Signal Detection - Psychophysics Lab

Background

Signal Detection Theory (Green & Swets, 1966) separates an observer’s sensitivity from their response bias. On each trial, a noisy display may or may not contain a faint signal. The observer says “present” or “absent.”

Four Outcomes

Every trial produces one of four outcomes:

Hit — signal present, observer says “present”
Miss — signal present, observer says “absent”
False Alarm — signal absent, observer says “present”
Correct Rejection — signal absent, observer says “absent”

d′ (d-prime)

d′ measures sensitivity: d′ = z(Hit Rate) − z(False Alarm Rate). Higher d′ means better detection ability, independent of response strategy.

Criterion (c)

Criterion measures response bias: c = −0.5 × [z(HR) + z(FAR)]. A positive c means the observer is conservative (tends to say “absent” more), while a negative c means the observer is liberal (tends to say “present” more).

References

Green, D.M. & Swets, J.A. (1966). Signal Detection Theory and Psychophysics. New York: Wiley.
Macmillan, N.A. & Creelman, C.D. (2005). Detection Theory: A User’s Guide (2nd ed.). Mahwah, NJ: Erlbaum.

Signal Detection Theory

A noisy display will flash briefly. On some trials, a faint circular pattern is hidden in the noise.

Press F if you think a signal was PRESENT, press J if ABSENT.

There are 100 trials (~6 minutes).

Don’t worry about being perfect — the analysis accounts for your response strategy.

Signal Detection Theory

Your Results