Weber's law and Fechner's law are both fundamental concepts in the field of psychophysics, but they differ in their focus and how they relate stimulus intensity to perceived sensation. In essence, Fechner's Law builds upon Weber's Law.
Here's a breakdown of the key differences:
Weber's Law
Weber's Law focuses on the just-noticeable difference (JND), also known as the difference threshold.
- Definition: Weber's Law states that the JND is a constant proportion of the initial stimulus intensity. This means that the amount a stimulus needs to change for a person to notice a difference is proportional to the original intensity of the stimulus.
- Formula: ΔI/I = k, where:
- ΔI is the increment threshold (JND).
- I is the initial stimulus intensity.
- k is the Weber fraction (a constant).
- Focus: The change in stimulus needed for detection.
- Example: If you're holding a 1 kg weight, you might need to add 0.1 kg to notice a difference (JND). If you're holding a 10 kg weight, you'd need to add 1 kg to notice a difference. The Weber fraction (k) would be 0.1 in both cases.
Fechner's Law
Fechner's Law aims to describe the relationship between the physical magnitude of a stimulus and the perceived intensity of the sensation.
- Definition: Fechner's Law states that the subjective sensation is proportional to the logarithm of the stimulus intensity.
- Underlying Assumption: As mentioned in the reference, Fechner built up his logarithmic law of sensation intensity from Weber's Law and the assumption that all jnds are subjectively equal. This is a crucial difference. Fechner assumed that each JND feels like an equal step in perceived sensation.
- Formula: S = k * log(I), where:
- S is the subjective sensation intensity.
- k is a constant that depends on the sensory modality.
- I is the physical intensity of the stimulus.
- Focus: The relationship between stimulus intensity and perceived sensation.
- Example: Doubling the intensity of a light stimulus doesn't double the perceived brightness. The perceived brightness increases, but by a smaller amount than the physical increase in intensity due to the logarithmic relationship.
Comparison Table
| Feature | Weber's Law | Fechner's Law |
|---|---|---|
| Focus | Just-Noticeable Difference (JND) | Relationship between stimulus and perceived sensation |
| Relationship | Proportional change in stimulus needed for JND | Logarithmic relationship between stimulus and sensation |
| Underlying Basis | Directly empirical observation | Derived from Weber's Law + assumption of equal JNDs |
| Key Contribution | Quantifies the relative difference threshold | Provides a scale for measuring subjective sensation |
Key Takeaways
- Weber's Law describes the relative change in stimulus needed for detection, while Fechner's Law attempts to quantify the absolute relationship between physical stimulus and perceived sensation.
- Fechner's Law is based on Weber's Law and makes the critical assumption that all JNDs are subjectively equal. This assumption is often questioned.
- Both laws are simplifications of complex perceptual processes, but they provide valuable frameworks for understanding sensory perception.
