A method to assess the functional impact of cerebral connections on targetpopulations of neurons

We describe an innovative and tested approach combining two individually po tent techniques to visualize simultaneously the functional impact of multip le projections on target populations of neurons in the brain. The rationale is simple: silence a defined set of efferent projections from one cortical region using cooling deactivation and then measure the impact of the deact ivation on activities in multiple target regions using 2-deoxyglucose (2DG) . This is a straightforward and sound approach because 2DG uptake by neuron s reflects levels of underlying neural activity. All distant modifications evoked by the silencing of the set of efferent projections are examined in anatomical tissue and simultaneously for the multiple target sites to provi de a global view of the functional impacts of the set of projections on the targets. With this method, downward adjustments of 2DG uptake levels ident ify removals of net excitatory signals, whereas upward adjustments identify net removals of suppressive influences. Future possible uses and modificat ions of the technique, including optical imaging, are discussed. Overall, t he technique has the potential to provide fundamental, new measures on cere bral network interactions that both complement and extend current static mo dels of cerebral networks and electrophysiological measures of functional i mpacts on individual neurons. (C) 1999 Elsevier Science B.V. All rights res erved.