# Sensation
Sensation is the process by which sensory receptors detect physical stimuli and convert them into neural signals. It is distinct from [[Perception]], which involves interpreting and organizing those signals into meaningful experiences. While sensation is largely automatic and physiological, perception is cognitive and influenced by experience, expectations, and context. The study of sensation began with psychophysics, founded by Gustav Fechner and Ernst Weber in the 19th century, establishing quantitative relationships between physical stimuli and psychological experience.
Each sensory system has specialized receptors tuned to specific forms of energy: photoreceptors for light, mechanoreceptors for pressure, chemoreceptors for molecules. The process of converting physical energy into neural signals is called transduction. Key concepts include absolute threshold (minimum detectable stimulus), difference threshold (smallest detectable change), and signal detection theory. Understanding sensation is foundational for [[Cognitive Psychology]], neuroscience, and practical applications in design, accessibility, and human factors engineering.
## Sensation vs Perception
```
┌─────────────────────────────────────────────────────────────┐
│ SENSATION vs PERCEPTION │
├─────────────────────────────────────────────────────────────┤
│ │
│ PHYSICAL SENSATION PERCEPTION │
│ STIMULUS → (Detection) → (Interpretation) │
│ │
│ ┌─────────┐ ┌─────────────┐ ┌─────────────────┐ │
│ │ Light │ → │ Photoreceptor│ → │ "I see a face" │ │
│ │ waves │ │ activation │ │ │ │
│ └─────────┘ └─────────────┘ └─────────────────┘ │
│ │
│ SENSATION PERCEPTION │
│ • Physiological • Cognitive │
│ • Automatic • Interpretive │
│ • Bottom-up • Top-down influenced │
│ • Detection • Recognition │
│ • "Something is there" • "That is my friend" │
│ │
└─────────────────────────────────────────────────────────────┘
```
## Sensory Systems
| System | Stimulus | Receptor | Location |
|--------|----------|----------|----------|
| **Vision** | Light (electromagnetic) | Rods, cones | Retina |
| **Audition** | Sound waves (pressure) | Hair cells | Cochlea |
| **Touch** | Pressure, vibration | Mechanoreceptors | Skin |
| **Pain** | Tissue damage | Nociceptors | Throughout body |
| **Temperature** | Heat/cold | Thermoreceptors | Skin |
| **Taste** | Chemicals (dissolved) | Taste receptor cells | Tongue |
| **Smell** | Chemicals (airborne) | Olfactory neurons | Nasal cavity |
| **Proprioception** | Body position | Muscle spindles | Muscles, joints |
| **Vestibular** | Head movement/gravity | Hair cells | Inner ear |
## Transduction Process
| Stage | Description |
|-------|-------------|
| **1. Reception** | Stimulus reaches sensory receptor |
| **2. Transduction** | Physical energy → neural signal |
| **3. Transmission** | Signal travels via sensory neurons |
| **4. Processing** | Brain interprets signals |
## Psychophysics Concepts
| Concept | Definition |
|---------|------------|
| **Absolute threshold** | Minimum stimulus detected 50% of time |
| **Difference threshold (JND)** | Smallest detectable difference |
| **Weber's Law** | JND proportional to stimulus intensity |
| **Fechner's Law** | Perceived intensity = log of stimulus |
| **Stevens' Power Law** | More accurate intensity relationship |
| **Signal detection theory** | Separates sensitivity from bias |
## Weber's Fractions
| Sense | Weber Fraction | Example |
|-------|----------------|---------|
| **Vision (brightness)** | 1/60 | Detect 1.7% change |
| **Hearing (pitch)** | 1/333 | Detect 0.3% change |
| **Touch (weight)** | 1/50 | Detect 2% change |
| **Taste (salt)** | 1/5 | Detect 20% change |
| **Smell** | 1/4 | Detect 25% change |
## Sensory Adaptation
| Type | Description | Example |
|------|-------------|---------|
| **Receptor adaptation** | Decreased response over time | Not feeling clothes |
| **Neural adaptation** | Reduced neural firing | Ignoring background noise |
| **Habituation** | Learned reduction in response | Tuning out ticking clock |
## Key Figures
| Person | Contribution |
|--------|--------------|
| Ernst Weber | Weber's Law (difference thresholds) |
| Gustav Fechner | Founded psychophysics |
| S.S. Stevens | Power Law, magnitude estimation |
| Hermann von Helmholtz | Sensory physiology |
| Johannes Müller | Doctrine of specific nerve energies |
## Sensory Thresholds in Design
| Application | Consideration |
|-------------|---------------|
| **Contrast** | Text must exceed visual threshold |
| **Audio feedback** | Sounds above hearing threshold |
| **Haptic feedback** | Vibration above touch threshold |
| **Color differences** | JND for color discrimination |
| **Accessibility** | Account for varied sensory abilities |
## References
- Fechner, G. (1860). *Elements of Psychophysics*
- Stevens, S.S. (1957). "On the Psychophysical Law"
- https://en.wikipedia.org/wiki/Sensation_(psychology)
## Related
- [[Perception]]
- [[Cognitive Psychology]]
- [[Psychophysics]]
- [[Neuroscience]]
- [[Accessibility (a11y)]]
- [[Signal Detection Theory]]