Positron emission tomography was used to investigate the functional anatomy of mental simulation of routes (MSR) in five normal volunteers. Normalized regional cerebral blood flow was measured while subjects mentally navigated between landmarks of a route which had been previously learned by actual navigation. This task was contrasted with both static visual imagery of landmarks (VIL) and silent Rest. MSR appears to be subserved by two distinct networks: a non-specific memory network including the posterior and middle parts of the hippocampal regions, the dorsolateral prefrontal cortex and the posterior cingulum, and a specific mental navigation network, comprising the left precuneus, insula and medial part of the hippocampal regions.

Positron emission tomography (PET) was used to monitor regional cerebral blood flow variations while subjects were constructing mental images of objects made of three-dimensional cube assemblies from auditorily presented instructions. This spatial mental imagery task was contrasted with both passive listening (LIST) of phonetically matched nonspatial word lists and a silent rest (REST) condition. All three tasks were performed in total darkness. Mental construction (CONS) specifically activated a bilateral occipitoparietal-frontal network, including the superior occipital cortex, the inferior parietal cortex, and the premotor cortex. The right inferior temporal cortex also was activated specifically during this condition, and no activation of the primary visual areas was observed. Bilateral superior and middle temporal cortex activations were common to CONS and LIST tasks when both were compared with the REST condition. These results provide evidence that the so-called dorsal route known to process visuospatial features can be recruited by auditory verbal stimuli. They also confirm previous reports indicating that some mental imagery tasks may not involve any significant participation of early visual areas.

The membrane integrity of a cell is a well-accepted criterion for characterizing viable (active or inactive) cells and distinguishing them from damaged and membrane-compromised cells. This information is of major importance in studies of the function of microbial assemblages in natural environments, in order to assign bulk activities measured by various methods to the very active cells that are effectively responsible for the observations. To achieve this task for bacteria in freshwater and marine waters, we propose a nucleic acid doublestaining assay based on analytical flow cytometry, which allows us to distinguish viable from damaged and membrane-compromised bacteria and to sort out noise and detritus. This method is derived from the work of S. Barbesti et al. (Cytometry 40:214-218, 2000) which was conducted on cultured bacteria. The principle of this approach is to use simultaneously a permeant (SYBR Green; Molecular Probes) and an impermeant (propidium iodide) probe and to take advantage of the energy transfer which occurs between them when both probes are staining nucleic acids. A full quenching of the permeant probe fluorescence by the impermeant probe will point to cells with a compromised membrane, a partial quenching will indicate cells with a slightly damaged membrane, and a lack of quenching will characterize intact membrane cells identified as viable. In the present study, this approach has been adapted to bacteria in freshwater and marine waters of the Mediterranean region. It is fast and easy to use and shows that a large fraction of bacteria with low DNA content can be composed of viable cells. Admittedly, limitations stem from the unknown behavior of unidentified species present in natural environments which may depart from the established permeability properties with respect to the fluorescing dyes.

We report four studies in which we investigated the production of spatial discourse designed to help people move around in unfamiliar environments. In Study 1, descriptions of three routes were collected from residents of the city of Venice. Analysis of the descriptions revealed the variety of ways used to describe each route. Typical features of route directions were found, in particular the uneven distribution of the landmarks mentioned, which tended to concentrate at critical points where an orientation problem had to be solved. Study 2 used individual protocols to construct more abstract (‘skeletal’) descriptions, reflecting the essentials needed for navigation. New subjects selected those units of information they judged necessary and sufficient to guide a person travelling along the routes. The contents of the skeletal descriptions were very similar, whether they were established by people familiar with Venice or complete strangers, suggesting that people can judge the relevance of information in route directions, regardless of their knowledge of the environment described. Study 3 showed that the ratings of the communicative value of the original individual protocols also resulted in very similar responses from familiar and unfamiliar judges. Finally, Study 4 assessed the value of individual descriptions for assisting navigation by testing the navigational performance elicited by these descriptions. Subjects unfamiliar with the city of Venice were given skeletal descriptions or descriptions which had been rated ‘good’ or ‘poor’ in the previous study. Navigation with good descriptions gave significantly lower error scores than navigation with poor descriptions, and skeletal descriptions gave scores similar to those of good descriptions. Poor descriptions also resulted in more errors from subjects who tended to use a survey perspective than from subjects expressing a preference for visual memories of landmarks. We suggest that the efficiency of route directions as navigational aids depends not only on their intrinsic characteristics but also on the mode of processing adopted by the users. Copyright © 1999 John Wiley & Sons, Ltd.

The bactericidal effect of photocatalysis with TiO2 is well recognized, although its mode of action is still poorly characterized. It may involve oxidation, as illuminated TiO2 generates reactive oxygen species. Here we analyze the bactericidal effect of illuminated TiO2 in NaCl-KCl or sodium phosphate solutions. We found that adsorption of bacteria on the catalyst occurred immediately in NaCl-KCl solution, whereas it was delayed in the sodium phosphate solution. We also show that the rate of adsorption of cells onto TiO2 is positively correlated with its bactericidal effect. Importantly, adsorption was consistently associated with a reduction or loss of bacterial membrane integrity, as revealed by flow cytometry. Our work suggests that adsorption of cells onto aggregated TiO2, followed by loss of membrane integrity, is key to the bactericidal effect of photocatalysis.

Precipitation of calcium carbonate by phytoplankton in the photic oceanic layer is an important process regulating the carbon cycling and the exchange of CO 2 at the ocean-atmosphere interface. Previous experiments have demonstrated that, under nutrient-sufficient conditions, doubling the partial pressure of CO 2 (pCO 2 ) in seawater -a likely scenario for the end of the centurycan significantly decrease both the rate of calcification by coccolithophorids and the ratio of inorganic to organic carbon production. The present work investigates the effects of high pCO 2 on calcification by the coccolithophore Emiliania huxleyi (Strain TW1) grown under nitrogen-limiting conditions, a situation that can also prevail in the ocean. Nitrogen limitation was achieved in NO 3 -limited continuous cultures renewed at the rate of 0.5 d -1 and exposed to a saturating light level. pCO 2 was increased from 400 to 700 ppm and controlled by bubbling CO 2 -rich or CO 2 -free air into the cultures. The pCO 2 shift has a rapid effect on cell physiology that occurs within 2 cell divisions subsequent to the perturbation. Net calcification rate (C ) decreased by 25% and, in contrast to previous studies with N-replete cultures, gross community production (GCP) and dark community respiration (DCR) also decreased. These results suggest that increasing pCO 2 has no noticeable effect on the calcification/photosynthesis ratio (C /P) when cells of E. huxleyi are NO 3 -limited.

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