Terminal Link Research Articles

SOME EFFECTS OF PROCEDURAL VARIATIONS ON CHOICE RESPONDING IN CONCURRENT CHAINS

The present research used pigeons in a three-key operant chamber and varied procedural features pertaining to both initial and terminal links of concurrent chains. The initial links randomly alternated on the side keys during a session, while the terminal links always appeared on the center key. Both equal and unequal initial-link schedules were employed, with either differential or nondifferential terminal-link stimuli across conditions. The research was designed to neutralize initial- and terminal-link spatial cues in order to gain a clearer understanding of the roles of conditioned reinforcement and delayed primary reinforcement in choice. With both equal and unequal initial links and with differential terminal-link stimuli, all pigeons reliably preferred the chain with the shorter terminal link. However, with equal initial links and nondifferential stimuli, all pigeons were indifferent. With unequal initial links and nondifferential stimuli, some pigeons were also indifferent, while others actually reversed and preferred the chain with the shorter initial link, even though it was followed by the longer terminal link. The decrease if not reversal of the previous preferences implies that preferences in concurrent chains are a function of the conditioned reinforcement afforded by terminal-link stimuli, rather than delayed primary reinforcement.

Effects of free food deliveries and temporal contiguity on choice under concurrent-chain schedules

Eight pigeons responded in a concurrent chain with variable-interval (VI) 10-sec and VI 20-sec terminal links. Free food deliveries were then added to the initial links according to a variable-time (VT) 20-sec schedule in two conditions that differed in terms of whether a differential reinforcement of other (DRO) contingency was also arranged, which ensured that those deliveries could not occur within 2-sec of a response. Preference for the VI 10-sec terminal link increased when VT food was added, but not when the DRO contingency was operative, showing that free food deliveries affected preference only when those deliveries could be temporally contiguous with choice responding. This finding suggests that Mazur's (2003) report of increased preference with added VT food, replicated here, was due to adventitious reinforcement. Current models for behavioral choice are limited because they are based entirely on temporal relations between stimuli and reinforcers and fail to take into account response-reinforcer contiguity.

Design of self-tuning controllers for a two terminal HVDC link

This paper presents the design of self-tuning controllers for a two terminal HVDC link. The controllers are designed utilizing a novel discrete-time converter model based on multirate sampling. The nature of converter firing system necessitates the development of a two-step ahead self-tuning control strategy. A two terminal HVDC system study has been carried out to show the effectiveness of the control strategies proposed which include the design of minimum variance controller, pole assigned controller and PLQG controller. The coordinated control of a two terminal HVDC system has been established deriving the signal from inverter end current and voltage which has been estimated based on the measurements of rectifier end quantities only realized through the robust reduced order observer. A well known scaled down sample system data has been selected for studies and the controllers designed have been tested for worst conditions. The performance of self-tuning controllers has been evaluated through digital simulation.

Effects of initial-link duration on preference and resistance to change in concurrent-chains schedules

Previous studies with concurrent-chains procedures have shown that preference for a terminal-link signaling a higher reinforcement rate decreases as initial-link durations increase. Using a concurrent-chains procedure, the present experiment examined the effects of manipulating initial-link duration on preference and resistance to disruption with rats nose poking for different rates of food reinforcement in the terminal links. Consistent with previous findings, preference for a terminal link with a higher reinforcement rate decreased with longer initial links. Conversely, relative resistance to disruption in the terminal link with a higher reinforcement rate increased with longer initial links. These findings are counter to the prediction of behavioral momentum theory that preference and resistance to change should be positively related.

