Using crowdsourcing to examine relations between delay and probability discounting

Although the extensive lines of research on delay and/or probability discounting have greatly expanded our understanding of human decision-making processes, the relation between these two phenomena remains unclear. For example, some studies have reported robust associations between delay and probability discounting, whereas others have failed to demonstrate a consistent relation between the two. The current study sought to clarify this relation by examining the relation between delay and probability discounting in a large sample of internet users (n = 904) using the Amazon Mechanical Turk (AMT) crowdsourcing service. Because AMT is a novel data collection platform, the findings were validated through the replication of a number of previously established relations (e.g., relations between delay discounting and cigarette smoking status). A small but highly significant positive correlation between delay and probability discounting rates was obtained, and principal component analysis suggested that two (rather than one) components were preferable to account for the variance in both delay and probability discounting. Taken together, these findings suggest that delay and probability discounting may be related, but are not manifestations of a single construct (e.g., impulsivity).     

Response bias is unaffected by delay length in a delay discounting paradigm

This study examined the contribution of response bias to measures of delay discounting in Long-Evans rats (n = 8) using the adjusting amount procedure. Under this procedure, we assessed preference for 150 μl of 10% sucrose solution delivered following a delay over a variable-amount alternative delivered immediately. Bias was calculated based on relative preference when reinforcers were delivered immediately from both alternatives. We extended this assessment procedure to examine preference when rewards from both alternatives were equally delayed (2, 4, 8, or 16 s) in addition to assessing a traditional delay discounting function. Relative preference was similar across delays and slightly larger than 150 μl. These results indicate that response bias was stable and suggests a relative aversion for the adjusting alternative, which may be due to the variability in reward size associated with that alternative.     

Within-subject differences in degree of delay discounting as a function of order of presentation of hypothetical cash rewards

Procedural variants in estimating delay discounting (DD) have been shown to yield significant differences in estimated degree of DD as well as variations in individual patterns of choice. For example, a recent study found significantly different degrees of DD between groups assessed using either an ascending or descending order of presentation of the immediately available rewards. The purpose of this study was to test for within-subject effects of order of presentation of the immediate rewards in a DD task. In a single session, college students (N = 29) were asked to complete two DD tasks, one with the immediate rewards presented in ascending order and one in descending order. Consistent with previous results, significantly larger mean area under the discounting curve (AUC) was observed when the descending sequence was used compared to the ascending order of presentation; and the correlation between both measurements was moderate. These results suggest that some DD assessment tasks may be sensitive to contextual variables such as order and range of the reward and delay values.     

Precision of allometric scaling equations for trees can be improved by including the effect of ecological interactions

Allometric scaling relationships of the form Y = aX ^b are widely utilized in many types of models and analyses of tree structure. They are often viewed as static relationships where both the scaling exponent (b) and the normalization constant (a) obtain empirical values that are fixed within a single set of data. Among different sets of data, their values can show environmental variability. However, there have been only few attempts to give a mechanistic interpretation for this variability. We used field data to demonstrate how the scaling relationships in trees can be modified by ecological interactions. Moreover, we show how such processes can be incorporated into the scaling models to improve the fit and the information content of the scaling equations. When fixed theoretical scaling exponents were used instead of empirical exponents and when the effect of competitive interactions between trees was described by separate submodels that predicted the value of the normalisation constant in the scaling equations, it was possible to obtain 4–10% improvement in the model fit of three different structural scaling relationships. Our results suggest that unexplained variation in the values of the scaling parameters can be substituted by an identified effect of ecological factors on the value of the normalisation constant. This agrees with recent theoretical suggestions stating that ecological factors can directly influence the value of normalisation constants.     

