Thèse soutenue

Corrélation entre l'excitation et l'inhibition dans les circuits corticaux visuels : conséquences fonctionnelles et plausibilité biologique
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Auteur / Autrice : Jens Oliver Kremkow
Direction : Guillaume S. MassonAdrianus Aertsen
Type : Thèse de doctorat
Discipline(s) : Neurosciences
Date : Soutenance en 2009
Etablissement(s) : Aix-Marseille 2 en cotutelle avec Albert-Ludwigs-Universität (Freiburg im Breisgau, Allemagne)
Partenaire(s) de recherche : autre partenaire : Université d'Aix-Marseille II. Faculté de médecine (1970-2011)

Résumé

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The primary visual cortex (V1) is one of the most studied cortical area in the brain. Together with the retina and the lateral geniculate nucleus (LGN) it forms the early visual system, which has become a common model for studying computational principles in sensory systems. Simple artificial stimuli (such as drifting gratings (DG)) have given insights into the neural basis of visual processing. However, recently more and more researchers have started to use more complex natural visual stimuli (NI), arguing that the low dimensional artificial stimuli are not sufficient for a complete understanding of the visual system. For example, whereas the responses of V1 neurons to DG are dense but with variable spike timings, the neurons respond with only few but precise spikes to NI. Furthermore, linear receptive field models provide a good fit to responses during simple stimuli, however, they often fail during NI. To investigate the mechanisms behind the stimulus dependent responses of cortical neurons we have built a biophysical, yet simple and comprehensible, model of the early visual system. We show how the spatial and temporal stimulus properties interact with the model architecture to give rise to the differential response behaviour. Our results show that during NI the LGN afferents show epochs of correlated activity. These temporal correlations induce transient excitatory synaptic inputs, resulting in precise spike timings in V1. Furthermore, the sparseness of the responses to NI can be explained by correlated and lagging inhibitory conductance, which is induced by the interactions of the thalamocortical circuit with the spatial-temporal correlations in the stimulus. We continue by investigating the origin of stimulus dependent nonlinear responses, by comparing models of different complexity. Our results suggest that adaptive processes shape the responses, depending on the temporal properties of the stimuli. The spatial properties can result in nonlinear inputs through the recurrent cortical network. We then study the functional consequences of correlated excitatory and inhibitory condutances in more details in generic models. These results show that: (1) spiking of individual neurons becomes sparse and precise, (2) the selectivity of signal propagation increases and the detailed delay allows to gate the propagation through feed-forward structures (3) and recurrent cortical networks are more stable and more likely to elicit in vivo type activity states. Lastly our work illustrates new advances in methods of constructing and exchanging models of neuronal systems by the means of a simulator independent description language (called PyNN). We use this new tool to investigate the feasibility of comparing software simulations with neuromorphic hardware emulations. The presented work gives new perspectives on the processing of the early visual system, in particular on the importance of correlated conductances. It thus opens the door for more elaborated models of the visual system.