The Theory of Signal Detection

An alternative to the conception of perceptual thresholds as neural barriers.

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The threshold concept has been (and is) widely studied in Psychophysics, the branch of psychology that seeks to establish the relationship between physical stimulation and perception. The threshold, in broad strokes, is understood to be the minimum amount of signal that must be present to be recorded.

We will know the theory of signal detection, also called response threshold theory, a proposal that seeks to know when a subject is able to detect a signal or stimulus.

Theory of signal detection: characteristics

Fechner was a researcher who considered the threshold as an almost constant point, above which stimulus differences were detectable and below which they could not be detected. According to him, the threshold was a kind of “neural barrier”.

Thus, Fechner characterized the pereptive experience as something discontinuous, and affirmed that the awareness of a stimulus or the changes that occur in it is acquired through a sudden jump that goes from not overcoming the barrier to overcome it (thus establishing the law of all or nothing).

After Fechner, other researchers supported the idea that the step to the detection or discrimination of a stimulus takes place by means of a smooth and slow transition, that is, they considered continuity in the detection (the subjects appreciate the continuous changes of stimulation).

Currently many authors believe that the idea of ​​an absolute measure of sensitivity to what is called threshold is not valid. Thus, different procedures have been proposed to study the detectability of stimuli that avoid the threshold concept. The most important theory is the theory of signal detection (TSD).

Experimental procedure of the TSD

The experimental procedure consists in that the observer (subject examined) must respond indicating whether during the observation interval, the signal (auditory stimulus) was present or not (if he has listened to it). That is, detect it when it appears.

The subject’s task, therefore, will no longer be to classify the stimuli above or below the threshold (as in previous models), but this will basically consist of a decision process.

Thus, according to the theory of signal detection, the response of a subject to a stimulus goes through two phases: the first is sensory (more objective) and the second is decisional (more cognitive).

The subject must decide if the magnitude of the sensation that provokes a stimulus of a certain intensity is enough to lean in favor of detecting its presence (positive response, detection) or not detecting it (negative response, absence).

Experimental paradigm: types of stimuli

Through the theory of signal detection, an experimental paradigm was developed with two types of auditory stimuli that could be presented to the individual examined:

1. Stimulus S (noise + signal)

This is composed of two elements: noise + signal. That is, the auditory stimulus (signal) appears superimposed on the noise (distractor).

2. Stimulus N (noise)

This is the same environment that accompanies the signal, but without it (without the auditory stimulus). That is, the distractor appears alone.

Response matrix

The responses of the observed subjects generate a matrix of possible answers with 4 possibilities. Let’s divide them into successes and mistakes:

1. Hit

They are the correct answers issued by the subject in the experimental paradigm:

1.1. Success

It is a correct decision, and consists of correctly detecting the S stimulus (noise + signal).

1.2. Right rejection

This is a success, not a correct detection; the subject rejects that the signal has appeared because, indeed, it has not appeared (stimulus N: noise).

2. Errors

They are the incorrect answers issued by the subject in the experimental paradigm:

2.1. False alarm

It is an error, and consists in answering that the signal has been heard when in fact it has not appeared, since it was the N (noise) stimulus.

2.2. Failure

This is also a mistake; it consists of an omission (failed detection), since the subject does not respond when the signal appears (in the stimulus S: noise + signal).

Graphical representation of the results

The representation of the results in the theory of signal detection is translated into a curve called COR (which detects the sensitivity and detectability of the person.) The graph shows two elements:

  • D ‘, d premium or sensitivity index: discriminability or detectability of the signal.
  • B (beta), subject response criteria or: high values ​​indicate a conservative subject, and low values, a liberal subject.

Types of subjects

The types of subjects that can be observed in the results of the theory of signal detection, as we have seen, are two:

1. Conservatives

On the one hand, conservative subjects do not take risks and respond less (that’s why they make more errors of omission, that is, they do not respond to the signal).

2. Liberals

Liberal subjects, on the other hand, have more false alarm errors (they respond that they have listened to the signal almost always) and have fewer omissions (for the same reason as the previous one).

Final comments

The theory of signal detection questions the validity of the threshold concept understood as a “neural barrier”. In addition, a single intensity of the stimulus is used and does not vary, as in previous psychophysical methods.

On the other hand, in each trial of the experimental paradigm, the subject can only answer YES or NO (dichotomous response).

Finally, the theory states that, in addition to sensitivity (concept of classical psychophysics), the response decision criterion also influences the response of the individual (conservatives vs. liberals).