Sensation and Perception
Sensation is the detection of physical stimuli by sensory receptors and their conversion into neural signals; perception is the brain’s organization and interpretation of that sensory information. The AAMC outline expects you to know sensory thresholds, transduction pathways for each sense, signal detection theory, and the Gestalt principles of perceptual organization. This is one of the most heavily tested topics in the Psych/Soc section.
Thresholds and Psychophysics
Psychophysics studies the relationship between physical stimuli and the sensations they produce.
- Absolute threshold — the minimum stimulus intensity detectable 50% of the time.
- Threshold of conscious perception — the level at which a stimulus reaches awareness; sub-threshold (subliminal) stimuli may still be processed.
- Difference threshold (just-noticeable difference, JND) — the minimum change needed to detect a difference between two stimuli.
- Weber’s law — the JND is a constant proportion of the original stimulus, not a fixed amount. Lifting 10 vs 11 lb is noticeable; 100 vs 101 lb is not.
Signal Detection Theory
Signal detection theory (SDT) explains how we detect faint stimuli against background noise, accounting for both sensitivity and response bias (expectations, motivation). Every trial yields one of four outcomes:
| Signal present | Signal absent | |
|---|---|---|
| Responded "yes" | Hit | False alarm |
| Responded "no" | Miss | Correct rejection |
SDT separates d’ (sensitivity) from the criterion (bias). A radiologist who is very cautious will have more hits but also more false alarms — a liberal criterion.
Sensory Adaptation and Transduction
Sensory adaptation is a decrease in response to a constant stimulus over time (you stop noticing your clothes). Transduction converts physical stimulus energy into electrochemical neural signals via receptor cells.
Light passes cornea → pupil → lens → retina. Rods (peripheral, dim light, no color) and cones (fovea, color, high acuity) contain photopigments. Signals pass bipolar cells → ganglion cells → optic nerve → thalamus (LGN) → visual cortex. The trichromatic theory (three cone types) explains color at the receptor level; the opponent-process theory (red–green, blue–yellow, black–white) explains afterimages.
Sound waves → tympanic membrane → ossicles (malleus, incus, stapes) → oval window → cochlea. Hair cells on the basilar membrane transduce vibration. Place theory explains high-frequency pitch (location of peak vibration); frequency/temporal theory explains low-frequency pitch (firing rate).
Perceptual Processing and Gestalt
Bottom-up processing builds perception from raw sensory data; top-down processing uses prior knowledge, context and expectations to interpret input. The Gestalt principles describe how we organize elements into wholes:
- Proximity
- Objects near each other are grouped together.
- Similarity
- Similar objects are grouped together.
- Continuity (good continuation)
- We perceive smooth, continuous lines rather than abrupt changes.
- Closure
- We fill in gaps to perceive a complete object.
- Figure–ground
- We distinguish an object (figure) from its background (ground).
- Prägnanz (simplicity)
- We perceive the simplest, most stable interpretation.
Depth perception uses binocular cues (retinal disparity, convergence) and monocular cues (relative size, interposition, linear perspective, motion parallax).
Worked MCQs
Five MCQs covering the high-yield testing patterns for sensation and perception.
Q1. A researcher finds that a subject can just detect the difference between a 100 g and a 105 g weight. According to Weber’s law, the just-noticeable difference for a 200 g weight would be about:
The JND is a constant proportion. 5 g out of 100 g is 5%, so for a 200 g weight the JND is 5% of 200 = 10 g. Weber’s law describes a constant ratio, not a fixed amount.
Q2. A radar operator reports a plane when none is present. In signal detection terms this is a:
Responding "yes" when the signal is absent is a false alarm. A miss is failing to detect a present signal; a hit is correctly detecting a present signal; a correct rejection is correctly saying "no" when absent.
Q3. After staring at a green image and then looking at a white wall, a person sees a red afterimage. This is best explained by:
Opponent-process theory posits red–green, blue–yellow and black–white channels. Fatiguing the green response leaves the red response dominant, producing a red afterimage. Trichromatic theory explains color coding at the cone level but not afterimages.
Q4. Perceiving a series of dots that are close together as a single group is an example of which Gestalt principle?
Proximity groups elements based on how near they are to one another. Similarity groups by shared features; closure fills gaps; continuity favors smooth continuous contours.
Q5. Reading a word with a smudged letter and still understanding it because of the surrounding context is an example of:
Using prior knowledge and context to interpret incomplete sensory data is top-down processing. Bottom-up processing would rely only on the raw features of the smudged letter.
Quick Recap
- Absolute threshold = detectable 50% of the time; difference threshold = JND.
- Weber’s law: JND is a constant proportion of the stimulus.
- Signal detection: hit, miss, false alarm, correct rejection; separates sensitivity from bias.
- Rods = dim light/no color; cones = color/acuity in the fovea.
- Trichromatic (cones) vs opponent-process (afterimages) color theories.
- Place theory (high pitch) vs frequency theory (low pitch) for hearing.
- Gestalt: proximity, similarity, continuity, closure, figure–ground, Prägnanz.
- Bottom-up (data-driven) vs top-down (concept-driven) processing.