(from Latin, stimulus: a goad) The term STIMULUS appears barely to need definition: a stimulus is something—an object, action, sound or whatever—that produces a response. In a broad sense this is clearly correct. However, for scientists interested in analysing behaviour in terms of relationships between stimuli and responses (see for example CONDITIONING), a critical task is to define the EFFECTIVE STIMULUS. Stimuli are very rarely pure: they are composites of many features—texture, size, orientation, movement, colour, sound and so on. Which element is important in establishing an association between a stimulus and a response or REINFORCEMENT? That is, what is the effective stimulus? For example, during the breeding season, male sticklebacks will defend their territory against other breeding males.
Breeding male sticklebacks have red bellies. The defending male will attack any more or less fish-shaped object that has a red underbelly, but will leave a non-breeding male (lacking the red coloration) alone. The effective stimulus to TERRITORIAL BEHAVIOUR is clearly very specific. It is also worth noting that stimulus properties may appear to be different under different circumstances, though in fact the stimulus never changes. Hall cites the example of moon rock, which appears to be very dark, almost as black as coal, even under bright illumination. The brightness of the moon seen in the sky (as opposed to a piece of it in a lab) is due to its illumination by the sun, but the extraordinary degree of brightness it seems to possess is a product of the contrast between it and the blackness of the night sky surrounding it.
Evidently, a stimulus is something that produces a response: but the effective component of a stimulus must be properly clarified, and one must be aware that stimulus properties may not be constant across all conditions.
