Signal Detection Theory

Question 1

what does signal detection theory allow us to predict

Answer 1

how the observer’s criterion, motivation, boredom, etc affect the detection of a stimulus

Question 2

what is the assumption of the signal detection theory

Answer 2

that there is inherent noise in the visual system

-noise is the randomly fluctuating neuronal activity in the absence of a stimulus (background noise)

Question 3

signal detection theory assumes that the stimulus a subject is trying to detect is….

Answer 3

always being detected in the presence of noise
-noise is not a problem most of the time but conditions near threshold (ex. weak stimulus) can severely affect the detectability of a stimulus

Question 4

what is detectability (d’)

Answer 4

the difference btwn the mean values of noise and noise+signal divided by the common std deviation of the 2 distributions
-if d’ is large there is no difficulty detecting a stimulus

Question 5

by setting a low criterion, your hit rate is ____, miss rate is ______, false alarm rate is _____, and correct rejections _______

Answer 5

hit rate high
miss rate low
false alarm high
correct rejection in btwn

Question 6

setting a high criterion, your correct rejections is ____, miss rate is ______, false alarm rate ______, hit rate ______

Answer 6

correct rejection is high
high miss rate
low false alarm rate
hit rate in btwn

Question 7

in the very lax, lax, strict, and very strict criteriorn: how are hit rate and false alarm rate related

Answer 7

very lax: hit rate is very high, false alarm rate veyr hihg
lax: hit rate is very hihg, fals alarm rate is high
strict: hit rate is high, false alarm rate is low
very strict: hit rate is very low, false alarm rate is very low

Question 8

you can caluclate the hit and false alarm rate for different criterion levels and create a _____ curve

Answer 8

receiver operating characteristic curve (ROC)

Question 9

the ROC curve is specific to what

Answer 9

a certain d’ value

ex. depends on the detectabilty btwn the noise and noise+signal distributions

Question 10

what do we have if d’=0
how is the ROC curve
how are false alarm and hit rate related

Answer 10

very weak stimulus and N+S overlap completely

• gives a linear ROC curve
• false alarm rate and hit rate increase linearly, are equal for all criterion values

Question 11

if the d’ is small but > 0

Answer 11

substatial overlap btwn distri but there is enough neural activity to separate them slightly, stimulus produces a signal just above noise level

• for strict crit: both the hit rate and false alarm rate are bow but hit rate is lsightly higher
• for lax: hit rate and false alarm rate high

Question 12

if d’ is large what happens

Answer 12

no overlap btwn N and N+S

-hit rate increases substantially before the false alarm rate increases (low)

Question 13

how do prey animals se ttheir criterion levels

Answer 13

very lax

-regonize a predator but have a very high false alarm rate

Question 14

what is the likelihood ratio

Answer 14

beta=stimulus freq/1-stimulus freq x $CR-$FA/$H-$M

Question 15

by showing the same payoff matrix to the subjects you know….

Answer 15

1. you have the same bias throughout the trials
2. all the subjects will try and set the same criterion levle and therefore you will have a consistence bias amont subjects

Question 16

signal detection theory and ROC are used int he medical field to…

Answer 16

1. asses the validity of a clincal diagnostic procedure

2. for rational decision making

Question 17

it is important to know the sensitivty and specificity of diagnostic tests. what are these things

Answer 17

sensitivty: the %age of pts correctly identified having the disease you are testing for (hits)
specificity: the %age of pts who are healthy and identified as healthy (correct reject)

Question 18

for example, if you have a machine that tests for glauc, what will a machine with strict criterion give for sensitivty and specificty? how about a lax criterion machine

Answer 18

strict: sensitivty (correctly id disease) falls but specificity incresaes (id healthy)
lax: sensitivty increaes but speciicity decreases

Question 19

if the disease is mild, self healing, and won’t result in vision loss, how will the clinician’s criterion be? what about if the diease is vision threatening

Answer 19

mild: criterion will be strict-no big deal to miss some cases
vision threat: lax-to ensure not misdiagnosing

Question 20

if the treatment for dieases is really expensive but the disease has mild consequcenes, what criterion?
if the treatment is cheap and the disease is serious, what criterion?

Answer 20

expensive: strict to reduce false alarms and avoid cost
cheap: lax to avoid untreated cases