Ontogenetic differences in conditioning to context and CS as a function of context saliency and CS-US interval

Abstract

A series of experiments examined ontogenetically the effects of context saliency and CS-US interval on the learning of CS-US and context-US associations. Although adult rats generally showed an inverse relationship between CS and context learning that depended upon CS-US interval and context saliency, the opposite relationship held for preweanlings, Consistent with many contemporary learning theories, adult rats expressed CS-context competition in learning about the US. Preweanlings, on the other hand, exhibited potentiated responding to either the context or the CS as a result of strong learning about the other element. These results suggest that adult and infant rats process information in a qualitatively different manner.Infants and adults may process information in a qualitatively different manner. For example, young animals do not show adults' inclination for overshadowing when they are tested on a single element of a compound training stimulus (Kraemer, Lariviere, & Spear, 1988;Mellon, Kraemer, & Spear, 1991) but rather show potentiation, the opposite of overshadowing (Hinderliter & Misanin, 1988;Kucharski & Spear, 1985;Spear & Kucharski, 1984). Age-related differences also exist in the treatment of contextual stimuli (Lariviere, Chen, & Spear, 1990; McKinzie, Lee, Bronfen, Spear, & Spear, 1994;Solheim, Hensler, & Spear, 1980). For example, McKinzie and Spear (1991) found that preweanling rats expressed strong learning to both context and tone following tone-footshock pairings. Context conditioning in animals given pairings ofa conditional stimulus (CS) and an unconditional stimulus (US) exceeded that in preweanlings given unpaired presentations of the same events. Thus, pairing of a tone CS with the US potentiated responding to the context (see also Goodwin et aI., 1992).Adults are more likely to attend to stimuli in the selective manner indicative of overshadowing. Punctate CSs usually are chosen for their capacity to elicit an orienting reflex (Pavlov, 1927) and, therefore, are likely to overshadow contextual stimuli in a typical conditioning experiment. Associative competition between the context and CS occurs in procedures that vary contingency and contiguity of the CS-US relationship. For example, Odling- Smee (1975) found that, as CS-US contingency decreased, conditioning to the training context increased. Marlin (1981) found that an inverse relationship existed between context and CS learning as a function of CS-US interval. When the CS and US were temporally contiguous, responding to the CS overshadowed responding to the context. As CS-US interval increased, however, conditioning to the CS diminished, while context learning increased. At an intermediate trace interval, conditioning occurred to both the context and the CS, whereas only context conditioning occurred at long CS-US intervals. Marlin concluded that both context and CS stimuli potentially form associations with the US and that learning occurs to the better predictor of the forthcoming US.The purpose of the present series of experiments...

Contextual fear conditioning differs for infant, adolescent, and adult rats

“…The apparently special effectiveness of infants in learning redundant aspects of an episode is exemplified by their strong conditioning to context in a circumstance in which a CS is markedly more predictive of the US, whereas adults exhibit their weakest conditioning to context in this circumstance (Brasser & Spear, 2004;McKinzie & Spear, 1995). One possible reason is that infants are more likely to process the CS and context, for example, as a "single intense event" --an undifferentiated configural representation that is conditioned more effectively because of its greater perceived intensity.…”

“…In these cases, infants have seemed equally or more effective in contextual fear conditioning than older animals, including adults (e.g., Brasser & Spear, 2004;Carew & Rudy, 1991;Lariviere et al, 1990). And when a discrete auditory CS predicts the footshock within a salient olfactory context, contextual fear conditioning in the infant exceeds that in the adult, even when conditioning to the CS is held constant (Brasser & Spear, 2004;Mckinzie & Spear, 1995).…”

“…Second, adolescents again were shown to have a particularly strong disposition to learn about context (e.g., Barrett, Rizzo, N. E. Spear, & Spear, 1984), in that contextual fear conditioning was significant whether it included paired or unpaired presentations of the CS and US. Finally, adults were again shown to be relatively limited in contextual fear conditioning: such conditioning occurred when unpaired presentations of CS and US were presented within that context (Marlin, 1981;McKinzie & Spear, 1995;Odling-Smee, 1975), and not with paired CS and US--opposite the infants.In a general sense, these results are consistent with the idea that preweanling rats are less likely than adults to be selective in their learning and more likely to acquire memory attributes redundant to the particular learning task at hand (Spear, 1979(Spear, , 1984Spear, McKinzie, & Arnold, 1994;Spear, Kraemer, Molina, & Smoller, 1988). The apparently special effectiveness of infants in learning redundant aspects of an episode is exemplified by their strong conditioning to context in a circumstance in which a CS is markedly more predictive of the US, whereas adults exhibit their weakest conditioning to context in this circumstance (Brasser & Spear, 2004;McKinzie & Spear, 1995).…”

“…Finally, adults were again shown to be relatively limited in contextual fear conditioning: such conditioning occurred when unpaired presentations of CS and US were presented within that context (Marlin, 1981;McKinzie & Spear, 1995;Odling-Smee, 1975), and not with paired CS and US--opposite the infants.…”

“…In these cases, infants have seemed equally or more effective in contextual fear conditioning than older animals, including adults (e.g., Brasser & Spear, 2004;Carew & Rudy, 1991;Lariviere et al, 1990). And when a discrete auditory CS predicts the footshock within a salient olfactory context, contextual fear conditioning in the infant exceeds that in the adult, even when conditioning to the CS is held constant (Brasser & Spear, 2004;Mckinzie & Spear, 1995).It should not be a surprise that infant rats --in some cases--show more contextual fear conditioning than adults, given other instances in which infants have seemed more sensitive to context than older animals: CS conditioning is disrupted by a change in olfactory context more extremely for infants than adults (Solheim, Hensler, & Spear, 1980); Pavlovian trace conditioning is regularly facilitated in infant rats by a salient olfactory and visual context but unaffected by the same manipulations in adults (Brasser & Spear, 1998;McKinzie & Spear, 1995); the presence of a familiar context such as odor of the rat's home is more likely to affect infant learning than adult learning (Spear, Kucharski, & Hoffmann, 1985); and infants are more likely than older animals to show enhanced unconditioned responding to visual, auditory and tactile stimuli in the presence of a context rich in novel odor (Brasser & Spear, 1998;Kraebel, Vizvary, & Spear, 1998).Thus, the goal of the present series of experiments was to examine fear conditioning to an olfactory context, the type of situation where infant rats seem to show the strongest disposition for contextual conditioning. They were intended to replicate and extend previous experiments (viz., Brasser & Spear, 2004) suggesting that contextual fear conditioning in infants may exceed that of adults, particularly when an auditory CS within an olfactory context also predicts the US.…”