Sensation & Sensory Systems
- Distinguish sensation from perception and define transduction
- Explain absolute and difference thresholds and sensory adaptation
- Describe how the eye and ear transduce physical energy into neural signals
Sensation, transduction, and thresholds
Sensation is the process by which sensory receptors detect physical energy from the environment; perception is how the brain organizes and interprets it. The bridge between them is transduction — converting physical energy (light, sound waves) into neural impulses the brain can read. Detection has limits. The absolute threshold is the minimum stimulation needed to detect a stimulus 50% of the time. The difference threshold (or just-noticeable difference) is the smallest change you can detect — governed by Weber’s law, which says the change must be a constant proportion of the original, not a fixed amount.
Adaptation and attention
Sensory adaptation is our reduced sensitivity to constant, unchanging stimulation — you stop noticing the feel of your clothes or a steady hum, freeing attention for changes that matter. Attention is also selective: selective attention focuses awareness on one stimulus among many (the "cocktail-party effect" of hearing your name across a room), and dramatic failures like inattentional blindness (missing the gorilla in Simons and Chabris’s famous study) show we miss what we do not attend to. Signal detection theory adds that detecting a faint stimulus depends not just on its intensity but on our expectations, motivation, and fatigue.
Vision and hearing: two transducers
In the eye, light passes through the cornea and lens and lands on the retina, where rods (dim light, black-and-white, peripheral) and cones (color and detail in bright light) transduce it; signals leave via the optic nerve. Color is explained by two complementary theories: the trichromatic (Young–Helmholtz) theory (three cone types for red, green, blue) at the receptor level, and the opponent-process theory (red-green, blue-yellow, black-white pairs) that explains afterimages. In the ear, sound waves vibrate the eardrum and tiny bones, moving fluid in the cochlea, whose hair cells transduce the vibration into neural signals.
A barista adds sugar to a customer’s already very sweet 20-ounce drink. To taste any difference, she must add far more sugar than she would to a small, barely sweet drink. Explain this using a principle of sensation, and identify the principle.
- 1.Identify the phenomenon: the customer must detect a change in sweetness — a difference threshold, or just-noticeable difference.
- 2.Recall Weber’s law: the just-noticeable difference is a constant proportion of the original stimulus, not a fixed amount.
- 3.Apply it: because the drink is already very sweet, the amount of sugar needed to be noticeable is a proportion of a large baseline — so much more sugar is required than for a barely sweet drink.
- 4.Conclude: the stronger the original stimulus, the larger the absolute change needed to notice a difference.
After a few minutes in a room with a strong smell of fresh paint, a person no longer notices the odor, even though the paint is still present. This best illustrates:
Keep sensation (bottom-up detection of raw energy) separate from perception (top-down interpretation). Transduction belongs to sensation. Also do not confuse the absolute threshold (minimum to detect a stimulus) with the difference threshold (minimum to detect a change).
A person walks from bright sunlight into a dark movie theater and at first can see almost nothing, but after several minutes can make out the seats. Which cells are becoming more responsible for vision as the eyes adjust to the dark?
For color-vision items, pair the two theories: trichromatic (three cones) explains detection at the receptor level, while opponent-process explains afterimages and color pairs. Questions about staring at a color then seeing its complement are always opponent-process.
Answer the 2 checkpoints as you read.
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