Changes in the categorization of appetitive and aversive events during postnatal development of the rat.

Environmental stimuli are not clearly categorized into appetitive and aversive classes during the first postnatal week. Three- and 6-day-old rats are highly activated by nominally appetitive (milk infusions) and aversive (foot shock) stimuli. Both evoked the same generalized behavioral responses (rolling, curling). By 12 days of age, these stimuli were responded to differently; mouthing and probing occurred in response to milk while flinching and escape responses were observed to shock. The affective properties of mild shock were found to be hedonically opposite for 6- and 12-day-old pups. Six-day-olds showed an acquired preference for an odor paired with shock which increased with intensity (.1-.5 mA) similar to that observed with milk infusions. An acquired odor aversion was displayed by 12 days old using these shock parameters. These results suggest that young rat pups may not differentiate many appetitive and aversive events behaviorally or affectively until the second postnatal week.