Types of Sensitivity Thresholds
Sensory system sensitivity is characterized by two main thresholds:
- Absolute Threshold of Sensation: The minimum stimulus intensity required for an individual to detect it. Any stimulus below this threshold is termed subthreshold and does not elicit a conscious sensation, regardless of the sensory modality.
- Differential Threshold (Just-Noticeable Difference): This parameter measures the sensory system's ability to detect changes, representing the minimum change in stimulus intensity or quality that a person can notice.
Classical Laws of Perception
Mathematical formulas are used to describe the relationship between stimulus intensity and the resulting sensation.
Weber's Law (1831) applies to the differential threshold. It states that the increment in stimulus intensity needed to produce a just-noticeable difference in sensation is proportional to the initial stimulus intensity (ΔJ / J = K, where J is the initial intensity, ΔJ is the increment, and K is a constant).
The Weber-Fechner Law translates this relationship to the subjective level of sensation. It states that the magnitude of sensation increases not linearly with stimulus intensity, but proportionally to its logarithm (S = a × log R + b). Graphically, this appears as a logarithmic curve: as the stimulus grows stronger, the perceived intensity increases more slowly. In other words, for sensation to increase in arithmetic progression, the stimulus must increase in geometric progression.
Functional Mobility
Classical laws have limitations because they do not account for the dynamic state of the receptor apparatus. The concept of functional mobility explains that the body can alter the number of active receptors depending on incoming signals.
The threshold constant depends not only on stimulus intensity, but also on the area of stimulation and the number of active receptors at any given moment. A modified formula is expressed as: $J \times S \times (P - p) = \text{const}$, 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, p is the non-functioning ones).
Physiological Properties of Sensation Formation
The formation of sensations in the central nervous system has specific characteristics:
- Inertia: Sensations do not appear or disappear instantaneously. This is due to the propagation time of excitation within the central processing centers of the analyzer. It manifests as simultaneous and successive contrast, as well as the persistence of a sensation after the stimulus has ceased (e.g., a visual afterimage following a bright flash).
- Active Nature of Perception: The central nervous system does not merely accept signals passively; there is central modulation that filters incoming information at the level of sensory relay nuclei. An example is descending control, which can decrease response amplitudes in subcortical visual centers during light exposure.