Thresholds and laws of perception

Types of sensitivity thresholds

Sensory system sensitivity is characterized by two main thresholds:

• Absolute threshold of sensation: This is the minimum stimulus intensity required for a person to perceive it at all. All stimuli below this value are called subthreshold and do not elicit any sensation, regardless of their sensory modality.
• Differential threshold (just-noticeable difference): This parameter characterizes the sensory system's ability to detect changes. It reflects the minimum change in stimulus intensity or quality that a person is able to notice.

Classic laws of perception

Mathematical formulas are used to describe the relationship between stimulus intensity and sensation.

E. Weber's law (1831) relates to the differential threshold. Its core principle is that the increment in stimulus intensity required for a minimally noticeable increase in sensation is proportional to the initial stimulus intensity as a constant value (ΔJ / J = K, where J is the initial intensity, ΔJ is the increment, and K is a constant).

The Weber-Fechner law extends this relationship to the level of sensation. According to this law, the intensity of sensation grows not in proportion to the stimulus intensity, but in proportion to its logarithm (S = a × log R + b). Graphically, this appears as a logarithmic curve: the stronger the stimulus becomes, the more slowly the intensity of sensation increases. In other words, for the sensation to increase in an arithmetic progression, the stimulus must increase in a geometric progression.

Phenomenon of functional mobility

Classical laws have limitations because they do not account for the dynamics of the receptor apparatus. P.G. Snyakin introduced the concept of functional mobility, which explains that the body is capable of altering the number of active receptors depending on incoming signals.

The threshold sensation constant depends not only on stimulus intensity, but also on the area of stimulation and the number of currently active receptors. Snyakin proposed a modified formula: $J \times S \times (P - p) = \text{const}$, where J is the threshold intensity, S is the threshold area, and (P - p) is the number of active receptors (P is the total number, and p is non-functioning ones).

Physiological properties of sensation formation

The formation of sensations in the central nervous system has specific characteristics:

1. Inertia. Sensations do not arise and disappear instantaneously. This is due to how excitation propagates within the central portion of the analyzer. This manifests as simultaneous and successive contrast, as well as the persistence of sensation after the stimulus has ceased (e.g., a visual afterimage following a flash of light).
2. Active nature of perception. The central nervous system does not merely accept signals passively. There is a central regulatory mechanism that modulates incoming information at the level of sensory relays. An example is descending control, which can reduce response amplitudes in subcortical visual centers under light exposure.

Class and exam questions

What else to study on this topic

• Graphical representation of the logarithmic relationship of sensation magnitude
• The latent period of sensation and its dependence on stimulus intensity
• Mechanisms of central tuning and descending control in sensory systems
• Phenomena of simultaneous and successive contrast

Other topics in the section «Structural and functional organization of analyzers»

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.