Figure 1. Circuit-mapping strategies.

Examples of the three main approaches that have been used to study the connectivity of neurons. a | Single-cell staining by dye impregnation. The best-known of these techniques is the Golgi staining method, which was used by Ramón y Cajal to describe the principles of neuronal circuit organization³⁹. b | The introduction of diffusible or transportable labelling agents to discrete areas. In some cases the label crosses a synapse to reach a second-order target. A classical example is the identification of ocular dominance columns in the visual cortex following the injection of radioactive proline into one eye⁴⁰. c | Serial electron microscopy can be used to reconstruct neurons and their processes with the best attainable resolution. Shown here is a reconstruction from the adult rat barrel cortex that was segmented from a three-dimensional image stack obtained by serial block-face imaging¹⁶. Part a reproduced, with permission, from REF. 39 © (1914) Herederos de santiago Ramón y Cajal. Part b reproduced, with permission, from REF. 40 © (1977) royal society of London. Part c reproduced, with permission, from REF. 16 © (2006) Elsevier Sciences.