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| Alessandro E.P. Villa |
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Université Joseph Fourier, INSERM U 318, CHUG Michallon Pav. B - BP 217 , F-38043 Grenoble Cedex 9, France
e-mail: Alessandro.Villa@ujf-grenoble.fr |
Presentation: |
| 2002-10-07, 12:10-12:50 |
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| Thalamo-cortical information processing. |
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| Single unit spike trains were simultaneously recorded in the medial geniculate body (MGB) and thalamic reticular nucleus (RE) during and in the absence of reversible cooling of the primary auditory cortex. Studies were carried out in anesthetized cats, rats and guinea pigs. Each unit was studied during spontaneous activity and acoustically evoked activity. Average firing rate, burst size and burst duration during spontaneous activity, signal-to-noise ratio, frequency bandwidths of excitatory early- or late- transient onset and offset responses as well as inhibitory onset responses to pure tones and white noise bursts were the functional properties analyzed for each unit. Time domain analyses included cross renewal densities (crosscorrelations) and search for precise repetition of complex spatiotemporal firing patterns of reverberating thalamic circuits. Within a given anatomical subdivision, two populations of units could be defined according to the tendency of one property to increase, respectively to decrease during the cortical cooling. The functional selectivity for a property is the result of a distributed population coding based on the presence of such two distinct populations of neurons in the auditory thalamus. Several examples of this coding are provided and it appears that one cell may participate to more than one coding function. The cell assemblies coding for a functional property could act as adaptive filters, whose parameters are controlled by the cortex. Such filtering could selectively extract information from the incoming sensory signal, according to the cortical activity. The cortical cooling affected the majority of significant correlations between pairs of single units recoredd simultaneously and the strength of the interactions was increased twice as often as decreased. These results show that lower levels of cortical activity result in greater interactions between thalamic neurons and greater sharing of signals. During cortical deactivation the absence of cortical input would induce an intrathalamic spread of activity, thus characterizing a distributed processing state in which the thalamus speaks primarily with itself. The cortex exerts a dynamic control over the functional segregation of signals transmitted through the thalamus and theoretical models for the interpretation of these results are presented. |
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