After an organism has learned to respond to a conditioned stimulus, it must still learn when not to respond. Discrimination is that refinement. The dog salivates to a tone of one pitch but not to a slightly higher or lower tone. The difference is trained by reinforcing one stimulus and withholding reinforcement from the other.

In operant settings the same principle holds. A pigeon pecks a key illuminated green for food and ignores the red key. The process sharpens perception and conserves energy. Without discrimination, every similar cue would trigger the same response, producing wasteful or dangerous behavior.

Stimulus generalization is the opposing tendency: responding to stimuli that resemble the original. Discrimination training pushes the organism to draw finer boundaries. The steeper the generalization gradient, the sharper the discrimination.

• Learned ability to distinguish similar stimuli
• Achieved by differential reinforcement
• Opposite of stimulus generalization
• Measured by the slope of the generalization gradient

In everyday life the skill appears when a child learns that only certain adults may be approached for help, or when a driver stops at red lights but not at similarly colored store signs.