SINGLE‐SAMPLE DISCRIMINATION OF DIFFERENT SCHEDULES' REINFORCED INTERRESPONSE TIMES

Food-deprived rats in Experiment 1 responded to one of two tandem schedules that were, with equal probability, associated with a sample lever. The tandem schedules' initial links were different random-interval schedules. Their values were adjusted to approximate equality in time to completing each tandem schedule's response requirements. The tandem schedules differed in their terminal links: One reinforced short interresponse times; the other reinforced long ones. Tandem-schedule completion presented two comparison levers, one of which was associated with each tandem schedule. Pressing the lever associated with the sample-lever tandem schedule produced a food pellet. Pressing the other produced a blackout. The difference between terminal-link reinforced interresponse times varied across 10-trial blocks within a session. Conditional-discrimination accuracy increased with the size of the temporal difference between terminal-link reinforced interresponse times. In Experiment 2, one tandem schedule was replaced by a random ratio, while the comparison schedule was either a tandem schedule that only reinforced long interresponse times or a random-interval schedule. Again, conditional-discrimination accuracy increased with the temporal difference between the two schedules' reinforced interresponse times. Most rats mastered the discrimination between random ratio and random interval, showing that the interresponse times reinforced by these schedules can serve to discriminate between these schedules.

Single Nucleotide Variants of the TGACTCA Motif Modulate Energetics and Orientation of Binding of the Jun-Fos Heterodimeric Transcription Factor

The Jun-Fos heterodimeric transcription factor is the terminal link between the transfer of extracellular information in the form of growth factors and cytokines to the site of DNA transcription within the nucleus in a wide variety of cellular processes central to health and disease. Here, using isothermal titration calorimetry, we report detailed thermodynamics of the binding of bZIP domains of Jun-Fos heterodimer to synthetic dsDNA oligos containing the TGACTCA cis element and all possible single nucleotide variants thereof encountered widely within the promoters of a diverse array of genes. Our data show that Jun-Fos heterodimer tolerates single nucleotide substitutions and binds to TGACTCA variants with affinities in the physiologically relevant micromolar to submicromolar range. The energetics of binding are richly favored by enthalpic forces and opposed by entropic changes across the entire spectrum of TGACTCA variants in agreement with the notion that protein-DNA interactions are largely driven by electrostatic interactions and intermolecular hydrogen bonding. Of particular interest is the observation that the Jun-Fos heterodimer binds to specific TGACTCA variants in a preferred orientation. Our 3D atomic models reveal that such orientational preference results from asymmetric binding and may in part be attributable to chemically distinct but structurally equivalent residues R263 and K148 located within the basic regions of Jun and Fos, respectively. Taken together, our data suggest that the single nucleotide variants of the TGACTCA motif modulate energetics and orientation of binding of the Jun-Fos heterodimer and that such behavior may be a critical determinant of differential regulation of specific genes under the control of this transcription factor. Our study also bears important consequences for the occurrence of single nucleotide polymorphisms within the TGACTCA cis element at specific gene promoters between different individuals.

Response allocation in a rapid-acquisition concurrent-chains procedure: Effects of overall terminal-link duration

Four pigeons were exposed to a concurrent-chains procedure in which the initial link was constant (variable-interval 10s) while the terminal-link delays changed across sessions according to a pseudorandom binary series. In the short condition, the left terminal link was a fixed-interval 8s and the right terminal link was either fixed-interval 4s or fixed-interval 16s; in the long condition, the left terminal link was fixed-interval 16s and the right terminal link was either fixed-interval 8s or fixed-interval 32s. Multiple regression showed that response allocation was sensitive to the current session immediacy ratio, with greater sensitivity in the long than the short condition. This replicates the terminal-link effect [MacEwen, D., 1972. The effects of terminal-link fixed-interval and variable-interval schedules on responding under concurrent-chained schedules. J. Exp. Anal. Behav. 18, 253-261] in a pseudorandom binary series design. Grace and McLean's [Grace, R.C., McLean, A., 2006. Rapid acquisition in concurrent chains: evidence for a decision model. J. Exp. Anal. Behav. 85, 181-202] decision model provided a good description of the data and predicted the terminal-link effect. Estimates of the criterion parameter increased in the long condition but less than proportionally, and were close to the expected values based on the programmed schedule durations, as predicted by the decision model. These results provide additional support for the decision model as an account of choice in concurrent chains.

