Section: New Results

Modeling of brain development and brain functions

Organization of the visual cortex

Participants : Jonathan Touboul, Jérôme Ribot [CIRB] , Alberto Romagnoni [ENS] , Daniel Bennequin [IMG-PRG] , Chantal Milleret [CIRB] .

In the early visual cortex, information is processed within functional maps whose layout is thought to underlie visual perception. However, the precise organization of these functional maps as well as their interrelationships remains unresolved. We have investigated using new data acquisition and analysis as well as mathematical modeling, the inter-relationship between different visual maps in cat visual cortex.

We have shown in [25] that spatial frequency representation in cat areas 17 and 18 exhibits singularities around which the map organizes like an electric dipole potential. These singularities are precisely co-located with singularities of the orientation map: the pinwheel centers. We have first shown, using high resolution optical imaging, that a large majority (around 80%) of pinwheel centers exhibit in their neighborhood semi-global extrema in the spatial frequency map. These extrema create a sharp gradient that was confirmed with electrophysiological recordings. Based on an analogy with electromagnetism, a mathematical model of a dipolar structure has been proposed and accurately fitted to optical imaging data for two third of pinwheel centers with semi-global extrema. We have concluded that more than half of orientation pinwheel centers form spatial frequency dipoles in cat early visual cortex.

We have demonstrated mathematically in [27] that two natural principles, local exhaustivity of representation and parsimony, would constrain the orientation and spatial frequency maps to display co-located singularities around which the orientation is organized as a pinwheel and spatial frequency as a dipole. We have further focused on the theoretical implications of this structure. Using a computational model, we have shown that this architecture allows a trade-off in the local perception of orientation and spatial frequency, but this would occur for sharper selectivity than the tuning width reported in the literature. We therefore re-examined physiological data and have shown that indeed the spatial frequency selectivity substantially sharpens near maps singularities, bringing to the prediction that the system tends to optimize balanced detection between different attributes.

Modeling the timing of neurogenesis and control of the neuron pool : Enhanced abventricular proliferation compensates cell death in the embryonic cerebral cortex

Participants : Betty Freret-Hodara [IJM] , Yi Cui, Amélie Griveau [IJM] , Lisa Vigier [IJM] , Yoko Arai [IJM] , Jonathan Touboul, Alessandra Pierani [IJM] .

Loss of neurons in the neocortex is generally thought to result in a final reduction of cerebral volume. Yet, little is known on how the developing cerebral cortex copes with death of early-born neurons. We have tackled this issue by taking advantage of a transgenic mouse model in which, from early embryonic stages to mid-corticogenesis, abundant apoptosis is induced in the postmitotic compartment. Unexpectedly, the thickness of the mutant cortical plate at E18.5 was normal, due to an overproduction of upper layer neurons at E14.5. We have developed and simulated a mathematical model to investigate theoretically the recovering capacity of the system and found that a minor increase in the probability of proliferative divisions of intermediate progenitors (IPs) is a powerful compensation lever. Combined with our experimental observations, these results illustrate the remarkable plasticity of neocortical progenitors to adapt to major embryonic insults via the modulation of abventricular divisions thereby ensuring the production of an appropriate number of neurons.