342 research outputs found
High on Habits
The neural circuits involved in learning and executing goal-directed actions, which are governed by action-outcome contingencies and sensitive to changes in the expected value of the outcome, have been shown to be different from those mediating habits, which are less dependent on action-outcome relations and changes in outcome value. Extended training, different reinforcement schedules, and substances of abuse have been shown to induce a shift from goal-directed performance to habitual performance. This shift can be beneficial in everyday life, but can also lead to loss of voluntary control and compulsive behavior, namely during drug seeking in addiction. Although the brain circuits underlying habit formation are becoming clearer, the molecular mechanisms underlying habit formation are still not understood. Here, we review a recent study where Hilario et al. (2007) established behavioral procedures to investigate habit formation in mice in order to investigate the molecular mechanisms underlying habit formation. Using those procedures, and a combination of genetic and pharmacological tools, the authors showed that endocannabinoid signaling is critical for habit formation
Chemosymbiotic species from the Gulf of Cadiz
Previous work in the mud volcanoes from the Gulf
of Cadiz (South Iberian Margin) revealed a high number
of chemosymbiotic species, namely bivalves and siboglinid
polychaetes. In this study we give an overview of the distribution
and life styles of these species in the Gulf of Cadiz,
determine the role of autotrophic symbionts in the nutrition
of selected species using stable isotope analyses ( 13C, 15N
and 34S) and investigate the intra-specific variation of isotope
signatures within and between study sites. During our
studies, we identified twenty siboglinidae and nine bivalve
chemosymbiotic species living in fifteen mud volcanoes.
Solemyid bivalves and tubeworms of the genus Siboglinum
are widespread in the study area, whereas other species
were found in a single mud volcano (e.g. “Bathymodiolus”
mauritanicus) or restricted to deeper mud volcanoes (e.g.
Polybrachia sp., Lamelisabella denticulata). Species distribution
suggests that different species may adjust their position
within the sediment according to their particular needs,
and to the intensity and variability of the chemical substrata
supply. Tissue stable isotope signatures for selected
species are in accordance with values found in other studies,
with thiotrophy as the dominant nutritional pathway, and
with methanotrophy and mixotrophy emerging as secondary
strategies. The heterogeneity in terms of nutrient sources (expressed
in the high variance of nitrogen and sulphur values)
and the ability to exploit different resources by the different
species may explain the high diversity of chemosymbiotic
species found in the Gulf of Cadiz. This study increases the
knowledge on distributional patterns and resource partitioning
of chemosymbiotic species and highlights how trophic
fuelling varies on spatial scales with direct implications to
seep assemblages and potentially to the biodiversity of continental
margin
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Random walk on random walks: Low densities
We consider a random walker in a dynamic random environment given by a
system of independent simple symmetric random walks. We obtain ballisticity
results under two types of perturbations: low particle density, and strong
local drift on particles. Surprisingly, the random walker may behave very
differently depending on whether the underlying environment particles perform
lazy or non-lazy random walks, which is related to a notion of permeability
of the system. We also provide a strong law of large numbers, a functional
central limit theorem and large deviation bounds under an ellipticity
condition
Macrofaunal assemblages from mud volcanoes in the Gulf of Cadiz: abundance, biodiversity and diversity partitioning across spatial scales
The Gulf of Cadiz is an extensive seepage area in the south Iberian margin (NE Atlantic) encompassing over 40 mud volcanoes (MVs) at depths ranging from 200 to 4000 m. The area has a long geologic history and a central biogeographic location with a complex circulation ensuring oceanographic connectivity with the Mediterranean Sea, equatorial and North Atlantic regions. The geodynamics of the region promotes a notorious diversity in the seep regime despite the relatively low fluxes of hydrocarbon-rich gases. We analyse quantitative samples taken during the cruises TTR14, TTR15 and MSM01-03 in seven mud volcanoes grouped into Shallow MVs (Mercator: 350 m, Kidd: 500 m, Meknes: 700 m) and Deep MVs (Captain Arutyunov: 1300 m, Carlos Ribeiro: 2200 m, Bonjardim: 3000 m, Porto: 3900 m) and two additional Reference sites (ca. 550 m). Macrofauna (retained by a 500 mu m sieve) was identified to species level whenever possible. The samples yielded modest abundances (70-1567 individuals per 0.25m(2)), but the local and regional number of species is among the highest ever reported for cold seeps. Among the 366 recorded species, 22 were symbiont-hosting bivalves (Thyasiridae, Vesicomyidae, Solemyidae) and tubeworms (Siboglinidae). The multivariate analyses supported the significant differences between Shallow and Deep MVs: The environmental conditions at the Shallow MVs make them highly permeable to the penetration of background fauna leading to high diversity of the attendant assemblages (H': 2.92-3.94; ES(100): 28.3-45.0; J': 0.685-0.881). The Deep MV assemblages showed, in general, contrasting features but were more heterogeneous (H': 1.41-3.06; ES(100): 10.5-30.5; J': 0.340-0.852) and often dominated by one or more siboglinid species. The rarefaction curves confirmed the differences in biodiversity of Deep and Shallow MVs as well as the convergence of the latter to the Reference sites. The Bray-Curtis dissimilarity demonstrated the high beta-diversity of the assemblages, especially in pairwise comparisons involving samples from the Deep MVs. Diversity partitioning assessed for species richness, Hurlbert's expected number of species and Shannon-Wiener index confirmed the high beta-diversity across different spatial scales (within MVs, between MVs, between Deep and Shallow MVs). We suggest that historical and contemporary factors with differential synergies at different depths contribute to the high alpha-, beta- and gamma-diversity of the mud volcano faunal assemblages in the Gulf of Cadiz
Random walk on random walks
In this paper we study a random walk in a one-dimensional dynamic random environment consisting of a collection of independent particles performing simple symmetric random walks in a Poisson equilibrium with density ¿¿(0,8). At each step the random walk performs a nearest-neighbour jump, moving to the right with probability p° when it is on a vacant site and probability p· when it is on an occupied site. Assuming that p°¿(0,1) and p·¿12, we show that the position of the random walk satisfies a strong law of large numbers, a functional central limit theorem and a large deviation bound, provided ¿ is large enough. The proof is based on the construction of a renewal structure together with a multiscale renormalisation argument
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