Cortical feedback microcircuits in the visual system of the mouse.

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Integrated perception of the outside world emerges from the processing of sensory information in multiple areas of the cerebral cortex. It is generally believed that the multiple cortical areas are reciprocally interconnected by an intricate network of feedforward and feedback connections. Forward connections provide information from lower to higher areas for further (bottom-up) processing. Feedback connections send information from higher areas and modulate lower areas activity (top-down). From these types of connections a general hierarchical model of cortical processing has been proposed and has been applied to primates, cats and rodents. These types of cortical connections are defined by the distribution of the cells of origin and their terminal axons in the 6 layers of the cerebral cortex. This gross pattern of laminar distribution of axonal terminals is the result of the sum of a host of single axons. Observations of single axons show a variety of termination patterns some of which are common to both forward and backward connections. There is yet no in depth understanding of the microcircuits that constitute these cortical macrocircuits. The classical model of hierarchical cortical networks has recently been challenged and the cerebral cortex is now more perceived as a distributed rather than a hierarchical network. It is increasingly evident that a dichotomous classification of cortical connections cannot account for the broad spectrum of connection types. At a local level, there is clear evidence for cortical microcircuits that reflect the long range target of axons to cortical and subcortical structures. The objective of the present proposal is a study of the fine structure of corticocortical connections at the single axon level in order to define the building blocs of these connection patterns. In doing this, we will have the basis to devise a novel classification of cortical connection that will account for their diversity. We will also provide basic knowledge of how these long range cortical connections are related to local microcircuits.