The theory of signal detection: characteristics and elements
The concept of threshold has been (and is) widely studied in Psychophysics, the branch of psychology that seeks to establish the relationship between physical stimuli and perception. The threshold, broadly speaking, is understood to be the minimum amount of signal that must be present to be recorded.
Here we will know signal detection theory, or also called response threshold theory, a proposal that seeks to know when a subject is capable of detecting a signal or stimulus.
- Related article: "Psychophysics: the beginnings of Psychology"
Signal detection theory: 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”.
So, Fechner characterized the perceptive experience as something discontinuous, and stated that awareness of a stimulus or the changes that occur in it is acquired through of a sudden jump that goes from not overcoming the barrier to overcoming it (thus establishing the law of all or nothing).
After Fechner, other researchers supported the idea that the transition to the detection or discrimination of a stimulus takes place through a smooth and slow transition, that is, they considered continuity in detection (subjects appreciate continuous changes in stimulation).
Currently many authors believe that the idea of an absolute measure of sensitivity to be called a 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 (TDS).
- You may be interested in: "The 7 types of sensations, and what information they capture"
TDS Experimental Procedure
The experimental procedure consists in that the observer (examined subject) must answer indicating if during the observation interval, the cue (auditory stimulus) was present or not (whether you have listened). That is, detect it when it appears.
The subject's task, therefore, will no longer be to classify stimuli above or below the threshold (as in previous models), but this basically consist of a decision process. Thus, according to the signal detection theory, a subject's response 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 causes a stimulus of a certain intensity, is enough to incline in favor of detecting its presence (positive response, detection) or not detected (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 examined individual:
1. S stimulus (noise + signal)
This is made up 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 to say, the distractor only appears.
response matrix
The responses of the observed subjects generate a matrix of possible responses with 4 possibilities. Let's divide them into hits and misses:
1. Hits
Are the correct answers emitted by the subject in the experimental paradigm:
1.1. Success
It is a correct decision, and consists of correctly detecting the stimulus S (noise + signal).
1.2. correct rejection
This is a hit, a correct non-detection; the subject rejects that the signal has appeared because, indeed, it has not appeared (stimulus N: noise).
2. Mistakes
Are wrong answers emitted by the subject in the experimental paradigm:
2.1. False alarm
This is an error, and consists of responding that the signal has been heard when in fact it has not appeared, since it was the stimulus N (noise).
2.2. Failed
This is also a mistake; consists of an omission (failed detection), since the subject does not respond when the signal appears (in stimulus S: noise + signal).
Graphic representation of the results
The representation of the results in the theory of signal detection is translated into a curve called ROC (which detects the sensitivity and detectability of the person. Two elements can be seen in the graph:
- D’, d premium or sensitivity index: discriminability or detectability of the signal.
- B (beta), subject response criterion: 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 signal detection theory, as we have seen, are two:
1. conservatives
On the one hand, conservative subjects they don't take risks and respond less (That is why they make more errors of omission, that is, they do not respond to the signal).
2. liberals
Liberal subjects, for their part, have more false alarm errors (they respond that they have heard 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 concept of threshold understood as "neural barrier". In addition, in it a single intensity of the stimulus is used and it does not vary, as it happened in other previous psychophysical methods.
On the other hand, in each trial of the experimental paradigm, the subject can answer only YES or NO (dichotomous response).
Finally, the theory establishes that, in addition to sensitivity (classical psychophysics concept), the response decision criterion also influences the individual's response (conservatives vs. liberals).
Bibliographic references:
- Classical and contemporary psychophysics. The theory of signal detection. CRAI of the UB, Teaching Unit.
- Munar, E.; Rossello, J. and Sánchez-Cabaco, A. (1999). Attention and perception. Alliance. Madrid.
- Goldstein, E.B. (2006). Sensation and perception. 6th edition. Debate. Madrid.