Functional impact of cerebral projection systems

Typically, anatomical connections have been traced by injecting pathway-tracing chemicals into restricted portions of the brain. After a few days, the brains are fixed and the transported chemicals identified in histological sections. Orthograde tracers move forward along axons from cell body to axon terminals and retrograde tracers move backwards along axons from terminals to parent cell body. The use of both types of tracers has revealed origins and terminations of pathways and a massively complex network of connections between numerous functionally and anatomically distinct cerebral cortical and subcortical regions. In the monkey visual system alone more than 300 connections have been described between the 32 visual cortical areas. Even so, in the network descriptions neither anatomical strengths nor functional impacts of individual connections are identified. Yet, there is no doubt that knowledge about both aspects of connectivity is essential for developing accurate descriptions of network operations. We describe a new combination of a metabolic mapping and a reversible deactivation technique in an animal model to assess the functional impact of cerebral connections.