Signal detection theory

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SIGNAL DETECTION THEORY

(thery +criterion)

The theory of signal detection theory evolved from the development of communications and radar equipment in the first half of this century. It migrated to psychology, initially as part of sensation and perception, in the 50's and 60's as an attempt to understand some of the features of human behavior when detecting very faint stimuli that were not being explained by traditional theories of thresholds.

According to the theory, of absolute threshold a minimum stimulus energy is necessary for an observer to detect a stimulus.i.e. If there is a stimulus present we perceive the stimulus even if the intensity of the stimulus is minimum. But, these theories give importance to an observer’s sensory capacities. The probability that an observer detects the stimulus is not given importance.

The signal detection theory is different from traditional threshold theories. It says that the subject makes a decision, a cognitive act, as to whether the signal is present or not. This basic sensory act of determining if a stimulus occurred now is understood to have a cognitive component.

THEORY:

 The signal detection theory is a mathematically based theory which assumes that the observer is not a passive receiver of stimuli, but an active decision-maker who makes difficult perceptual judgements under conditions of uncertainty.

HYPOTHETICAL EXAMPLE:

The experimental setup for signal detection studies, uses two types of stimulus presentations:

A signal absent presentation, which is, like a catch trial on which no stimulus is presented and the observers see or hear only the noise generated by their sensory system.

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A signal present trial on which there is actual presentation of the target stimulus which is superimposed on the endogenous noise in the sensory system.( the stimulus is of one single low intensity)

Two responses are possible in the experiment: ‘yes’ indicates that an observer thinks a stimulus was present on a particular trial and ‘no’ indicates that the observer thinks the signal was absent.

The combination of two possible responses leads to four possible outcomes on a given trial. When the signal is present and the response is yes, the observer has made a hit. But if the observer responded yes when ...

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