \\\\ - //// which matches? 2AFC //// - //// which pair has same/different //// - \\\\ difference?

Psychophysics Methods: Nulling:

Psychophysics Methods: Nulling:

Psychophysics Supra-threshold methods: paired comparisons: order/% order/jnd triads: is central stimulus closer to left or right? Magnitude estimation magic number 7

Psychophysics Other methods: Response timing: generally, observers respond faster when the task is easier. Magnitude estimation magic number 7

Psychophysics Noise: In world In Brain In mind

Signal Detection Theory Internal response = firing rate of neuron If have large range of uncertainty, how can we determine how good is the system?

Signal Detection Theory Probability Internal response

Signal Detection Theory d' = z (hit rate) – z (false alarm rate) Probability Internal response

Receiver Operating Characteristic Signal Detection Theory Receiver Operating Characteristic A B C Effect of shifting the criterion

Receiver Operating Characteristic Signal Detection Theory 2-Aletrnative Forced Choice Receiver Operating Characteristic A B C Effect of shifting the criterion 2 alternative forced choice method