Effects of NMDA antagonist dizocilpine (MK-801) are modulated by the number of distractor stimuli in the rodent odor span task of working memory.

The rodent odor span task (OST) uses an incrementing non-matching to sample procedure in which a series of odors is presented and selection of the session-novel odor is reinforced. An OST is frequently used to test the effects of neurobiological variables on memory capacity as the number of odors to remember increases during the course of the session. In this regard, one important finding has been that NMDA receptor antagonists selectively impair OST performance at doses that spare accuracy on control tasks. However, in many versions of the odor span task the number of stimuli to remember is confounded with the number of distractor odors presented to the rat on each trial. The present study compared the effects of the NMDA antagonist dizocilpine when the number of choices was held constant at two (one novel odor-S+ and one previously presented distractor odor-S-) and when the number of choice stimuli was permitted to increase up to 10 (one S+ and 9 S-). Dizocilpine impaired OST accuracy at doses that had no effect on a reference memory control task in both 2-choice and 10-choice conditions; however, the dose-response function was shifted to the left in the 10-choice tests. The impairments produced by dizocilpine were exacerbated as the memory load increased in both 2- and 10-choice conditions. These findings support the hypothesis that NMDA antagonism reduces the number of stimuli that rats can remember accurately, but the interaction between the effective DZP dose and the number of distractors shows that drug effects on OST performances may involve attentional factors in addition to memory capacity. The findings also demonstrate that variations in number of OST distractors can be used to alter sensitivity of the task.

Generalized identity in a successive matching-to-sample procedure in rats: Effects of number of exemplars and a masking stimulus.

Two experiments examined the emergence of generalized identity matching in rats using a successive discrimination procedure with olfactory stimuli. Trials consisted of the presentation of two odors separated by a 1-s interstimulus interval. Responses during the second odor presentation were reinforced only if the two odors were identical. In Experiment 1, rats were trained with two odors and then exposed to sessions that included unreinforced probe trials with novel odors. There was evidence of higher response rates on matching probe trials in some rats, but matching did not approach baseline levels. Additional training with four exemplars produced transfer to novel odors that was equivalent to baseline levels. Experiment 2 tested the possibility that detection of stimulus change, rather than generalized identity, was responsible for the transfer seen in Experiment 1. Thus, a masking odor was inserted during the 1-s interstimulus interval so that stimulus change occurred on all trials. Although response rates on probe trials were lower than baseline rates, above chance transfer to novel stimuli was still observed in four of the five animals tested. These findings support the hypothesis that transfer of matching to novel odors in this successive matching-to-sample paradigm is based on a generalized identity relation.

Abstraction, Multiple Exemplar Training and the Search for Derived Stimulus Relations in Animals.

Symmetry and other derived stimulus relations are readily demonstrated in humans in a variety of experimental preparations. Comparable emergent relations are more difficult to obtain in other animal species and seem to require certain specialized conditions of training and testing. This article examines some of these conditions with an emphasis on what animal research may be able to tell us about the nature and origins of derived stimulus relations. We focus on two areas that seem most promising: 1) research generated by Urcuioli's (2008) theory of the conditions necessary to produce symmetry in pigeons, and 2) research that explores the effects of multiple exemplar training on emergent relations. Urcuioli's theory has successfully predicted emergent relations in pigeons by taking into account their apparent difficulty in abstracting the nominal training stimulus from other stimulus properties such as location and temporal position. Further, whereas multiple exemplar training in non-humans has not consistently yielded arbitrarily-applicable relational responding, there is a growing body of literature showing that it does result in abstracted same-different responding. Our review suggests that although emergent stimulus relations demonstrated in non-humans at present have not yet shown the flexibility or generativity apparent in humans, the research strategies reviewed here provide techniques that may permit the analysis of the origins of derived relational responding.

Social interactions following parturition in stumptail macaques.

The social behavior of six female stumptail macaques (Macaca arctoides) living in a social group of 23-29 individuals was monitored before and after parturition and was compared to the social interactions of six nonpregnant females. Pregnant and control females did not differ in terms of affiliative and most types of agonistic behavior performed or received. After parturition, new mothers interacted with more group members than did control females. New mothers received more grooming initiations but did not differ from control females in the duration of time spent grooming. New mothers also received more frequent contact aggression from adult males and increased frequency of contact aggression toward juveniles. Overall, new mothers received more submissive responses relative to control females. New mothers did not show elevations in overall dominance status, as measured by the direction of agonistic interactions, and still received aggression from dominant individuals at normative rates. These data indicate that new mothers may become focal points for group interactions, but they do not receive more affiliation overall or less aggression than nonmothers.