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6A · Sensing, perceiving, and responding
Sensing the environment
How raw physical energy becomes neural signals (sensation), how the brain organizes those signals into a usable world (perception), the limits of what we can detect (thresholds), and the machinery of each sense.
Sensory processing & psychophysics
The study of how physical stimuli relate to the sensations we experience — thresholds, the just-noticeable difference, and signal detection.
Transduction is the conversion of physical/chemical stimulus energy into an electrochemical (neural) signal by a sensory receptor. Bottom-up processing builds perception from the raw sensory data up; top-down processing uses prior knowledge and expectations to interpret. Key receptor types recur across senses: photoreceptors (light), hair cells (sound, balance), chemoreceptors (taste, smell), mechanoreceptors (touch, pressure), thermoreceptors, and nociceptors (pain).
Thresholds & Weber's law
The absolute threshold is the minimum stimulus detectable 50% of the time; the difference threshold (JND) is the smallest detectable change, and by Weber's law it's a constant percentage of the original stimulus, not a fixed amount.
Subliminal stimuli fall below the absolute threshold (below conscious detection). The threshold of conscious perception is distinct from the absolute threshold. Sensory adaptation is the decline in receptor sensitivity to a constant stimulus (you stop feeling your clothes). Weber's law explains why adding one candle to a dark room is obvious but adding one to a bright room isn't — the JND scales with intensity.
Don't confuse
Sensory adaptation (receptor-level, a constant stimulus) vs. habituation (a behavioral/cognitive decrease, from 7C). AAMC offers them as a pair.
Signal detection theory
Detection depends not just on the stimulus but on the observer's decision process. Outcomes are hits, misses, false alarms, and correct rejections, split into sensitivity (d′) and response bias.
Signal detection theory says there's no fixed threshold; whether you report a stimulus depends on sensitivity (how well you actually discriminate signal from noise) and response bias/criterion (your willingness to say "yes," shaped by expectations, motivation, and the costs/rewards of each outcome). A radiologist primed to find tumors raises hits and false alarms.
How AAMC tests it
A passage manipulates payoffs or expectations and asks how hits/false alarms shift — that's response bias, not a change in true sensitivity.
Vision
Light is focused onto the retina, transduced by rods and cones, and routed to the visual cortex; color is explained by two complementary theories.
Light passes the cornea → pupil (sized by the iris) → lens (which changes shape, accommodation, to focus) → onto the retina. Photoreceptors transduce light: rods (very light-sensitive, peripheral, night/dim vision, no color) and cones (color and fine acuity, concentrated in the fovea). Signals pass through bipolar and ganglion cells, whose axons form the optic nerve (the exit point is the optic disc / blind spot). The pathway runs via the optic chiasm and LGN of the thalamus to the visual cortex; feature detectors (Hubel & Wiesel) respond to specific features, and parallel processing handles color, motion, form, and depth simultaneously.
Trichromatic vs. opponent-process theory
Trichromatic (Young-Helmholtz) theory: three cone types (red/green/blue) combine to make all colors. Opponent-process theory: color is coded in opposing pairs (red–green, blue–yellow, black–white), explaining afterimages.
Both are correct at different stages: trichromatic at the cone (receptor) level, opponent-process at the ganglion/LGN (neural processing) level. Negative afterimages (stare at green, see red) are the classic opponent-process evidence; color blindness (missing/defective cones) is trichromatic evidence.
Don't confuse
Which theory explains afterimages? → opponent-process. Which explains color blindness from missing cones? → trichromatic.
Hearing (audition)
Sound waves are funneled through the ear, vibrate the ossicles and cochlear fluid, and bend hair cells; pitch is encoded by two theories.
Path: pinna → auditory canal → tympanic membrane (eardrum) → the ossicles (malleus, incus, stapes) → oval window → cochlea, where fluid waves move the basilar membrane and bend hair cells (the organ of Corti) to transduce sound. Loudness is coded by amplitude; pitch by the two theories below. Conductive hearing loss (a mechanical problem in the outer/middle ear) vs. sensorineural loss (damage to the cochlea/hair cells or auditory nerve) is worth knowing.
Place vs. frequency theory
Place theory — pitch is coded by which spot on the basilar membrane vibrates most (explains high frequencies). Frequency (temporal) theory — pitch is coded by the rate of neural firing matching the sound's frequency (explains low frequencies); the volley principle extends it.
The other senses
Touch/pain (somatosensation), taste, smell, body position (kinesthetic), and balance (vestibular).
- Somatosensation — touch, pressure, temperature, and pain. The gate control theory of pain: a neural "gate" in the spinal cord can block or allow pain signals, explaining why rubbing an injury or distraction reduces pain.
- Taste (gustation) — chemoreceptors in taste buds on papillae; five basic tastes (sweet, sour, salty, bitter, umami).
- Smell (olfaction) — olfactory chemoreceptors; the only sense not routed through the thalamus (it projects directly to the cortex and limbic system, tying smell tightly to memory and emotion). Pheromones signal between members of a species.
- Kinesthetic sense (proprioception) — position and movement of body parts.
- Vestibular sense — balance and spatial orientation, via the semicircular canals and otolith organs of the inner ear.
Perception & perceptual organization
How the brain organizes sensations into coherent objects and scenes — Gestalt grouping, depth, and constancy.
Gestalt principles describe how we group elements into wholes: proximity (near things group), similarity, good continuation, closure (we fill gaps), and figure-ground (we separate an object from its background). Depth perception uses binocular cues — retinal (binocular) disparity and convergence — and monocular cues — relative size, interposition (overlap), linear perspective, texture gradient, and motion parallax. Perceptual constancy keeps size, shape, and color stable despite changing retinal images.
How AAMC tests it
Name the Gestalt principle or depth cue at work in a described image, or identify constancy when an object is perceived as stable despite a changed sensory input.
Worked question
A radiologist is told the next set of mammograms comes from a high-risk clinic. She then reports more suspected tumors than usual, including several findings later confirmed to be healthy tissue. This shift is best explained by a change in her: