Sensation and Perception
What to know
Sensation is detecting physical energy; perception is interpreting it. Psychophysics links them: absolute threshold (weakest detectable stimulus), difference threshold/just noticeable difference, and Weber's law (JND is a constant proportion of the original stimulus). Signal detection theory separates sensitivity from response bias. Sensory adaptation is declining responsiveness to constant stimulation.
Vision and hearing
Light enters the eye through the cornea and pupil, is focused by the lens, and strikes the retina, where rods (dim light, peripheral, no color) and cones (color, detail, fovea) transduce it. Ganglion cells' axons form the optic nerve, cross at the optic chiasm, and reach the occipital lobe. Color theories: trichromatic (three cone types) and opponent-process (red-green, blue-yellow, black-white pairs-explains afterimages). Sound is frequency (pitch) and amplitude (loudness); the cochlea's hair cells transduce, and place vs. frequency theories explain pitch.
Organizing perception
Gestalt principles-proximity, similarity, closure, continuity, figure-ground-describe grouping. Depth cues are binocular (retinal disparity, convergence) or monocular (interposition, linear perspective, relative size). Perceptual constancies keep size, shape, and color stable. Top-down processing uses expectations; bottom-up uses raw data.
Traps
Don't swap absolute and difference thresholds. Opponent-process explains afterimages; trichromatic explains initial color mixing. Perceptual set and context can bias perception-questions often test this.
Rule of thumb
Ask "is this detecting (sensation) or interpreting (perception)?" Then match the theory to the phenomenon-afterimages signal opponent-process.