Effects of unpredictable changes in initial-link duration on choice and timing

Four pigeons responded in a concurrent-chains procedure in which terminal-link schedules were fixed-interval (FI) 10s and FI 20s. Across sessions, the location of the shorter terminal-link changed according to a pseudorandom binary sequence. Each session, the variable-interval initial-link schedule value was sampled from a uniform distribution that ranged from 0.01 to 30s. On some terminal links, food was withheld to obtain measures of temporal control. Terminal-link delays determined choice (log initial-link response ratios) and timing (start and stop times on no-food trials) measures, which stabilized within the 1st half of each session. Preference for the shorter terminal-link delay was a monotonically decreasing function of initial-link duration. There was no evidence of control by initial-link durations from previous sessions.

The probability of small schedule values and preference for random-interval schedules.

Preference for working on variable schedules and temporal discrimination were simultaneously examined in two experiments using a discrete-trial, concurrent-chains arrangement with fixed interval (FI) and random interval (RI) terminal links. The random schedule was generated by first sampling a probability distribution after the programmed delay to reinforcement on the FI schedule had elapsed, and thus the RI never produced a component schedule value shorter than the FI and maintained a rate of reinforcement half that of the FI. Despite these features, the FI was not strongly preferred. The probability of obtaining the smallest programmed delay to reinforcement on the RI schedule was manipulated in Experiment 1, and the interaction of this probability and initial link length was examined in Experiment 2. As the probability of obtaining small values in the RI increased, preference for the schedule increased while the discriminated time of reinforcer availability in the terminal link decreased. Both of these effects were attenuated by lengthening the initial links. The results support the view that in addition to the delay to reinforcement, the probability of obtaining a short delay is an important choice-affecting variable that likely contributes to the robust preferences for variable, as opposed to fixed, schedules of reinforcement.

Miljøgifter i morsmelk

Breast milk is very important to ensure infants a well-composed and safe diet during the first year of life. However, the quality of breast milk seems to be affected by an increasing amount of environmental toxins (particularly so-called Persistent, Bioaccumulative Toxins [PBTs]). Many concerns have been raised about the negative effects this may have on infant health. The article is a review of literature (mainly review articles) identified through a non-systematic search in PubMed. The concentration of PBTs in breast milk is mainly caused by man's position as the terminal link in the nutritional chain. Many breast-fed infants have a daily intake of such toxins that exceed limits defined for the population in general. Animal studies demonstrate effects on endocrine function and neurotoxicity in the offspring, and a number of human studies seem to point in the same direction. However the "original" optimal composition of breast milk still seems to protect against long-term effects of such toxicity. There is international consensus about the need to monitor breast milk for the presence of PBTs. Such surveillance will be a good indicator of the population's general exposure to these toxins and may also contribute to identifying groups as risk who should not breast-feed their children for a long time.

Zooplankton in different types of estuaries (using Curonian and Vistula estuaries as an example)

Based upon the results of long-term monitoring of zooplankton in the Curonian and Vistula estuaries carried out by the author, specific features of the structure and functioning of the zooplankton communities are studied. It is shown that the zooplankton communities in the open (Vistula Bay) and closed (Curonina Bay) types of estuaries differ in the species structures, numbers, biomasses, and functioning. The similarity of the zooplankton communities of these estuaries has to do with the commonness of their trophic structure. The zooplankton community in the Vistula estuary is more affected by the gradient of abiotic factors than the Curonian bay community. The mechanism providing the stability of the zooplankton communities in the open-type estuaries is the energy flow through the omnivorous species and the increase in the number of facultative predators at the terminal links of the trophic chain. It is shown that the Vistula estuary zooplankton community has a higher specific productivity rate and a higher rate of its circulation and use.

