Sensation and Perception
Explain thresholds, sensory adaptation, vision, hearing, touch, taste, smell, perceptual organization, depth cues, and top-down processing.
A lesson of AP Psychology, in Biological Bases of Behavior, about 10 minutes.
By the end of this lesson, you will be able to explain the difference between sensation and perception, describe how the major sensory systems work, and analyze the principles of perceptual organization.
Sensation vs. Perception
Sensation: the process by which sensory receptors and the nervous system receive and represent stimulus energies from the environment. The "raw data" of experience.
Perception: the process of organizing and interpreting sensory information, enabling us to recognize meaningful objects and events. The brain's interpretation of sensation.
Bottom-up processing: perception driven by the sensory data itself (feature detection, then pattern recognition).
Top-down processing: perception driven by prior knowledge, expectations, and context. Explains why we can read misspeled words or see faces in clouds.
Key Concepts
Absolute threshold: the minimum stimulus intensity detectable 50% of the time.
Just Noticeable Difference (JND) / Difference threshold: the minimum detectable change in a stimulus (50% of the time).
Weber's Law: the JND is a constant fraction of the original stimulus. To notice a change in a 100g weight, you need ~2g; to notice a change in a 1000g weight, you need ~20g. The fraction remains constant (~2% for weight).
Signal Detection Theory: detecting a stimulus depends not just on the stimulus intensity but also on the observer's criteria (willingness to say "yes"). No fixed absolute threshold — context, motivation, and experience matter.
Sensory adaptation: diminished sensitivity after constant exposure to a stimulus. You stop noticing your clothes, a constant smell, background noise. Allows focus on changes in the environment.
Subliminal stimuli: stimuli below the conscious threshold; can prime responses without awareness; effects are small and context-dependent.
Vision
The eye:
- Cornea: transparent protective layer; bends (refracts) light entering the eye
- Pupil: opening that controls light entry; iris regulates pupil size
- Lens: flexible; changes shape to focus (accommodation)
- Retina: light-sensitive tissue at the back; contains photoreceptors
Photoreceptors:
- Rods: 120 million; low-light, black-and-white vision; peripheral; not color