Approches multiéchelles du cerveau visuel : des échos synaptiques à la perception des formes et du mouvement  (série : Colloquium Jacques Morgenstern)

Résumé en anglais seulement,mais la conférence est en français.The field of neuromorphic computation has grown from the idea that  inspiration for future computational architectures can be gained from a  better understanding of information processing in biological neural  networks. Information coding in our brain is both digital,in terms of  output spike timing,and analogue,produced by the slower,subthreshold  changes in membrane voltage resulting from a continual barrage of  synaptic inputs. These small and ever-changing voltage fluctuations in  the neuronal membrane potential of the single neuron,control its  excitability and spiking reliability. The reverse engineering analysis  of these synaptic echoes allows to retrieve the functional effective  connectivity of the contextual network within which each cell is  embedded.   I will review work from my laboratory (UNIC-CNRS) on spatio-temporal  features of the processing realized by the early visual system.  Multiscale recordings in the mammalian visual cortex of ongoing and  evoked dynamics have been compared using electrophysiological  intracellular (single cell) and multiple electrode recording (assembly)  techniques. By varying and controlling the visual statistics simulated  by a virtual oculomotor exploration of our visual environment,we were  able to show that the time precision of the code,the reliability of the  evoked dynamics of the visual cortical network and the functional  organization of visual cortical receptive fields all adapt to the  statistics of the sensory signals. Our observations are best explained  by an homeostatic representation principle,where complexities of the  input statistics and of V1 receptive fields covary inversely.  Generalized recruitment by the stimulus of center-surround interactions  and local non-linearities tend to reduce the contextual noise in  subthreshold dynamics of the single cortical neuron through a divisive  shunt effect. Dynamic full field interactions are shown to regularize  the functionally expressed organization of V1 receptive fields,making  them more linear and ?Simple?-like.   A second illustration of the predictive power of multiscale studies of  visual processing has been obtained by comparing intracellular and  network imaging (voltage sensitive dye) while exploring the ?silent?  periphery of visual cortical neurons. Using apparent motion noise at  saccadic speed,we have inferred from the synaptic echoes (recorded  intracellularly) the existence of long-distance propagation of visually  evoked activity through lateral (and possibly feedback) connectivity  outside the classical receptive field. VSD imaging has been used to  visualize,at the mesoscopic level,the propagation patterns travelling  at the speed inferred from our microscopic recordings. Our results  demonstrate the propagation at the V1 map level of intracortical  depolarizing waves,broadcasting an elementary form of collective  ?belief? to distant parts of the network. The functional features of  these slow waves support the hypothesis of a dynamic perceptual  association field,facilitating synaptic modulation in space and time  during oculomotor exploration. They may serve as a substrate for  implementing the psychological Gestalt principles of common fate and  axial collinearity.   We conclude from this review that the early visual system is far from  being understood,and that  the functional dynamics of visual cortical  networks show a much higher level of complexity than initially thought.  Comparison between different levels of integration not only shows how  limited is our understanding of the emergence of feature selective maps  in primary visual areas,but reveals unexpected immergence processes  through which collective order regulates more microscopic properties in a  top down fashion.

Disciplines : Simulation informatique,Cerveau
Mots-clés : neurobiologie,réseaux neuronaux,neurosciences cognitives,neurosciences computationnelles,vision bas niveau
Type de document : image en mouvement
Type de ressource : lecture
Niveau : master,doctorat
Auteurs ou éditeurs : INRIA (Institut national de recherche en informatique et automatique),UNS,CNRS - Centre National de la Recherche Scientifique,Région PACA,INRIA (Institut national de recherche en informatique et automatique),Yves FRÉGNAC