Anterior Hippocampus and Goal-Directed Spatial Decision Making

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Establishing the boundaries: the hippocampal contribution to imagining scenes

When we visualize scenes, either from our own past or invented, we impose a viewpoint for our “mind's eye” and we experience the resulting image as spatially coherent from that viewpoint. The hippocampus has been implicated in this process, but its precise contribution is unknown. We tested a specific hypothesis based on the spatial firing properties of neurons in the hippocampal formation of rats, that this region supports the construction of spatially coherent mental images by representing the locations of the environmental boundaries surrounding our viewpoint. Using functional magnetic resonance imaging, we show that hippocampal activation increases parametrically with the number of enclosing boundaries in the imagined scene. In contrast, hippocampal activity is not modulated by a nonspatial manipulation of scene complexity nor to increasing difficulty of imagining the scenes in general. Our findings identify a specific computational role for the hippocampus in mental imagery and episodic recollection

Consolidation of complex events via reinstatement in posterior cingulate cortex

It is well-established that active rehearsal increases the efficacy of memory consolidation. It is also known that complex events are interpreted with reference to prior knowledge. However, comparatively little attention has been given to the neural underpinnings of these effects. In healthy adult humans, we investigated the impact of effortful, active rehearsal on memory for events by showing people several short video clips and then asking them to recall these clips, either aloud (Experiment 1) or silently while in an MRI scanner (Experiment 2). In both experiments, actively rehearsed clips were remembered in far greater detail than unrehearsed clips when tested a week later. In Experiment 1, highly similar descriptions of events were produced across retrieval trials, suggesting a degree of semanticization of the memories had taken place. In Experiment 2, spatial patterns of BOLD signal in medial temporal and posterior midline regions were correlated when encoding and rehearsing the same video. Moreover, the strength of this correlation in the posterior cingulate predicted the amount of information subsequently recalled. This is likely to reflect a strengthening of the representation of the video's content. We argue that these representations combine both new episodic information and stored semantic knowledge (or "schemas"). We therefore suggest that posterior midline structures aid consolidation by reinstating and strengthening the associations between episodic details and more generic schematic information. This leads to the creation of coherent memory representations of lifelike, complex events that are resistant to forgetting, but somewhat inflexible and semantic-like in nature

Increasing Mercury in Yellow Perch at a Hotspot in Atlantic Canada, Kejimkujik National Park

In the mid-1990s, yellow perch (Perca flavescens) and common loons (Gavia immer) from Kejimkujik National Park and National Historic Site (KNPNHS), Nova Scotia, Canada, had among the highest mercury (Hg) concentrations across North America. In 2006 and 2007, we re-examined 16 lakes to determine whether there have been changes in Hg in the loon’s preferred prey, yellow perch. Total Hg concentrations were measured in up to nine perch in each of three size classes (5−10 cm, 10−15 cm, and 15−20 cm) consumed by loons. Between 1996/97 and 2006/07, polynomial regressions indicated that Hg in yellow perch increased an average of 29% in ten lakes, decreased an average of 21% in three, and were unchanged in the remaining three lakes. In 2006/07, perch in 75% of the study lakes had Hg concentrations (standardized to 12-cm fish length) equal to or above the concentration (0.21 μg·g−1 ww) associated with a 50% reduction in maximum productivity of loons, compared with only 56% of these lakes in 1996/97. Mercury contamination currently poses a greater threat to loon health than a decade ago, and further reductions in anthropogenic emissions should be considered to reduce its impacts on ecosystem health

The many weak instruments problem and Mendelian randomization.

Instrumental variable estimates of causal effects can be biased when using many instruments that are only weakly associated with the exposure. We describe several techniques to reduce this bias and estimate corrected standard errors. We present our findings using a simulation study and an empirical application. For the latter, we estimate the effect of height on lung function, using genetic variants as instruments for height. Our simulation study demonstrates that, using many weak individual variants, two-stage least squares (2SLS) is biased, whereas the limited information maximum likelihood (LIML) and the continuously updating estimator (CUE) are unbiased and have accurate rejection frequencies when standard errors are corrected for the presence of many weak instruments. Our illustrative empirical example uses data on 3631 children from England. We used 180 genetic variants as instruments and compared conventional ordinary least squares estimates with results for the 2SLS, LIML, and CUE instrumental variable estimators using the individual height variants. We further compare these with instrumental variable estimates using an unweighted or weighted allele score as single instruments. In conclusion, the allele scores and CUE gave consistent estimates of the causal effect. In our empirical example, estimates using the allele score were more efficient. CUE with corrected standard errors, however, provides a useful additional statistical tool in applications with many weak instruments. The CUE may be preferred over an allele score if the population weights for the allele score are unknown or when the causal effects of multiple risk factors are estimated jointly.This is the final version. It was first published by Wiley at onlinelibrary.wiley.com/doi/10.1002/sim.6358/abstrac

Earth observation as a tool for tracking progress towards the Aichi Biodiversity Targets

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Derivation of screening benchmarks for dietary methylmercury exposure for the common loon ( Gavia immer ): Rationale for use in ecological risk assessment

