Information processing in large-scale cerebral networks: the causal connectivity approach

Today, cognitive functions are considered to be the offspring of the activity of large-scale networks of functionally interconnected cerebral regions. The interpretation of cerebral activation data provided by functional imaging has therefore recently moved to the search for the effective connectivity of activated regions, which aims at understanding the role of anatomical links in the activation propagation. Our assumption is that only causal connectivity can offer a real understanding of the links between brain and mind. Causal connectivity is based on the anatomical connection pattern, the information processing within cerebral regions and the causal influences that connected regions exert on each other. In our approach, the information processing within a region is implemented by a causal network of functional primitives, which are the interpretation of integrated biological properties. Our choice of a qualitative representation of information reflects the fact that cerebral activation data are only the approximate view, provided by imaging techniques, of the real cerebral activity. This explicit modeling approach allows the formulation and the simulation of functional and physiological assumptions about activation data. Two alternative models explaining results of the striate cortex activation described by Fox and Raichle (Fox PT, Raichle ME (1984) J. Neurophysiol 51:1109–1120; Fox PT, Raichle ME (1985) Ann Neurol 17:303–305) are provided as an example of our approach. Received: 22 December 1998 / Accepted in revised form: 23 June 1999     

Control of seed dormancy in Nicotiana plumbaginifolia: post-imbibition abscisic acid synthesis imposes dormancy maintenance

The physiological characteristics of seed dormancy in Nicotiana plumbaginifolia Viv. are described. The level of seed dormancy is defined by the delay in seed germination (i.e the time required prior to germination) under favourable environmental conditions. A wild-type line shows a clear primary dormancy, which is suppressed by afterripening, whereas an abscisic acid (ABA)-deficient mutant shows a non-dormant phenotype. We have investigated the role of ABA and gibberellic acid (GA₃) in the control of dormancy maintenance or breakage during imbibition in suitable conditions. It was found that fluridone, a carotenoid biosynthesis inhibitor, is almost as efficient as GA₃ in breaking dormancy. Dry dormant seeds contained more ABA than dry afterripened seeds and, during early imbibition, there was an accumulation of ABA in dormant seeds, but not in afterripened seeds. In addition, fluridone and exogenous GA₃ inhibited the accumulation of ABA in imbibed dormant seeds. This reveals an important role for ABA synthesis in dormancy maintenance in imbibed seeds. Received: 31 December 1998 / Accepted: 9 July 1999     

Nomimoscolex suspectus n. sp. (Eucestoda: Proteocephalidea: Zygobothriinae) with morphological and molecular phylogenetic analyses of the genus

We describe a new species of Nomimoscolex from the Amazon siluriform fishes Brachyplatystoma filamentosum, B. flavicans and B. vaillanti. It differs from N. piraeeba in a lower mean number of testes, the paramuscular position of the vitelline follicles, the ovarian width/proglottis width ratio and the cirrus-pouch length/proglottis width ratio. Protein electrophoresis assays performed for 25 enzymatic systems showed that specimens of N. suspectus n. sp. from the three host species form a homogenous population which was genetically isolated from N. piraeeba and N. dorad. Moreover, the latter two species, synonymised by Rego (1991) because of their close morphological similarity, could be separated at eight loci. We thus restore N. dorad as a valid species. We finally examined the composition of the genus Nomimoscolex using DNA sequences from the 5.8S rRNA, ITS-2 and 28S rRNA nuclear ribosomal genes and a matrix of 24 morphological characters. Phylogenetic relationships were inferred for nine species of the genus, five members of other monticelliid genera and two outgroup species. The results of the phylogenetic analyses performed on morphological and molecular characters converged with those from allozyme studies and showed that N. suspectus, N. piraeeba and N. dorad clustered in a distinct clade that excluded other members of the genus. We therefore recognised them as an aggregate of species to reflect an isolation supported by both morphological and genetic data. Because relationships among the remaining Nomimoscolex representatives and other genera were generally poorly resolved, regardless of the database analysed, no action was taken to reorganise them into alternative groupings.     

A Matrix Model to Study the Colonization by Brown Trout of a Virgin Ecosystem in the Kerguelen Islands

We present a matrix model for the study of the population dynamics of brown trout Salmo trutta L., introduced in the '60s in the virgin aquatic ecosystems of the Kerguelen Islands. This species clearly acclimatized very well: a portion of the population became migratory and spent a part of its life cycle in the sea, which allowed the rapid colonization of two rivers close to the stream of origin in the same bay (Baie Norvégienne).These migratory trout can become a smolt at 2, 3, or 4 years of age. The model takes into account age and smolt age structures and in a first step considers the fish from the Baie Norvégienne as belonging to a single population. The transition matrix looks like a 32 × 32 Leslie matrix in which some survival rates are not on the subdiagonal. They represent survival after the first sea migration and are particularly important for the dynamics of the whole population.The estimate of demographic parameters was obtained from a data base containing information collected in the field since 1970. The model was calibrated on the population size estimate and the stock structure of the migratory trout in 1979. Population size was estimated by tagging-recapture and monitoring of the migratory trout in freshwater when they returned to overwinter or reproduce. Under the hypothesis of a constant survival rate for all ages and categories of fish, it was possible to determine relationships between the annual population growth rate and survival rates at first downstream migration, for which no direct estimate was available.These constraints on the model induced paradoxical results. For instance, an increase in survival rate reduced migratory trout numbers in the first years of colonization. These data suggested that the average survival rate should be around 0.3 and not 0.5 as surmised in previous studies. However, the model systematically underestimated numbers of migratory trout during the first years of development. Thus, to improve the model it will be necessary to introduce survival rates varying with time. Another possible approach would be to consider the population as three subunits corresponding to the three colonized rivers of the Baie Norvégienne.     

Zooplankton food limitation and grazing impact in a eutrophic brackish-water tropical pond (Côte d'Ivoire, West Africa)

Gut fluorescence, feeding and egg production rates of zooplankton assemblages were measured in a shallow, eutrophic brackish-water pond for 24 days. Brachionus plicatilis, Hexarthra intermedia and Apocyclops panamensis successively developed and exhibited differences in food selectivity. Rotifers selected small particles but also had a preference for larger particles (15–21 μm, Equivalent Spherical Diameter, ESD). B. plicatilis appeared less selective than H. intermedia, which fed mostly on particles <6 μm. A. panamensis adults showed a selectivity for 6–21 μm ESD particles. Laboratory experiments suggested that A. panamensis adults were able to shift from seston to carnivorous feeding, depending on the availability of these food resources. Measurements of gut fluorescence and grazing gave comparable ingestion rates. Rotifers displayed the highest ingestion rates (up to 486% body C d-1). Despite high total phytoplankton and seston biomasses, the high ingestion rates and selective grazing of rotifers induced auto food-limitation phenomena and caused major changes in seston abundance and size structure. Grazing impact was less important when A. panamensis dominated. This revised version was published online in July 2006 with corrections to the Cover Date.