Studies of conditioning essentially look at how various unconditioned and conditioned stimuli influence responses under different arrangements of time and space. Following are a few general findings.
Pavlovian conditioning tends to be readily established when stimuli or responses or both are strong rather than weak. For example, in response to a near-drowning experience, some people promptly learn to fear such conditioned stimuli as the sights of water, boats, palm trees, bathing suits, and so on. In such cases, relevant stimuli and responses (panic) are presumably quite strong.
Conditioned stimuli are most likely to elicit conditioned responses when unconditioned and conditioned stimuli are paired consistently. If a mother always hums when she rocks her infant daughter to sleep, humming is likely to become a potent and reliable CS which soothes and comforts her daughter. This outcome is less likely if mother hums only occasionally.
When several stimuli precede a US, the one most often paired with the US will likely emerge as the strongest CS. If, for example, both parents threaten to punish their young son, but only father always carries out the threats, father's threats are more likely than mother's to evoke apprehension in the child.
For some responses, such as eye blinking, conditioned stimuli tend to be strongest when they precede the US by about one-half second. The optimal interval for other responses varies from seconds to fractions of seconds: A neighbor's dog barks immediately before little Sophie falls from her swing, bumping her nose very hard. She cries. If the dog's bark subsequently makes Sophie feel uneasy, the bark is functioning as a CS. This outcome becomes less and less likely as the bark and fall increasingly separate in time.
Conditioned responses are usually not established if a US and CS occur together (simultaneous conditioning)—the potency of the UC overshadows the potential CS—or when a neutral stimulus follows the US (backward conditioning).
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