Discriminative and reinforcing stimulus properties of music for rats

We trained rats to discriminate music by Bach from that by Stravinsky using operant conditioning. The rats successfully learned the discrimination and transferred their discrimination to novel music by the same artists. Then, we trained rats on concurrent-chain schedule in which the terminal links were associated with different music, Bach or Stravinsky. The rats did not show strong preference for either style of music, although one subject showed a preference for Bach and another subject preferred Stravinsky. Finally, we examined the validity of the concurrent-chain procedure as a method of preference measurement with conspecific vocalization evoked by an aversive experience. Most of the rats preferred white noise to the conspecific vocalization. Therefore, music has a discriminative stimulus property but not a clear reinforcing property for rats.

Choice and the Initial Delay to a Reinforcer

Pigeons were trained in two experiments that used the concurrent-chains procedure. These experiments sought to identify the variables controlling the preference of pigeons for a constant duration over a variable duration of exposure to an aperiodic, time-based, terminal-link schedule. The results indicated that two variables correlated with the constant-duration terminal link combined to control preference: (a) a shorter initial delay to a reinforcer; and (b) the probabilistic occurrence of multiple reinforcers. Grace and Nevin (2000) trained pigeons on a concurrent-chains procedure with equal variable-interval (VI) schedules in the initial links and equal VI schedules in the terminal links. The terminal links differed in that one ended after a single reinforcer, which they called “variable-duration” terminal link, whereas the other ended after a fixed period of exposure equal to the average interreinforcement interval (IRI) of the schedule, which they called “constantduration” terminal link. As Grace and Nevin identified, and as discussed at some length below, an important feature of the constant-duration terminal link is that it probabilistically yielded 0, 1, or multiple reinforcers per entry, although it provided the same average rate of reinforcement overall as the variable-duration terminal link. Grace and Nevin (2000) found that three of four pigeons clearly preferred the constant-duration terminal link. In their words, the data of a fourth pigeon “demonstrated a consistent right-key bias” (p. 178), and the present conclusion is that its data are more difficult to interpret. In any case, an important question is what variables caused the preference. Ordinarily, one would have expected the pigeons to be indifferent, since the schedules in effect during the alternatives were identical, and each alternative yielded the same overall rate of reinforcement. Grace and Nevin (2000) initially pondered the role of multiple reinforcers in the constant-duration terminal link, because research has shown that subjects may well prefer a choice alternative associated with multiple reinforcers rather than a single reinforcer per terminal-link entry (e.g., Fantino & Herrnstein, 1968; Mazur, 1986; McDiarmid & Rilling, 1965; Moore, 1979; Poniewaz, 1984; Shull, Mellon, & Sharp, 1990; Shull, Spear, & Bryson, 1981). In particular, Grace and Nevin discussed their findings from the view Correspondence concerning this article may be addressed to Dr. J. Moore, Dept. of Psychology,

Signals, resistance to change, and conditioned reinforcement in a multiple schedule

The effect of signals on resistance to change was evaluated using pigeons responding on a three-component multiple schedule. Each component contained a variable-interval initial link followed by a fixed-time terminal link. One component was an unsignaled-delay schedule, and two were equivalent signaled-delay schedules. After baseline training, resistance to change was assessed through (a) extinction and (b) adding free food to the intercomponent interval. During these tests, the signal stimulus from one of the signaled-delay components (SIG-T) was replaced with the initial-link stimulus from that component, converting it to an unsignaled-delay schedule. That signal stimulus was added to the delay period of the unsignaled-delay component (UNS), converting it to a signaled-delay schedule. The remaining signaled component remained unchanged (SIG-C). Resistance-to-change tests showed removing the signal had a minimal effect on resistance to change in the SIG-T component compared to the unchanged SIG-C component except for one block during free-food testing. Adding the signal to the UNS component significantly increased response rates suggesting that component had low response strength. Interestingly, the direction of the effect was in the opposite direction from what is typically observed. Results are consistent with the conclusion that the signal functioned as a conditioned reinforcer and inconsistent with a generalization-decrement explanation.

