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Absolute and Difference Threshold, Scaling Studies, Signal Detection Theory.docx

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University of Guelph
PSYC 2390
Lana Trick

Absolute Threshold 9/6/2012 3:30:00 PM Psychophysics:  One of earliest kinds of psychology  Quantitative method to measure relationships between stimuli (physics) and perception (psychology)  Proving that mental processes can be measured quantitatively*** Gustav Fechner: Goal  1801-1887  physicist and philosopher  studied how physics of stimulus causes internal reaction (psychological reaction)  Elements of Psychophysics Ways to Measure Perception: describing (asking person to describe, phenomenological method), recognizing (categorizing, used with brain damage patients), detecting, perceiving magnitude, searching Detection: to sense it or not sense it  Absolute Threshold: smallest amount of stimulus energy necessary to detect a stimulus o High Threshold: stimulus must be really intense to be recognized  “high threshold for pain” o Low Threshold: low stimulus intensity needed to be sensed o As you increase the threshold, you decrease the sensitivity Findings Related to Absolute Thresholds:  Drugs (smoking) o Nicotine, carbon monoxide o Smokers increase their absolute threshold for taste (stronger cheese needed so they can taste the cheese)  Alcohol o Increase absolute threshold for bitter tastes o Scotch doesn’t taste as bitter after a while  Sex Hormones o Men have higher thresholds than women o Olfaction  Women have more acute sense of smell  Adaptive? Preparing food?  Goes down during menstruation  Testosterone increases threshold for sense of smell  Need intense smells o Vision  Men have lower threshold in daylight conditions  Can see dimmer light  Women have lower threshold in night conditions How is Absolute Threshold Measured: Classical Psychophysical Methods  Differences in accuracy and time needed to perform Method of Limits  Present stimuli in either ascending order (intensity increases) or descending order  Continues until participant no longer hears, sees etc. stimulus  Cross over point when answer turns from yes (I can see or hear) to no (I cannot see or hear)  Do experiment multiple times and average cross over points to get threshold o Ascending never saw light and will say no longer before saying yes o Descending saw light and will say yes longer before saying no  Slow and painful Intensity of Light Do you see it? Do you see it? 20 Y *ascending trial 19 Y Y 18 Y Cross Y 17 N over pt. Y Cross 16 N N over pt. 15 N N 14 *descending trial N Method of Adjustment  Experimenter adjusts the stimulus intensity (via dial) continuously until the observer can barely detect stimulus  Repeat several times and take an average of the setting  Like method of limits but faster o Fastest method  Less reliable than method of limits Method of Constant  Experimenter presents 5-9 stimuli with different intensities in random order o 100 trials of each intensity  Observer asks if they can perceive the stimulus at the given intensities  Repeat randomly and plot the frequency (percentage) of times the stimulus was perceived by the observer vs. the intensity  Threshold is intensity that results in detection on 50% of trials  Most accurate test o Random order, many observations o Time consuming Intensity Sound % of Times Heard It 1 10% 2 20% 3 40% 4 50% Absolute Threshold 5 70% 6 80% Difference Threshold 9/6/2012 3:30:00 PM Measuring Discrimination: the Difference Threshold  Described by Ernst Weber  Two different things you are comparing o Standard (what you start with) vs. Comparison (what you increase intensity to) Definition:  Difference Threshold: smallest difference between two stimuli that a person can detect  “Just Noticeable Difference” Finds Related to Difference Thresholds:  Smoking o Nicotine and especially carbon monoxide causes JND to increase (less sensitive to differences) for contrast sensitivity  Need a bigger difference in brightness / contrast sensitivity  At greater risk for collisions when driving at night (cannot distinguish between black and dark grey, cannot see pedestrians)  Colour discrimination (colours of the rainbow) o Marijuana and LSD users have higher JND for colour discrimination  Need bigger difference in colour in order for it to be detected How is the difference threshold is measured?  Same as absolute threshold but instead of seeing if they detect stimulus, see if observer detects a difference  Method of constants (for difference threshold) o 100 mg  standard o Have range of comparisons (97, 98, 99, 100, 101, 102, 103) o Is the comparison heavier?  Y or N answer  Even if both times were given 100 mg (comparison can equal standard)  Often will say comparison is heavier (if given equal weights)  Things that are near to you in time are considered more intense  “negative time error”  remember pain more vividly the next day than you do weeks after o Present each comparison 100 times  Give them standard, then you give them the comparison  Never at same time (not in each hand, one arm stronger than other, use same hand during 100 trials)  Point of subjective equality: 50% of the time perceive, means they can’t tell the difference  Difference threshold = (comparison intensity judged more intense 75% of the time – comparison intensity judged more intense 25% of the time)/2  E.g. (102-98)/2 = 2 mg is difference threshold % of times person said comparison was Comparison heavier Weight 97 10% 98 25% 99 50% Point of Subjective Equality 100 60% Negative Time Error 101 70% 102 75% 103 100% Classic Result in Difference Threshold: Weber’s Law  Weber found that if the magnitude of the stimulus increases, so does the size of the difference threshold  The difference threshold is bigger for bigger standards o Birthday cake example (difference between 1 candle and 2 candles is big, difference between 60 candles and 61 candles is not very noticeable) o Ratio of difference threshold to standard stimulus is constant  Weber’s Law: o (difference threshold) / (value of standard stimulus) = K o K is a constant (Weber’s fraction)  Each sensory judgment as own constant o E.g. weights: K is 0.02 (always) and say the standard or comparison weight is 200g, plug into equation and see that 4g must be added in order for a difference to be detected  E.g. found that JND for 8lbs weight is 4 lbs, K=4/8 = 0.5 o If standard is 16 lbs, what is JND? 0.5=JND/16 , JND = 8 lbs o When you double the standard, you will double the JND Magnitude Estimation and Scaling Studies 9/6/2012 3:30:00 PM Magnitude Estimation (Scaling Studies)  S.S. Stevens (1957)  Measuring how perceived intensity increases above the threshold  Accurately measures relationship between intensity and magnitude  Technique: o Take standard intensity (e.g. volume 4)  Call it a 10 perceived intensity o Present another intensity (e.g. volume 8)  “If the first was 10, what is this one?”  If individual is accurate, perceived intensity would be 20  Individuals are accurate in determining lengths of lines  Findings: o Doubling intensity does not make the stimulus appear twice as
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