Compound presentation

Abstract: We studied the ability of honeybees to discriminate between single odorants and binary olfactory mixtures. We analyzed the effect of the number of common elements between these two stimulus classes on olfactory discrimination. We used olfactory conditioning of the honeybees' proboscis extension reflex (PER), a paradigm in which odors can be associated with a reinforcement of sucrose solution. Bees were asked to discriminate reinforced from nonreinforced olfactory stimuli. They were trained with two elemental odors (A and B) versus a binary olfactory mixture. The mixture was either AB (group 2CE, two common elements), AC (group 1CE, one common element A), or CD (group 0CE, no common element). Three groups followed a positive patterning schedule (mixture reinforced and elements nonreinforced: groups 2CE+, 1CE+, and 0CE+) and three other groups a negative patterning schedule (mixture nonreinforced and elements reinforced: groups 2CE-, 1CE-, and 0CE-). We showed that a reduction of similarity (number of common elements) between elemental odors and compounds enhanced the ability to discriminate elements from compounds and that the kind of compound processing used by the bees supports theories that assume nonelemental compound processing (i.e., that exclude the mere summation of the elemental associative strengths upon compound presentation).

Abstract: Current theories of associative learning suggest that extinction is not the forgetting or “unlearning” of the original association, but rather involves active new learning involving the prediction of nonreward (Pavlov 1927; Delamater 1996; Rescorla 1996, 1997). Extinction learning is notoriously labile and easily disrupted. Even following extinction training, which may far exceed the duration of the original excitatory conditioning, the expression of extinction is easily disrupted by the passage of time (spontaneous recovery) (Rescorla 1997), stress or arousal (disinhibition) (Pavlov 1927; Horns and Heron 1940), the presentation of the previously trained reinforcer or stimuli associated with that reinforcer (reinstatement) (Rescorla and Heth 1975), or changes in the test context (renewal) (Bouton and King 1983). The aim of the current series of experiments was to examine approaches that might enhance extinction learning. The impact of a reinforcing event, such as reward delivery, depends on the degree to which that reinforcer is well signaled; a reinforcer that is preceded by a well-conditioned excitatory stimulus has a reduced ability to produce new learning (Kamin 1968). Observations such as this have lead to the development of error-correction models of learning (e.g., Rescorla and Wagner 1972), which postulate that learning will occur when there is a discrepancy between what is expected based on the presence of predictors (i.e., conditioned stimuli; CS) and the reinforcer that actually occurs (reward, in appetitive conditioning). These models make predictions not only about what will happen with reinforcement, but also about what learning will occur in the case of nonreinforcement, or extinction. Within this framework, the concurrent presence of an excitatory stimulus at a time when a response is nonreinforced would result in a particularly large prediction error, and hence should enhance the extinction of that response when it is not reinforced. In line with that prediction, Rescorla (2000) demonstrated that extinguishing a previously rewarded response in the presence of an additional excitatory stimulus resulted in significantly less subsequent responding at test than extinguishing the response either alone or in the presence of a neutral stimulus. Reberg (1972) made the interesting observation that even when two stimuli have themselves been extinguished individually, presenting them together, in compound, yielded greater responding than was observed to either stimulus alone, suggesting again that extinction does not entirely eliminate the original excitatory conditioning, and compound presentation potentially reintroduces a prediction error, which could in turn lead to further learning. This possibility was explored by Rescorla (2006) where he reported that nonreinforcement of a compound of stimuli augmented extinction when the components were later tested compared with a stimulus that was presented alone for the same number of extinction trials, suggesting that nonreinforcement of a compound stimulus further engaged learning mechanisms at the time of nonreinforcement, and thus, deepened the resultant extinction learning. While error correction models focus on the conditions under which learning will occur, they do not necessarily specify the mechanism. When an animal is confronted with a discrepancy between what is expected and what occurs, in addition to generating a prediction error, the experience itself may be arousing and engage the attentional and neural mechanisms responsible for establishing long-term memories. Emotional memories are often better remembered than neutral ones (e.g., Dolan 2002), and neutral stimuli may be better remembered when encoded in an emotional context (Sterpenich et al. 2006). A hallmark of many emotional experiences is the release of neural and hormonal modulators that produce characteristic arousing effects. Many of these neural and hormonal modulators, including norepinephrine (NE), have been proposed to modulate learning and memory formation (McGaugh 2004; Arnsten 2009). For example, pharmacological treatments that increase the NE response to stimuli facilitate episodic memory formation (O'Carroll et al. 1999; Southwick et al. 2002). In contrast, treatments that block postsynaptic NE effects decrease the memory benefit typically seen with emotional stimuli (Cahill et al. 1994; Berlau and McGaugh 2006). Importantly, arousal and associated noradrenergic effects may be particularly important for modifying performance when reward contingencies or reward value change as is the case in extinction (e.g., Sara et al. 1994; Usher et al. 1999). The aim of the current series of experiments was to examine ways that extinction learning might be enhanced through manipulations of stimulus presentation and by pharmacological modulation of the noradrenergic system. Rats were trained in a discriminative stimulus paradigm wherein performance of a lever-press response in the presence of one of three discrete stimuli resulted in the delivery of food reward. Thereafter, rats were extinguished and the efficacy of the extinction training was later assessed by examining spontaneous recovery of responding. We show that extinction of a stimulus compound improves extinction of the elements of that compound relative to extinction of a single stimulus. Further, we attempted to either mimic or block the benefit to extinction found with compound stimulus presentation by administering drugs that modulate noradrenergic signaling. To the extent that the benefit observed with compound stimulus presentation involves noradrenergic signaling, we predicted that drugs that increase NE should improve, whereas drugs that block NE should impair extinction revealed as changes in the degree of spontaneous recovery of responding at test. Consistent with these predictions we demonstrate that administration of the α2-antagonist yohimbine or the norepinephrine transporter blocker atomoxtetine augmented the extinction of a single stimulus. In contrast, administration of the β-adrenergic antagonist propranolol diminishes the benefit of compound–stimulus extinction. These findings suggest that extinction of reward-paired stimuli can be modulated by manipulations of noradrenergic signaling.

Abstract: An infant born in a compound presentation had ischemic forearm necrosis requiring amputation. Bilateral renal vein thrombosis predating labor and delivery was also diagnosed. The possibility of congenital ischemic necrosis in an extremity should not affect the obstetrical management of compound presentation, but if this complication occurs a search for an underlying coagulopathy is important.

Abstract: Four magazine approach experiments were performed with rats to explore the decrement produced by reinforcing a compound of two previously conditioned stimuli. Compound presentation produced the usual over-expectation decrement in responding to the individual stimuli. However, testing in an alternative, but similarly treated, context allowed renewal of the initial responding to the individual stimuli. This renewal is similar to that seen after a decrement produced by nonreinforcement. It joins other results in suggesting that the decrement produced in extinction and overexpectation may be due to the same mechanism.