RAPID ACQUISITION OF PREFERENCE IN CONCURRENT CHAINS WHEN ALTERNATIVES DIFFER ON MULTIPLE DIMENSIONS OF REINFORCEMENT

Pigeons responded in a concurrent-chains procedure in which terminal-link reinforcer variables were changed unpredictably across sessions. In Experiment 1, the terminal-link schedules were fixed-interval (FI) 8 s and FI 16 s, and the reinforcer magnitudes were 2 s and 4 s. In Experiment 2 the probability of reinforcement (100% or 50%) was varied with immediacy and magnitude. Multiple-regression analyses showed that pigeons' initial-link response allocation was determined by current-session reinforcer variables, similar to previous studies which have varied only immediacy (Grace, Bragason, & McLean, 2003). Sensitivity coefficients were positive and statistically significant for all reinforcer variables in both experiments. Analyses of responding within individual sessions showed that final levels of preference for dominated sessions, in which all reinforcer variables favored the same terminal link, were more extreme than for tradeoff sessions in which at least one reinforcer variable favored each alternative. This result implies that response allocation was determined by multiple reinforcer variables within individual sessions, consistent with the concatenated matching law. However, in Experiment 2, there was a nonlinear (sigmoidal) relationship between response allocation and relative value, which suggests the possibility that reinforcer variables may interact during acquisition, contrary to the matching law.

Design and Implementation of a Visual Feedback Control System Driven by Wires

This paper introduces a visual feedback control method for a multiple degree of freedom mechanical system driven by wires. This system is a small serial-link robot. It consists of a small-diameter tube and two pairs of wires, which are connected to the interior of the terminal link, which pass through the interior of every other link. The mechanical system is driven by electrical motors via tension in the wires. One pair of wires is for side-to-side motion while the other pair is for up-and-down motion. Such a mechanism is frequently used in medical and industrial devices, such as endoscopes. Furthermore, a compact CCD camera is mounted on the terminal link and functions as a vision sensor. The control system consists of a visual feedback control loop and a minor loop that directly controls the sum of all the joint angles. The control law of the minor loop is designed based on a Lyapunov's theory. On the other hand, the outer visual feedback control loop is designed to track a target point detected by the vision sensor. Each loop is connected by an image Jacobian number and its inverse, which are continually updated during operations. This control method is evaluated experimentally. It is anticipated that this system will be used in a variety of applications in the future.

Effect of unsignaled delays between stimuli in a chain schedule on responding and resistance to change

Behavioral momentum theory is an evolving theoretical account of the strength of behavior. One challenge for the theory is determining the role of signal stimuli in determining response strength. This study evaluated the effect of an unsignaled delay between the initial link and terminal link of a two-link chain schedule on resistance to change using a multiple schedule of reinforcement. Pigeons were presented two different signaled delay to reinforcement schedules. Both schedules employed a two-link chain schedule with a variable interval 120-s initial link followed by a 5-s fixed time terminal link schedule. One of the schedules included a 5-s unsignaled delay between the initial link and the terminal link. Resistance to change was assessed with two separate disruption procedures: extinction and adding a variable time 20-s schedule of reinforcement to the inter-component interval. Baseline responding was lower in the schedule with the unsignaled delay but resistance to change for the initial link was unaffected by the unsignaled delay. The results suggest that not all unsignaled delays are equal in their effect on resistance to change.

Choice Behavior in Pigeons Maintained With Probabilistic Schedules of Reinforcement

Pigeons were trained in three experiments with a two-key, concurrent-chains choice procedure. The initial links were equal variable-interval schedules, and the terminal links were randomtime schedules with equal average interreinforcement intervals. Across the three experiments, the pigeons either stayed in a terminal link until a reinforcer was delivered or were returned to the initial links if a reinforcer was not delivered after the first unit time interval. The results showed that choice responding in concurrent chains is a complex function of (a) local factors, such as the immediacy of reinforcement in a terminal link, and (b) overall factors, such as the overall interreinforcement interval as calculated over both initial and terminal links of the chain. However, the local factors are generally stronger when the two factors are directly opposed.