Determinate growth and the scaling of photosynthetic energy intake in the solitary coral Fungia concinna (Verrill)

For many marine invertebrates, the maximum size of an individual is influenced heavily by environmental factors and may be limited by energetic constraints. In this study, an energetic model developed originally for anemones was applied to the free-living scleractinian Fungia concinna (Verrill) from Moorea (French Polynesia) to test the hypothesis that energetic constraints limit the size of this solitary coral. The modified model assumed that photosynthesis was the primary source of metabolic energy, and that metabolic costs were represented by aerobic respiration; these sources and sinks of energy were compared using daily energy budgets that were analyzed using double logarithmic regressions of energy against coral size. With this approach, energy limitation is characterized by a scaling exponent for energetic cost (b_cost) that is larger than the scaling exponent for energy intake (b_intake). For the size range of F. concinna studied, b_intake = 0.73 ± 0.09 and b_cost = 0.46 ± 0.10, thereby demonstrating that large individuals accumulated an energetic surplus, even when the expenditure associated with host tissue and symbiont growth was included in the model. The surplus of energy that this coral acquires as it grows appears to be driven by the scaling of traits associated functionally with the scaling of respiration and photosynthesis. Specifically, tissue biomass displayed a strong positive allometry with respect to surface area (i.e., b > 1), and this constraint on surface area may be the mechanistic basis of the low scaling exponent for metabolic cost. In contrast, the capacity for autotrophy - defined indirectly as Symbiodinium population density and chlorophyll content - increased isometrically with surface area, and likely contributed to the higher scaling exponent for intake relative to cost. Our results suggest that growth in F. concinna is not limited strictly by energy, but instead maximum size must be determined by alternative physiological or ecological constraints.     

Percent body fat is related to delay and probability discounting for food in humans

This study describes delay and probability discounting patterns for hypothetical food and money in relation to percent body fat (PBF). Sixty university students completed four computerized discounting tasks in which they were asked to make a series of hypothetical decisions between (a) 10 dollars after one of several different delays (1, 2, 30, 180, and 365 days) or a smaller amount of money available immediately; (b) 10 bites of food after one of several delays (1, 2, 5, 10, and 20 h) or a smaller number of bites available immediately; (c) $10 at one of several probabilities (0.9, 0.75, 0.5, 0.25, 0.1) or a smaller amount of money to be received for sure; and (d) 10 bites of food at one of several probabilities (0.9, 0.75, 0.5, 0.25, 0.1) or a smaller number of bites to be received for sure. Median indifference points for all participants across each task were well described using the hyperbolic discounting function. Results suggest that percent body fat predicted discounting for hypothetical food, but not money, using regression analyses with the entire sample and when comparing individuals in the high and low quartiles for PBF. None of the other dietary variables (body mass index, subjective hunger, and time since last meal or snack) were related to discounting patterns. This suggests that individuals with high PBF may exhibit heightened sensitivities to delay and probability when making decisions about food.     

Thermodynamics of Loop Formation in the Denatured State of Rhodopseudomonas palustris Cytochrome c′: Scaling Exponents and the Reconciliation Problem

The observation that denatured proteins yield scaling exponents, ν, consistent with random-coil behavior and yet can also have pockets of residual or nonrandom structure has been termed the “reconciliation problem”. To provide greater insight into the denatured state of a foldable sequence, we have measured histidine-heme loop formation equilibria in the denatured state of a class II c-type cytochrome, cytochrome c′ from Rhodopseudomonas palustris. We have prepared a series of variants that provide His-heme loop stabilities, pK_loop(His), for loop sizes ranging from 10 to 111 residues at intervals of 7 to 11 residues along the sequence of the protein. We observe a scaling exponent for loop formation, ν₃, of 2.5 ± 0.3. Theoretical values for ν₃ range from 1.8 to 2.4; thus, the observed ν₃ is consistent with random-coil behavior. However, in contrast to data for loop formation as a function of loop size obtained with peptides of homogeneous sequence, we observe considerable scatter about the linear dependence of loop stability on loop size. Thus, foldable sequences behave very differently from homogeneous peptide sequences. The observed scatter suggests that there is considerable variation in the conformational properties along the backbone of a foldable sequence, consistent with alternating compact and extended regions. With regard to the reconciliation problem, it is evident that a scaling exponent consistent with a random coil is necessary but not sufficient to demonstrate random-coil behavior.     

Plant height–crown radius and canopy coverage–density relationships determine above-ground biomass–density relationship in stressful environments

Debate continues in theoretical ecology over whether and why the scaling exponent of biomass–density (M–N) relationship varies along environmental gradients. By developing a novel geometric model with assumptions of allometric growth at the individual level and open canopy at the stand level, we propose that plant height–crown radius and canopy coverage–density relationships determine the above-ground M–N relationship in stressful environments. Results from field investigation along an aridity gradient (from eastern to western China) confirmed our model prediction and showed that the above-ground M–N scaling exponent increased with drought stress. Therefore, the ‘universal’ scaling exponents (−3/2 or −4/3) of the M–N relationship predicted by previous models may not hold for above-ground parts in stressful environments.     

