Sensation and Perception

Sensation vs. Perception

Sensation is the detection of stimuli by sensory receptors (bottom-up processing); perception is the brain's interpretation of that input (top-down). The absolute threshold is the minimum stimulus detected 50 percent of the time; the difference threshold (just noticeable difference) is the minimum detectable change. Weber's Law states the JND is proportional to baseline stimulus intensity -- you notice one candle added to ten, not one added to a thousand.

Signal Detection Theory

Detection depends on both sensitivity (d-prime, how well you discriminate signal from noise) and response bias (your criterion for saying "yes"). The 2x2 matrix: hit, miss, false alarm, correct rejection. A conservative criterion yields fewer false alarms but more misses; a liberal criterion does the reverse -- crucial for understanding radiologists and diagnostic screening.

Adaptation and Gestalt Principles

Sensory adaptation is decreased sensitivity to constant stimuli (you stop smelling your own house), but it spares pain for survival reasons. Gestalt principles organize perception: proximity, similarity, closure, continuity, and figure-ground.

Key Takeaways

• Sensation is bottom-up detection; perception is top-down interpretation
• Weber's Law: the JND scales with stimulus intensity
• Signal detection separates sensitivity (d-prime) from response bias (criterion)
• Gestalt principles describe how the brain groups sensory input