Neuronal assembly detection and cell membership specification by principal component analysis

In 1949, Donald Hebb postulated that assemblies of synchronously activated neurons are the elementary units of information processing in the brain. Despite being one of the most influential theories in neuroscience, Hebb's cell assembly hypothesis only started to become testable in the past two decades due to technological advances. However, while the technology for the simultaneous recording of large neuronal populations undergoes fast development, there is still a paucity of analytical methods that can properly detect and track the activity of cell assemblies. Here we describe a principal component-based method that is able to (1) identify all cell assemblies present in the neuronal population investigated, (2) determine the number of neurons involved in ensemble activity, (3) specify the precise identity of the neurons pertaining to each cell assembly, and (4) unravel the time course of the individual activity of multiple assemblies. Application of the method to multielectrode recordings of awake and behaving rats revealed that assemblies detected in the cerebral cortex and hippocampus typically contain overlapping neurons. The results indicate that the PCA method presented here is able to properly detect, track and specify neuronal assemblies, irrespective of overlapping membership.     

A Hippocampus-Accumbens Tripartite Neuronal Motif Guides Appetitive Memory in Space

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Labelling halophilic Archaea using 13C and 15N stable isotopes: a potential tool to investigate haloarchaea consumption by metazoans

The use of stable isotope (SI) labelling and tracing of live diets is currently considered one of the most comprehensive tools to detect their uptake and assimilation by aquatic organisms. These techniques are indeed widely used in nutritional studies to follow the fate of specific microbial dietary components, unraveling trophic interactions. Nevertheless, to the current date our understanding of aquatic trophic relationships has yet to include a whole domain of life, the Archaea. The aim of the present research was, therefore, to describe a halophilic Archaea (haloarchaea) labelling procedure, using the SI ¹³C and ¹⁵N, to enable the application of SI tracing in future studies of haloarchaea consumption by aquatic metazoans. To this end, three ¹³C enriched carbon sources and two ¹⁵N enriched nitrogen sources were tested as potential labels to enrich cells of three haloarchaea strains when supplemented to the culture medium. Our overall results indicate ¹³C-glycerol as the most effective carbon source to achieve an efficient ¹³C enrichment in haloarchaea cells, with Δδ¹³C values above 5000‰ in all tested haloarchaea strains. As for ¹⁵N enriched nitrogen sources, both (¹⁵NH₄)₂SO₄ and ¹⁵NH₄Cl seem to be readily assimilated, also resulting in efficient ¹⁵N enrichment in haloarchaea cells, with Δδ¹⁵N values higher than 20,000‰. We believe that the proposed methodology will allow for the use of SI labelled haloarchaea biomass in feeding tests, potentially providing unambiguous confirmation of the assimilation of haloarchaea biomass by aquatic metazoans.