The current understanding of methylmercury (MeHg) toxicity to avian species has improved considerably in recent years and indicates that exposure to environmentally relevant concentrations of MeHg through the diet can adversely affect various aspects of avian health, reproduction, and survival. Because fish‐eating birds are at particular risk for elevated MeHg exposure, the authors surveyed the available primary and secondary literature to summarize the effects of dietary MeHg on the common loon ( Gavia immer ) and to derive ecologically relevant toxic thresholds for dietary exposure to MeHg in fish prey. After considering the available data, the authors propose three screening benchmarks of 0.1, 0.18, and 0.4 µg g −1 wet weight MeHg in prey fish. The lowest benchmark (0.1 µg g −1 wet wt) is the threshold for adverse behavioral impacts in adult loons and is close to the empirically determined no observed adverse effects level for subclinical effects observed in captive loon chicks. The remaining benchmarks (0.18 and 0.4 µg g −1 wet wt) correspond to MeHg levels in prey fish associated with significant reproductive impairment and reproductive failure in wild adult loons. Overall, these benchmarks incorporate recent findings and reviews of MeHg toxicity in aquatic fish‐eating birds and provide the basis for a national ecological risk assessment for Hg and loons in Canada. Environ. Toxicol. Chem. 2012; 31: 2399–2407. © 2012 SETACPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/93756/1/1971_ftp.pd

Liver Parasites and Body Condition in Relation to Environmental Contaminants in Caribou (Rangifer tarandus) from Labrador, Canada

Over the last several decades, elders and hunters of the Innu Nation in Labrador, Canada, have expressed concerns over perceived declines in environmental health and the integrity of country food, including caribou. The primary objective of this study was to determine links between specific health parameters and contaminants found in caribou from the George River herd. Twenty-seven caribou killed by local Innu hunters between February and December 2001 were evaluated for gross and microscopic pathology, body condition, liver parasitology, and contaminant levels in kidney and fat. Overall, the sampled caribou appeared to be in adequate body condition for the time of year, and no clinically significant lesions were found. Concentrations of selenium, metals (Hg, Cd, and Pb), 20 organochlorine pesticides (HCB, a-HCH, g-HCH, aldrin, dieldrin, methoxychlor, mirex, a- and b-endosulfan, heptachlor, heptachlor epoxide, g-CHL, cis-CHL, trans-nonachlor, and o,p'- and p,p'-DDD, DDE, DDT), and 24 PCB congeners were within the ranges reported for caribou in Canada. In general, contaminant levels were relatively low, with the exception of cadmium in kidneys (geometric mean: 6.5 μg/g wet weight; range: 1.5–44.0 μg/g). Two types of liver parasites were found: the liver fluke Fascioloides magna (prevalence: 78%; geometric mean abundance: 4.2 flukes/caribou) and a tapeworm larva consistent with Taenia hydatigena (prevalence: 50%; geometric mean abundance: 0.6 larvae/caribou). Using multiple variable regression analysis, we found renal concentrations of cadmium to be positively associated, and selenium to be negatively associated, with F. magna abundance.Ces dernières décennies, les aînés et les chasseurs de la nation montagnaise du Labrador, au Canada, ont exprimé des inquiétudes au sujet du déclin de la santé de l’environnement et de l’intégrité de la nourriture provenant de la campagne, telle que le caribou. L’objectif principal de cette étude consistait à déterminer les liens qui existent entre certains paramètres de santé précis et les contaminants se trouvant dans le caribou du troupeau de la rivière George. Vingt-sept caribous ayant été tués par les chasseurs montagnais de la région entre les mois de février et de décembre 2001 ont subi des examens pathologiques macroscopiques et microscopiques, en plus d’avoir été évalués pour en déterminer l’état du corps, la parasitologie du foie et les taux de contaminants dans le foie et le gras. Dans l’ensemble, l’état des corps de caribous échantillonnés semblait adéquat pour cette période de l’année et aucune lésion clinique importante n’a été signalée. Les concentrations de sélénium, de métaux (Hg, Cd et Pb), de 20 pesticides organochlorés (HCB, a-HCH, g-HCH, aldrine, dieldrine, méthoxychlore, mirex, a- et b-endosulfane, heptachlore, heptachlorépoxyde, g-CHL, cis-CHL, trans-nonachlore ainsi que o,p'- et p,p'-DDD, DDE, DDT) et de 24 congénères de PCB s’établissaient dans les étendues signalées pour le caribou au Canada. En général, les niveaux de contaminants étaient relativement faibles, à l’exception du cadmium se trouvant dans les reins (moyenne géometrique : 6,5 μg/g poids humide; étendue : 1,5–44,0 mg/g). Deux types de parasites du foie ont été trouvés : la douve Fascioloides magna (prévalence : 78 %; abondance moyenne géométrique : 4,2 douves/caribou) et un cestode du genre Taenia hydatigena (prévalence : 50 %; abondance moyenne géométrique : 0,6 larves/caribou). Nous avons également réalisé une analyse de régression à variables multiples qui nous a permis de constater que les concentrations de cadmium sont positivement associées et celles de sélénium sont négativement associées à l’abondance de F. magna