A ditopic receptor for cation binding and facilitated transport through a supported liquid membrane

The binding properties of an artificial receptor towards a series of cations including Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺, Fe²⁺ and Al³⁺ in acetonitrile are described. The receptor comprises a photo-responsive pyrene unit connected via a short spacer to a 2,2′:6′,2″-terpyridine metal ion binding site. Interaction of cations with the receptor was monitored by changes in absorption profile and the association constants calculated for 1:1 and 1:2 cation:ligand binding fall within the range log β = 3-12. The receptor is highly fluorescent and quenching of the emission is observed upon cation binding. The potassium picrate transport properties of the membrane-bound receptor are also described. This receptor when immobilised in a polymer support, which separates two aqueous solutions, has been shown to transport potassium ions in the dark with a flux rate of 1.5 × 10⁸ mol/s m². In contrast, when the membrane-bound receptor is selectively illuminated with light (λ > 400 nm), the flux increases to 2.0 × 10⁸ mol/s m². The transport efficiency depends on the nature of the trap used in the receiver phase.     

Effect of delay and social distance on the perceived value of social interaction

Temporal discounting is a process by which the perceived value of an outcome decreases as delay increases. Social discounting is a process by which the perceived value of an outcome decreases as the social distance between the decision-maker and recipient increases. Both temporal and social discounting are well established for monetary outcomes. However, little is known regarding the effect of delay-to-occurrence or increased social distance on the perceived value of social interactions. The current study demonstrates that the decrease in perceived value for delayed social interactions is well described by the same hyperbola-like function used for monetary outcomes and that the rate of discounting decreases (indicating increased preference for larger, more delayed outcomes) as social distance increases. Conceptualizing the current discounting procedure as entailing both potential gains and losses, suggest that under certain circumstances, higher discount rates may not reflect increased preference.     

A cyano-bridged FeRe(CN)2 cluster incorporating two high-magnetic anisotropy building units

The pentagonal bipyramidal high-spin iron(II) complex, [(TPA^2C(O)NHtBu)Fe(CF₃SO₃)]⁺, is shown to exhibit a high-anisotropy ground state, with fits to dc magnetization data providing an axial zero-field splitting parameter of D = − 7.9 cm⁻¹. The utility of this compound as a building unit is demonstrated, as its reaction with [ReCl₄(CN)₂]²⁻ affords the cyano-bridged dinuclear cluster (TPA^2C(O)NHtBu)FeReCl₄(CN)₂. dc magnetic susceptibility measurements reveal intracluster ferromagnetic exchange interactions between Fe^II and Re^IV centers, with J = +3.0 cm⁻¹, giving rise to a spin ground state of S = 7/2. Moreover, fits to dc magnetization data obtained for the FeRe cluster show the presence of strong axial anisotropy, with D = −2.3 cm⁻¹. Finally, variable-frequency ac susceptibility measurements reveal the onset of slow magnetic relaxation at low temperature, suggesting that the FeRe cluster is a single-molecule magnet.     

Intertemporal choice in lemurs

Different species vary in their ability to wait for delayed rewards in intertemporal choice tasks. Models of rate maximization account for part of this variation, but other factors such as social structure and feeding ecology seem to underly some species differences. Though studies have evaluated intertemporal choice in several primate species, including Old World monkeys, New World monkeys, and apes, prosimians have not been tested. This study investigated intertemporal choices in three species of lemur (black-and-white ruffed lemurs, Varecia variegata, red ruffed lemurs, Varecia rubra, and black lemurs, Eulemur macaco) to assess how they compare to other primate species and whether their choices are consistent with rate maximization. We offered lemurs a choice between two food items available immediately and six food items available after a delay. We found that by adjusting the delay to the larger reward, the lemurs were indifferent between the two options at a mean delay of 17 s, ranging from 9 to 25 s. These data are comparable to data collected from common marmosets (Callithrix jacchus). The lemur data were not consistent with models of rate maximization. The addition of lemurs to the list of species tested in these tasks will help uncover the role of life history and socio-ecological factors influencing intertemporal choices.     