THE EFFECT OF CONDITIONED REINFORCEMENT RATE ON CHOICE: A REVIEW

We review the nature of conditioned reinforcement, including evidence that conditioned reinforcers maintain choice behavior in concurrent schedules and that they elevate responding in the terminal links of concurrent-chains schedules. A question has resurfaced recently: Do theories of choice in concurrent-chains schedules need to include a term reflecting greater preference for higher rates of conditioned reinforcement? The review of several studies addressing this point suggests that such a term is inappropriate. Elevated rates of conditioned reinforcement (and responding) in the terminal links of concurrent-chains schedules do not lead to greater preference in the initial link leading to the higher rate of conditioned reinforcement. If anything, the opposite preference is likely to occur. This result is not surprising, since the additional putative conditioned reinforcers in the terminal link are not correlated with a reduction in time to primary reinforcement nor with an increase in value.

Persistent preference in concurrent-chain schedules.

Twelve pigeons, eight naive and four with a history of choice between different variable interval Schedules were exposed to concurrent-chains schedules to examine the effect of history on choice patterns. The initial links were always fixed-interval (FI) 3s schedules, and the terminal links were 5-valued variable-ratio (VR) schedules that varied by condition. During baseline, terminal link requirements were identical VR 60 schedules (equal alternative conditions) for both the red and white keys, producing indifference in all but one instance. Preferences were established by making the response requirements larger or smaller (depending on the condition) on the red key alternative (unequal alternative conditions). After preferences were established, 4 subjects were exposed to the equal alternative conditions, and 8 subjects were exposed to forced choice sessions with the same equal response requirements as during baseline before exposure to the equal alternative conditions. It was found that 10 of the 12 pigeons showed preferences that persisted during returns to baseline that were primarily influenced by either the immediately preceding unequal alternative condition or another particular unequal alternative condition. Two of the pigeons with prior histories did not show any shifts in preference with changes in terminal link response requirements. Keywords: choice, persistence, preference, concurrent-chains schedules, pigeons ********** Given two response alternatives where one reinforcement rate is higher (rich) relative to the other (lean), behavior is allocated toward the richer alternative (Hernstein, 1970). That is, behavior is allocated between concurrently available sources of reinforcement so as to match the relative rates of reinforcement obtained from those alternatives. In addition, it has been shown that preference exists for variable over fixed schedules or reinforcement (Ahearn, Hineline, & David, 1992; Field, Tonneaau, Ahearn, & Hineline, 1996), and for the smaller ratio in concurrent variable-ratio (VR) schedules (Hernstein and Loveland, 1975; MacDonall, 1988; McMillan, Hardwick & Li, 2002). There is considerable evidence suggesting that prior conditions effect later performance. Using human subjects, Weiner (1964, 1969) found that responding under identical FI schedules differed as a function of previous schedule exposure. Specifically, response rates during a FI condition were similar to either a low or high response rate previously produced by either a fixed-ratio (FR) schedule or differential-reinforcement-of-low-rate (DRL) schedule. Additionally, research conducted by LeFrancois and Metzger (1993) provided further support for the influence of history on later responding. They trained rats to lever press under a DRL schedule before changing to a FI schedule of reinforcement. With a second group of rats, lever presses were trained first under a DRL schedule and later shifted to a FR schedule, before a FI schedule was put in place. Low rates of responding during the FI condition were only found with the first group of rats where the DRL immediately preceded the FI condition, suggesting that immediate history had more of a direct influence on FI responding than remote history in this case. Similar effects of schedule history can be found in research by Cohen, Perdersen, Kinney & Myers (1994), Urbain, Poling, Millam, & Thompson, (1978), Freeman & Lattal (1992), and Wanchisen, Tatham, & Mooney. (1989). An important question involves persistence after changes in reinforcement schedules. Research by Poppen (1982) with young adult university students involved lever pressing producing points programmed on a concurrent schedule. After training with one set of concurrent schedules, all subjects were shifted to a second schedule with different response requirements. The investigations showed that for many of the schedules, naive subjects performed differently than subjects with prior histories. …