In classical conditioning, the stimulus that elicits an unconditioned response.

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The stimulus in a stimulus-response chain that is naturally occurring as opposed to learned.

In classical conditioning, the stimulus that produces what is initially the unconditioned response. unconventional air pollutants

a stimulant which induces an unconditioned reaction, as in withdrawal from a hot stovetop, contracting of the pupil upon exposure to light, or mouth-watering whenever food is placed inside the mouth.

In classical conditioning, an unconditioned stimulus (US or UCS) is any stimulus that can evoke a response without the organism going through any previous learning; the response to the US (the uncondi [..]

See classical conditioning.

All organisms are born with a set of reflexes (US - UR). These relationships are invariant and biologically based. The eliciting event is called the unconditioned stimulus (US).

A stimulus, the capacity of which to elicit a response does not depend upon its having been paired with another stimulus possessing this capacity.

A stimulus that produces the unconditioned response without previous pairing with another stimulus.