Robustly estimating the marginal likelihood for cognitive models via importance sampling.

Recent advances in Markov chain Monte Carlo (MCMC) extend the scope of Bayesian inference to models for which the likelihood function is intractable. Although these developments allow us to estimate model parameters, other basic problems such as estimating the marginal likelihood, a fundamental tool in Bayesian model selection, remain challenging. This is an important scientific limitation because testing psychological hypotheses with hierarchical models has proven difficult with current model selection methods. We propose an efficient method for estimating the marginal likelihood for models where the likelihood is intractable, but can be estimated unbiasedly. It is based on first running a sampling method such as MCMC to obtain samples for the model parameters, and then using these samples to construct the proposal density in an importance sampling (IS) framework with an unbiased estimate of the likelihood. Our method has several attractive properties: it generates an unbiased estimate of the marginal likelihood, it is robust to the quality and target of the sampling method used to form the IS proposals, and it is computationally cheap to estimate the variance of the marginal likelihood estimator. We also obtain the convergence properties of the method and provide guidelines on maximizing computational efficiency. The method is illustrated in two challenging cases involving hierarchical models: identifying the form of individual differences in an applied choice scenario, and evaluating the best parameterization of a cognitive model in a speeded decision making context. Freely available code to implement the methods is provided. Extensions to posterior moment estimation and parallelization are also discussed.

Toward a model-based cognitive neuroscience of mind wandering.

People often "mind wander" during everyday tasks, temporarily losing track of time, place, or current task goals. In laboratory-based tasks, mind wandering is often associated with performance decrements in behavioral variables and changes in neural recordings. Such empirical associations provide descriptive accounts of mind wandering - how it affects ongoing task performance - but fail to provide true explanatory accounts - why it affects task performance. In this perspectives paper, we consider mind wandering as a neural state or process that affects the parameters of quantitative cognitive process models, which in turn affect observed behavioral performance. Our approach thus uses cognitive process models to bridge the explanatory divide between neural and behavioral data. We provide an overview of two general frameworks for developing a model-based cognitive neuroscience of mind wandering. The first approach uses neural data to segment observed performance into a discrete mixture of latent task-related and task-unrelated states, and the second regresses single-trial measures of neural activity onto structured trial-by-trial variation in the parameters of cognitive process models. We discuss the relative merits of the two approaches, and the research questions they can answer, and highlight that both approaches allow neural data to provide additional constraint on the parameters of cognitive models, which will lead to a more precise account of the effect of mind wandering on brain and behavior. We conclude by summarizing prospects for mind wandering as conceived within a model-based cognitive neuroscience framework, highlighting the opportunities for its continued study and the benefits that arise from using well-developed quantitative techniques to study abstract theoretical constructs.

Physiological effects of whole cottonseed in the diet of lactating dairy cows.

With 32 lactating Holstein cows we evaluated physiological effects of gossypol and cyclopropenoid fatty acids in diets containing 18.5% whole cottonseed (dry matter) based on corn, corn silage, and soybean meal. All cows consumed a control diet for the first 2 wk of lactation and then were assigned to either control or whole cottonseed diet for the remainder of their lactation. Milk production, milk fat and protein percentages, and daily dry matter intake were measured. Dry matter intakes were less for cows consuming the whole cottonseed diet, but net energy intake was similar for all diets. Milk fat from cows consuming whole cottonseed contained detectable concentrations of cyclopropene fatty acids. Total lipid in plasma, total serum cholesterol, serum gossypol, and apparent liver gossypol concentrations were greater in cows fed whole cottonseed. Gossypol and cyclopropenoid fatty acids appeared to be absorbed from the gut of cows fed whole cottonseed. Small amounts of gossypol in serum and liver tissue and small amounts of cyclopropene fatty acids in adipose tissue lipids and milk fat indicate a need to elucidate the significance of these physiologically active compounds in the human diet and their biological effects on lactating dairy cows.

Physiological responses of lactating cows to gossypol from cottonseed meal rations.

We evaluated the potential for gossypol intoxication and resulting effects of feeding large amounts of cottonseed meal to dairy cows in early lactation. Twenty-four Holstein cows were grouped by age, prior production, and days postpartum and randomly assigned to one of three diets. After 14-day standardization cows individually were fed a blended corn-corn silage ration supplemented with screw-pressed cottonseed meal, direct solvent extracted cottonseed meal, or soybean meal during a 14-wk comparison. Packed cell volume, copper in plasma, activities of transaminases, and gross composition of milk were not affected by ration. Hemoglobin was depressed, and total protein of plasma was elevated by the 9th wk in cows fed the solvent meal. Erythrocyte fragility was detected in those cows by the 7th wk and appeared later in cows fed the pressed meal. Gossypol was identified and quantitated in plasma and liver but was not detected in erythrocytes or milk from cows fed cottonseed meal. Elevated ambient temperatures precipitated increased respiration rates in the cows fed solvent meal. Physiological changes and gossypol in tissues of cows suggest that intoxication is possible in mature ruminants consuming cottonseed meal containing high free gossypol.

Growth and morphology of Streptococcus bovis and of mixed rumen bacteria in the presence of aflatoxin B1, in vitro.

The metabolism of aflatoxin B1 to other fluorescent compounds was studied in rumen organisms. No fluorescent metabolites were found by chromatographic procedures which separate aflatoxin B1 from them. Rumen bacteria (mixtures and Streptococcus bovis) incubated with aflatoxin B1 grew in long chains whereas cells in control cultures were mainly doublets with some short chains. Growth of mixed rumen bacteria and Streptococcus bovis cultures was inhibited by aflatoxin B1 as measured by change in optical density and by incorporation of [carbon-14] adenine into nucleic acids.

Activity of lipoprotein lipase in adipose tissues from steers.

Mean lipoprotein lipase activities (mueq fatty acids released/h per g tissue) and lipid content(%) in adipose tissue of steers from brisket, perirectal, subcutaneous abdominal, and mesenteric areas were 32, 32, 30, 50, and 73, 77, 64, and 85. The enzyme activity was higher in mesenteric tissue than in the other sites which did not differ from each other. Mesenteric adipose tissue had higher and subcutaneous abdominal adipose tissue had lower lipid content than did brisket and perirectal adipose tissue. Lipid content and enzyme activity were correlated .38.