Malagasy dialects and the peopling of Madagascar.

The origin of Malagasy DNA is half African and half Indonesian, nevertheless the Malagasy language, spoken by the entire population, belongs to the Austronesian family. The language most closely related to Malagasy is Maanyan (Greater Barito East group of the Austronesian family), but related languages are also in Sulawesi, Malaysia and Sumatra. For this reason, and because Maanyan is spoken by a population which lives along the Barito river in Kalimantan and which does not possess the necessary skill for long maritime navigation, the ethnic composition of the Indonesian colonizers is still unclear. There is a general consensus that Indonesian sailors reached Madagascar by a maritime trek, but the time, the path and the landing area of the first colonization are all disputed. In this research, we try to answer these problems together with other ones, such as the historical configuration of Malagasy dialects, by types of analysis related to lexicostatistics and glottochronology that draw upon the automated method recently proposed by the authors. The data were collected by the first author at the beginning of 2010 with the invaluable help of Joselinà Soafara Néré and consist of Swadesh lists of 200 items for 23 dialects covering all areas of the island.

Exploring structure-function relationships in neocortical networks by means of neuromodelling techniques.

Determining the neuronal architecture underlying certain visual functions is of fundamental importance for understanding how sensory processing is implemented in the brain. The wealth of anatomical, physiological and biophysical data that is being currently acquired on the neocortex could be used to constrain its functional architecture. However, given the intrinsic complexity and diversity of the data, it is difficult to provide a comprehensive framework to use these data in order to characterize structure-function relationships. Here, we discuss the use of biophysically plausible models of dynamics of neuronal networks, constructed to reflect the known properties of neocortical connectivity and modularity, as a tool to bring together anatomy and physiology. We illustrate the utility and rationale of the neuro-dynamics modelling approach by considering recent studies on the relationship between functional structure of the visual cortex and its response timing, and on the cellular and network origin of neuronal oscillations in the gamma frequency range. We also critically discuss how an interaction between theory and experiments could help this approach to become directly relevant for clinical applications.

Decoding neuronal population activity in rat somatosensory cortex: role of columnar organization.

The present study asks in what way the activity of a neuronal population responding to a sensory stimulus could be most efficiently decoded, or 'read off', by the target neurons. A simple solution to this problem has been proposed - pooling the activity of responding neurons. However, pooling can be inefficient if sensory information is encoded by the 'label' of each neuron firing a spike. We have tested the efficiency of pooling by quantifying the extent to which information about a sensory stimulus is diminished when the identity of the individual neurons is lost by pooling. Analyzing the response of small groups of neurons in rat barrel cortex to single-whisker deflection, we found that pooling neurons within the same column is efficient for representing stimulus position; it causes a loss of only 1% of the information about whether the principal whisker was stimulated, and a loss of 5-12% of the finer information about which of nine possible whiskers (the principal and its neighbors) was stimulated. Cross-column pooling led to larger information losses, in the range of 25-55%. Thus, to decode stimulus position from the discharge of barrel cortex populations, 'downstream' neurons could pool the activity arising from neurons of the same column, while maintaining the activity arising from neurons of separate columns at least partially segregated. Since such parcellation is present in some of the projections from barrel cortex, these findings suggest that columnar organization of barrel cortex serves to facilitate decoding of the location of the stimulated whisker.

A critical assessment of different measures of the information carried by correlated neuronal firing.

Information theoretic measures have been proposed as a quantitative framework to clarify the role of correlated neuronal activity in the brain. In this paper we review some recent methods that allow precise assessments of the role of correlation in stimulus coding and decoding by the nervous system. We present new results that make explicit links between types of encoding and decoding mechanisms based on correlations. We illustrate the concepts by showing that the spike trains of pairs of neurons in rat somatosensory cortex can be decoded almost perfectly without including knowledge of correlation in the read-out model, although in this neural system correlations between spike times contribute appreciably to stimulus encoding.