stimulus

Meaning: A stimulus (plural stimuli) is any event, object, or energy in the environment or body that can be detected and can evoke a response. In sensation, it is physical energy, such as light or sound waves, that receptors transduce into neural signals; it must reach the absolute threshold to be detected.

In learning, stimuli are named by role: an unconditioned stimulus (UCS) triggers a response without training, a neutral stimulus at first does not, and a conditioned stimulus (CS) is a former neutral stimulus that triggers a conditioned response after pairing with the UCS. In operant conditioning, a discriminative stimulus signals that reinforcement or punishment is available. In biology, a stimulus can also mean the input that drives a neuron toward threshold.

On the MCAT: Passages describe an experiment, such as a tone before food, and ask you to label each stimulus or predict the effect of extinction, generalization, or discrimination. Sensation items ask whether a stimulus is above or below threshold.

Common confusion: Students swap the CS and UCS. The UCS elicits its response with no training; the CS does so only after pairing. A stimulus is the energy or event itself, not the sensation or perception built from it.