sensory adaptation

Meaning: When a stimulus stays constant, sensory receptors fire less over time, so the perceived intensity fades. Examples are not noticing a smell after minutes in a room, the feel of clothing on skin, or adjusting to a hot bath. The system is tuned to detect change, which is more useful than monitoring constants. Receptors differ in how fast they adapt. Rapidly adapting (phasic) receptors, such as Pacinian corpuscles for vibration, respond mainly when a stimulus starts or stops. Slowly adapting (tonic) receptors, such as pain receptors and Merkel cells, keep firing during a sustained stimulus.

On the MCAT: A passage may show a firing-rate graph over time during a steady stimulus. Adaptation looks like a high initial rate that declines, with a burst again when the stimulus changes in intensity. Questions ask you to predict that a constant stimulus gives a reduced response, or to identify an example. In vision, dark and light adaptation involve photopigment changes and pupil size, and the continuous image on the retina fades without eye movements (stabilized retinal images).

Common confusion: Sensory adaptation is not habituation. Adaptation occurs at the level of the receptor or sensory pathway and does not need attention or learning. Habituation is a learned decrease in behavioral response to a repeated stimulus, a form of non-associative learning involving the nervous system more centrally. Adaptation is also not fatigue or damage. The receptor responds again when the stimulus changes.