sensation

Meaning: Sensation is the process by which sensory receptors detect stimuli such as light, sound waves, pressure or odorant molecules and transduce them into neural signals. The signals travel along afferent pathways, usually through the thalamus (olfaction is the main exception), to primary sensory cortex. This is bottom-up processing. Perception is the later interpretation and organization of that input, drawing on memory, expectation and context.

Related sensation topics include absolute threshold (the minimum stimulus detected about half the time), the difference threshold or just-noticeable difference (by Weber's law, a constant proportion of the original stimulus), signal detection theory, and sensory adaptation.

On the MCAT: Passages describe a receptor type, such as rods and cones, hair cells or olfactory neurons, and ask where transduction occurs or which pathway carries the signal. Threshold experiments ask you to predict how detection changes with stimulus intensity, noise or response bias. Other items ask whether a finding reflects sensation or perception.

Common confusion: Sensation and perception are not the same. Sensation is detection and encoding by receptors and sensory pathways; perception is interpretation. Two people with the same receptor input can perceive it differently, and an optical illusion is a perception effect, not a failure of transduction.