Psychophysics

Absolute threshold

The minimum stimulus energy necessary for an observer to detect a stimulus.

Bias

A measure for the extent to which one response is more probable than another (e.g., a subject may be more likely to respond that a stimulus is present). Bias is independent of sensitivity.

Correct rejection

In a signal detection experiment, saying “No, I don’t detect a signal” on a trial in which the signal is not presented (a correct response).

Difference threshold

The minimal detectable difference between two stimuli. Also called just noticeable difference (JND).

False alarm

In a signal detection experiment, saying “Yes, I detect the signal” on a trial in which the signal is not presented (an incorrect response).

Fechner’s law

A principle describing the relationship between a stimulus and the resulting sensation that says the magnitude of subjective sensation increases proportionally to the logarithm of the stimulus intensity.

Hit

In a signal detection experiment, saying “Yes, I detect the signal” on a trial in which the signal is present (a correct response).

Miss

In a single detection experiment, saying “No, I don’t detect a signal” on a trial in which the signal is present (an incorrect response).

Psychophysics

Traditionally, the term psychophysics refers to quantitative methods of measuring the relationship between properties of the stimulus and the subjects experience

Receiver operating characteristic (ROC) Curve

In a signal detection experiment, the graphical plot of the hit rate as a function of the false alarm rate. If these are the same, observers cannot distinguish between signal and noise. As the observer’s sensitivity increases, the curve bows toward the upper left corner –a point that represents perfect sensitivity.

Response criterion

In a signal detection experiment, the subjective magnitude of a stimulus above which the participant will indicate that the stimulus is present.

Sensitivity

A measure for how hard or easy it is for a subject to detect that a target stimulus is present. It can be computed based on hit- and false alarm rates. Also call d’ (pronounced “dee prime”).

Signal detection theory

A theory stating that the detection of a signal depends both on the participant’s sensitivity to the signal and on the participant’s response bias.

Weber’s law

A law stating that the ratio of the difference threshold (I) to the stimulus intensity (I) is constant. According to this relationship, doubling the value of the stimulus intensity will cause a doubling of the difference threshold. The ratio I/I is called Weber’s fraction.