Impulsivity, risk taking, and timing

This study examined the relations among measures of impulsivity and timing. Impulsivity was assessed using delay and probability discounting, and self-report impulsivity (as measured by the Barratt Impulsiveness Scale; BIS-11). Timing was assessed using temporal perception as measured on a temporal bisection task and time perspective (as measured by the Zimbardo Time Perspective Inventory). One hundred and forty three college students completed these measures in a computer laboratory. The degree of delay discounting was positively correlated with the mean and range of the temporal bisection procedure. The degree of delay and probability discounting were also positively correlated. Self-reported motor impulsiveness on the BIS-11 was positively correlated with present hedonism and negatively correlated with future orientation on the ZTPI. Self-reported non-planning on the BIS-11 was positively correlated with fatalism on the ZTPI. These results show that people who overestimate the passage of time (perceive time as passing more quickly) hold less value in delayed rewards. They also confirm previous results regarding the relation between delay and probability discounting, as well as highlight similarities in self-report measures of impulsivity and time perspective.     

Modeling local and global intracellular calcium responses mediated by diffusely distributed inositol 1,4,5-trisphosphate receptors

Considerable insight into intracellular Ca²⁺ responses has been obtained through the development of whole cell models that are based on molecular mechanisms, e.g., single channel kinetics of the inositol 1,4,5-trisphosphate (IP₃) receptor Ca²⁺ channel. However, a limitation of most whole cell models to date is the assumption that IP₃ receptor Ca²⁺ channels (IP₃Rs) are globally coupled by a “continuously stirred” bulk cytosolic [Ca²⁺], when in fact open IP₃Rs experience elevated “domain” Ca²⁺ concentrations. Here we present a 2N+2-compartment whole cell model of local and global Ca²⁺ responses mediated by N=100,000 diffusely distributed IP₃Rs, each represented by a four-state Markov chain. Two of these compartments correspond to bulk cytosolic and luminal Ca²⁺ concentrations, and the remaining 2N compartments represent time-dependent cytosolic and luminal Ca²⁺ domains associated with each IP₃R. Using this Monte Carlo model as a starting point, we present an alternative formulation that solves a system of advection-reaction equations for the probability density of cytosolic and luminal domain [Ca²⁺] jointly distributed with IP₃R state. When these equations are coupled to ordinary differential equations for the bulk cytosolic and luminal [Ca²⁺], a realistic but minimal model of whole cell Ca²⁺ dynamics is produced that accounts for the influence of local Ca²⁺ signaling on channel gating and global Ca²⁺ responses. The probability density approach is benchmarked and validated by comparison to Monte Carlo simulations, and the two methods are shown to agree when the number of Ca²⁺ channels is large (i.e., physiologically realistic). Using the probability density approach, we show that the time scale of Ca²⁺ domain formation and collapse (both cytosolic and luminal) may influence global Ca²⁺ oscillations, and we derive two reduced models of global Ca²⁺ dynamics that account for the influence of local Ca²⁺ signaling on global Ca²⁺ dynamics when there is a separation of time scales between the stochastic gating of IP₃Rs and the dynamics of domain Ca²⁺.     

Assessing the Jarman–Bell Principle: Scaling of intake,digestibility, retention time and gut fill with body mass in mammalian herbivores

