Attenuation and enhancement of instrumental responding by signals for reinforcement on a variable interval schedule.

In three experiments we used pigeons responding on a variable interval schedule to examine the effects of signaling reinforcement on instrumental performance. Using a four-component multiple schedule, we found in Experiment 1 that when reinforcement was delayed by 0.5 s for two of the components, response rates were attenuated by signaling reinforcement relative to performance in the unsignaled delay condition. A delay of 3 s in the remaining two components resulted in an enhancement of responding when the delay was signaled. An enhancement of responding occurred when we used a scheduled 0.5-s delay when the procedure ensured that responding was prevented during the delay interval by extinguishing the keylight (Experiment 2) or by resetting the delay timer when responses were emitted during the delay (Experiment 3). These results are discussed with reference to the theories of signaling reinforcement during instrumental responding. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The plant cDNA LCT1 mediates the uptake of calcium and cadmium in yeast

Nonessential metal ions such as cadmium are most likely transported across plant membranes via transporters for essential cations. To identify possible pathways for Cd2+ transport we tested putative plant cation transporters for Cd2+ uptake activity by expressing cDNAs in Saccharomyces cerevisiae and found that expression of one clone, LCT1, renders the growth of yeast more sensitive to cadmium. Ion flux assays showed that Cd2+ sensitivity is correlated with an increase in Cd2+ uptake. LCT1-dependent Cd2+ uptake is saturable, lies in the high-affinity range (apparent KM for Cd2+ = 33 microM) and is sensitive to block by La3+ and Ca2+. Growth assays demonstrated a sensitivity of LCT1-expressing yeast cells to extracellular millimolar Ca2+ concentrations. LCT1-dependent increase in Ca2+ uptake correlated with the observed phenotype. Furthermore, LCT1 complements a yeast disruption mutant in the MID1 gene, a non-LCT1-homologous yeast gene encoding a membrane Ca2+ influx system required for recovery from the mating response. We conclude that LCT1 mediates the uptake of Ca2+ and Cd2+ in yeast and may therefore represent a first plant cDNA encoding a plant Ca2+ uptake or an organellar Ca2+ transport pathway in plants and may contribute to transport of the toxic metal Cd2+ across plant membranes.     

Testing response generation rules.

Robbins (1988) reported data that he viewed as inconsistent with Miller and Schachtman's (1985a) comparator hypothesis of conditioned response generation. Here we explain why we do not find his experiments a compelling test of the comparator hypothesis. We also briefly review other studies that tested the same predictions of the comparator hypothesis that Robbins examined. We conclude that there is considerable evidence that following excitatory or inhibitory conditioning with a target conditioned stimulus (CS) and unconditioned stimulus (US), extinction of other cues that were present during CS training ordinarily increases excitatory responding and decreases inhibitory responding to the CS. However, consistent with Robbins's conclusion, there is scant evidence that after CS–US training, enhancing the associative value of other cues that were present during CS training influences excitatory or inhibitory responding to the CS. The implications of these conclusions for the comparator hypothesis as an explanation of differences in acquired behavior and as a heuristic tool are considered. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The comparator hypothesis of conditioned response generation: Manifest conditioned excitation and inhibition as a function of relative excitatory strengths of CS and conditioning context at the time of testing.

Water-deprived rats were used in a conditioned lick suppression paradigm to test and further develop Rescorla's (1968) contingency theory, which posits that excitatory associations are formed when a conditioned stimulus (CS) signals an increase in unconditioned stimulus (US) likelihood and that inhibitory associations develop when the CS signals a decrease in US likelihood. In Exp I we found that responding to a CS varied inversely with the associative status of the context in which the CS was trained and that this response was unaltered when testing occurred in distinctively dissimilar context with a different conditioning history, provided associative summation with the test context was minimized. Results suggest that manifest excitatory and inhibitory conditioned responding is modulated by the associative value of the training context rather than that of the test context. Exp II demonstrated that postconditioning decreases in the associative value of the CS training context reduced the effective inhibitory value of the CS even when testing occurred outside of the training context. This contextual deflation effect was specific to the CS training context. These studies support the comparator hypothesis, which states that conditioned responding is determined by a comparison of the associative strengths of the CS and its training context that occurs at the time of testing rather than at the time of conditioning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Mechanisms underlying retarded emergence of conditioned responding following inhibitory training: Evidence for the comparator hypothesis.

The comparator hypothesis posits that conditioned responding is determined by a comparison at the time of testing between the associative strengths of the conditioned stimulus/stimuli (CS) and stimuli proximal to the CS at the time of conditioning. The hypothesis treats all associations as being excitatory and treats conditioned inhibition as the behavioral consequence of a CS that is less excitatory than its comparator stimuli. Conditioned lick suppression by rats was used to differentiate 4 possible sources of retarded responding to an inhibitory CS. These include habituation to the unconditioned stimulus/stimuli (UCS), latent inhibition to the CS, blocking of the CS-UCS association by the conditioning context, and enhanced excitatory associations to the comparator stimuli. Prior research has demonstrated the 1st 3 phenomena. Therefore, we employed parameters expected to highlight the 4th one—the comparator process. In Exp I, our negative contingency training produced a conditioned inhibitor that passed inhibitory summation and retardation tests. In Exp II we found transfer of retardation from an inhibitory CS to a novel stimulus when the location where retardation-test training occurred was excitatory. In Exp III, extinction of the conditioning context attenuated retardation regardless of whether extinction occurred before or after the CS-UCS pairings of the retardation test. Exp IV demonstrated that habituation to the UCS did not contribute to retardation in the present case. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Learned irrelevance exceeds the sum of CS-preexposure and US-preexposure deficits.

Barpress suppression by water-deprived rats was used to examine the retarded emergence of excitatory responding when a tone conditioned stimulus (CS) and a shock unconditioned stimulus (US) were paired following uncorrelated exposure to the CS and US. Experiment 1{a} established parameters whereby the retardation resulting from preconditioning CS-alone presentations (latent inhibition) was eliminated by presenting unpredictable, nontarget neutral stimuli (clicks) after each CS during the preconditioning phase, a treatment thought to maintain attention to the CS. Experiment 1{b} established parameters whereby the retardation resulting from preconditioning US-alone presentations was eliminated by preceding each US with a 2nd nontarget cue (a light) during the preconditioning phase, which presumably reduced acquisition of context–US associations. In Experiment 1{c}, the techniques to attenuate CS-preexposure and US-preexposure effects were imposed on a random schedule of CS and US presentations. Although this procedure reduced subsequent retardation, an appreciable response deficit remained. In Experiment 2 a context shift between CS-alone or US-alone presentations and subsequent CS–US pairings eliminated retardation, but retardation arising from uncorrelated exposures to the CS and US, albeit significantly reduced, transferred between contexts. These results suggest that the deficit resulting from preconditioning, uncorrelated exposures to the CS and US is composed of a CS preexposure effect, a US preexposure effect, and learned irrelevance. (PsycINFO Database Record (c) 2010 APA, all rights reserved)