The regulation of circadian clocks by light in fruitflies and mice

Abstract
Variability is an important but neglected aspect of connectional neuroanatomy. The quantitative density of the ‘same’ corticocortical or thalamocortical connection may vary by over two orders of magnitude between different injections of the same tracer. At present, however, the frequency distribution of connection densities is unknown. Therefore, it is unclear what kind of sampling strategies or statistical methods are appropriate for quantitative studies of connectivity. Nor is it clear if the measured variability represents differences between subjects, or if it is simply a consequence of intra–individual differences resulting from experimental technique and the exact placement of tracers relative to local spatial and laminar variation in connectivity.