Sensation and Perception

From physical energy to experience

Sensation is detecting raw energy; perception is organising and interpreting it. Transduction converts light, sound or pressure into neural impulses.

• Absolute threshold — the intensity detected 50% of the time.
• Difference threshold (just noticeable difference) — Weber's law says it is a constant proportion, not a constant amount: $\Delta I / I = k$. Adding one candle to ten is obvious; adding one to a hundred is not.
• Sensory adaptation — constant stimulation fades from awareness, which is why you stop feeling your watch.
• Signal detection theory — detection depends on expectation, motivation and fatigue, not just intensity: hits, misses, false alarms, correct rejections.

Vision and hearing

Light passes cornea, pupil, lens (which accommodates) and lands on the retina. Rods sit in the periphery, work in dim light and see no colour; cones cluster in the fovea and handle colour and detail. Signals leave by the optic nerve, creating the blind spot. Feature detectors respond to lines and angles, and the brain uses parallel processing.

Colour needs two theories at two stages: the trichromatic theory (three cone types, red/green/blue) explains the retina; the opponent-process theory (red–green, blue–yellow, black–white pairs) explains what happens after, including afterimages. In hearing, place theory accounts for high pitches and frequency theory for low ones. Conduction deafness is mechanical; sensorineural deafness is hair-cell or nerve damage and is the type a cochlear implant addresses.

Organising the input

Bottom-up processing builds from the raw data; top-down processing uses expectation and experience — a perceptual set. Gestalt principles group the parts: figure-ground, proximity, similarity, closure, continuity. Depth comes from binocular cues (retinal disparity, convergence) and monocular cues (relative size, interposition, linear perspective, texture gradient, relative height). Constancies keep size, shape and colour stable as the image on the retina changes.