1) A discrimination in which two or more discriminative stimuli are presented at the same time rather than successively (e.g., see JUMPING STAND) and which therefore involves two or more alternative responses. The locations of the stimuli are ordinarily at or close to those of the alternative responses (e.g., stimuli on each of two pigeon keys) and the organism is said to respond to one or the other stimulus.

2) In simultaneous discrimination, the SD and S' are presented at the same time and the organism is reinforced for responding to the relative properties of one or the other. For example, a pigeon may be presented with two keys, both illuminated with white lights, but one light is brighter than the other. The bird is reinforced for pecking the dimmer of the two keys. Pecks to the other key are placed on extinction. After training, the pigeon will peck the darker of any two keys.