Threshold methods of detection and discrimination...


As a result of studying this chapter, students will:


• the main threshold methods of sensory psychophysics and their varieties;

• methods of mathematical processing of protocols reflecting the results of estimating absolute and difference thresholds;

be able to

• prepare a measuring tool that provides a correct assessment of sensory thresholds;

• determine absolute and differential thresholds, uncertainty interval, subjective equality point, constant error;

• build a psychophysical function;


• the skills of assessing sensory thresholds, the methods of minimal changes and the constancy of other stimuli.

As we already noted in the previous chapter, the methods of threshold psychophysics became the first in the history of psychology to quantify the sensations. They were developed in the middle of the XIX century. German researcher G. Fechner. He deserves credit not only for creating concrete methods for measuring sensations, but also for developing an understanding of how sensations can be measured in principle.

The problem of Fechner was to find the quantitative relationship between the physical energy of the stimulus and the magnitude of the sensation. The methodology of the German researcher suggested that only an indirect, indirect way of assessing sensations is possible. In other words, it was believed that it is possible to express the sensation only by characterizing the physical energy of the stimulus acting on the sense organ, and the sensation itself can not express itself directly. Fehner designated this value, which gives the sensation, the term touch threshold & quot ;. The incentives or differences between them, which do not reach this value by force of impact, are subthreshold, and their impact on the senses is not felt, they are outside the consciousness. Fechner assumed that, knowing the magnitude of the thresholds, we can estimate the magnitude of any sensation-both close to the threshold value, and substantially exceeding it. That is why all that is connected with the measurement and definition of thresholds, is the main content and essence of classical psychophysics, called threshold psychophysics.

In the second half of XX century. A new, more modern approach to understanding how the sensation arises was proposed. This point of view, based on the application of some principles of the statistical theory of decision-making, was proposed in 1954 by American psychophysicists Tanner and Light, and later developed in detail in the signal detection theory proposed by Greene and Light in 1966. It interprets the sensation process as more complex as a process associated with generating an internal representation of the stimulus and making a decision based on an estimate of the likelihood ratio.

Nevertheless, the methodology of classical threshold psychophysics remains relevant to this day. This chapter is devoted to its discussion.

Sensory thresholds and their types

The touch threshold is the point in the continuum of stimuli that affect the senses, which divides them into two classes: those that are capable of causing sensation, and those that do not cause sensation. Thus, the entire continuum of stimuli is divided into two zones - subthreshold (sub-sensory) and supra-threshold (sensory).

Since the threshold relates to the physical characteristics of stimuli that can cause sensations, they are expressed in physical units that reflect the different characteristics of the stimuli, such as, for example, the intensity of the light flux, the frequency and amplitude of the sound oscillations, the degree of concentration of certain substances, causing a taste or smell.

The ability of the sense organ to respond to the impact of physical stimulation is indicated in sensory psychophysics, which relates to the measurement of sensory thresholds, the term sensitivity. This is the reciprocal of the threshold value. Thus, sensitivity appears to be related to the concept of sensory threshold by reciprocal relations: the higher the threshold, the lower the sensitivity. Conversely, the higher the threshold, the lower the sensitivity.

All sensory thresholds are divided into two classes - absolute and difference. Both of them, in turn, can be subdivided into lower and upper thresholds.

Absolute thresholds (they are denoted as RL - the abbreviation of Reiz Limen) are related to the absolute value stimulus. They characterize the ability of the sensory receptor, the sensory organ, to react to physical action in principle. In this case, the lower absolute threshold corresponds to the minimum amount of physical impact that can cause the reaction of the corresponding receptor. The upper absolute threshold corresponds to the limiting value of the stimulus, which causes a specific reaction of the sense organ. If this stimulus value is exceeded, the sensation either qualitatively changes or completely disappears.

An example of a qualitative change in sensation can be pain in the eyes when exposed to too bright light or a feeling of vibration when the hearing organs are exposed to high-frequency sound waves. An example of the disappearance of a sensation when passing through a point corresponding to the upper absolute threshold may be the fact that the visual sensations disappear as the wavelength of electromagnetic oscillations increases beyond 720 nm. Such electromagnetic phenomena are called infrared, and they are inaccessible to the human eye.

Difference or differential thresholds (DL - the abbreviation of Dijferenz Limeri) are related to our ability detect differences between the two stimuli. If the physical difference of the stimuli turns out to be invisible to our consciousness, it is considered subthreshold. Otherwise, it turns out to be above threshold. The boundary that separates the subliminal and supra-threshold differences of the two stimuli by any of their characteristics is called the difference threshold.

As already mentioned, the difference thresholds, just like absolute ones, can be divided into lower and upper thresholds. The fact is that the same stimulus (let's call it a standard) can be compared with both smaller stimuli and larger ones. Accordingly, in the first case we speak of a lower differential threshold, which determines the ability to distinguish smaller stimuli from the standard, and in the second case, to the upper one, which determines the ability to distinguish from the standard large stimuli. It should be noted, however, that such a distinction is not always made and in a number of cases it turns out to be unprincipled.

The concept of touch threshold was first proposed in the works of the German philosopher, the creator of sensory psychophysics, G. Fechner, who has already repeatedly mentioned us. He also developed the basic methods for assessing thresholds: the method of minimal changes, the method of mean error and the method of true and false cases, which in modern psychology is often called the method of constant stimuli. In psychophysics, some other methods for estimating thresholds have also been made known, these three methods are the main ones. All other methods are derived from them. Therefore, knowledge of these methods and their practical possession is an important task for the training of future psychologists.

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