Differences in allometric scaling of physiological characters have the appeal to explain species diversification and niche differentiation along a body mass (BM) gradient — because they lead to different combinations of physiological properties, and thus may facilitate different adaptive strategies. An important argument in physiological ecology is built on the allometries of gut fill (assumed to scale to BM^1.0) and energy requirements/intake (assumed to scale to BM^0.75) in mammalian herbivores. From the difference in exponents, it has been postulated that the mean retention time (MRT) of digesta should scale to BM^1.0–0.75 = BM^0.25. This has been used to argue that larger animals have an advantage in digestive efficiency and hence can tolerate lower-quality diets. However, empirical data does not support the BM^0.25 scaling of MRT, and the deduction of MRT scaling implies, according to physical principles, no scaling of digestibility; basing assumptions on digestive efficiency on the thus-derived MRT scaling amounts to circular reasoning. An alternative explanation considers a higher scaling exponent for food intake than for metabolism, allowing larger animals to eat more of a lower quality food without having to increase digestive efficiency; to date, this concept has only been explored in ruminants. Here, using data for 77 species in which intake, digestibility and MRT were measured (allowing the calculation of the dry matter gut contents (DMC)), we show that the unexpected shallow scaling of MRT is common in herbivores and may result from deviations of other scaling exponents from expectations. Notably, DMC have a lower scaling exponent than 1.0, and the 95% confidence intervals of the scaling exponents for intake and DMC generally overlap. Differences in the scaling of wet gut contents and dry matter gut contents confirm a previous finding that the dry matter concentration of gut contents decreases with body mass, possibly compensating for the less favorable volume–surface ratio in the guts of larger organisms. These findings suggest that traditional explanations for herbivore niche differentiation along a BM gradient should not be based on allometries of digestive physiology. In contrast, they support the recent interpretation that larger species can tolerate lower-quality diets because their intake has a higher allometric scaling than their basal metabolism, allowing them to eat relatively more of a lower quality food without having to increase digestive efficiency.     

Intracellular Transport Dynamics of Endosomes Containing DNA Polyplexes along the Microtubule Network

We have explored the transport of DNA polyplexes enclosed in endosomes within the cellular environment by multiple particle tracking (MPT). The polyplex-loaded endosomes demonstrate enhanced diffusion at short timescales (t < 7 s) with their mean-square displacement (MSD) 〈Δx(t)²〉 scaling as t^1.25. For longer time intervals they exhibit subdiffusive transport and have an MSD scaling as t^0.7. This crossover from an enhanced diffusion to a subdiffusive regime can be explained by considering the action of motor proteins that actively transport these endosomes along the cellular microtubule network and the thermal bending modes of the microtubule network itself.     

A broad spectrum anticancer nucleoside with selective toxicity against human colon cells in vitro

2′,3′-Bis-O-tert-butyldimethylsilyl-5′-deoxy-5′-[N-(methylcarbamoyl)amino]-N⁶-(N-phenylcarbamoyl)adenosine, a new member of the N⁶,5′-bis-ureidoadenosine class of anticancer nucleosides, is found to exhibit broad spectrum antiproliferative activity. A majority of the cell lines in the NCI-60 are inhibited with an average GI₅₀ = 3.13 μM. Selective toxicity against human colon cancer cell lines (COLO 205, HCC-2998, HCT-116, HT29, KM12) was also exhibited (LC₅₀’s = 6-10 μM).     

Within-subject comparison of degree of delay discounting using titrating and fixed sequence procedures

Different procedures are often used across experiments to estimate the degree of delay discounting, a common measure of impulsivity. In all procedures, participants indicate their choice between a reward available immediately and one available after a delay. The present experiment determined whether there are differences in the degree of discounting for a hypothetical $100 produced by a procedure that titrates the immediate amount (titrating sequence procedure) versus a procedure that presents a fixed sequence of immediate amounts (fixed sequence procedure) using a within-subject design. The adult human participants showed no significant differences in degree of discounting between procedures as assessed by a hyperboloid model and the Area Under the Curve. Furthermore, the Area Under the Curve values from the two procedures showed a strong positive correlation. These findings suggest there may be no systematic difference between the degree of delay discounting as estimated by the titrating sequence and fixed sequence procedures. Given the apparent similarities in the results, it appears researchers may be justified in basing their choice of which procedure to use on convenience.     

Prediction of moisture, calorific value, ash and carbon content of two dedicated bioenergy crops using near-infrared spectroscopy

The potential of near infrared spectroscopy in conjunction with partial least squares regression to predict Miscanthus xgiganteus and short rotation coppice willow quality indices was examined. Moisture, calorific value, ash and carbon content were predicted with a root mean square error of cross validation of 0.90% (R² = 0.99), 0.13 MJ/kg (R² = 0.99), 0.42% (R² = 0.58), and 0.57% (R² = 0.88), respectively. The moisture and calorific value prediction models had excellent accuracy while the carbon and ash models were fair and poor, respectively. The results indicate that near infrared spectroscopy has the potential to predict quality indices of dedicated energy crops, however the models must be further validated on a wider range of samples prior to implementation. The utilization of such models would assist in the optimal use of the feedstock based on its biomass properties.