Introduction to the special issue on from maps to circuits: Models and mechanisms for generating neural connections

This Special Issue grew out of the meeting “From Maps to Circuits: Models and Mechanisms for Generating Neural Connections” held at the University of Edinburgh in July 2014 (see maps2014.org for more details). It brings together work presented at the meeting along with other closely related contributions. Neural maps form a key organizing principle of wiring in the nervous system. While understanding the development of these maps has been a critical target for both experimental and theoretical work for many decades, the meeting and this Special Issue highlight many key questions that remain only partially answered. Neural maps can take a variety of forms in different systems. In the retinotectal/retinocollicular projection, neighboring points in the retina map to the neighboring points in the tectum. While the same is true on a broad scale for the mapping from the retina to the thalamus to the primary visual cortex, on a finer scale maps of higher order features emerge, including ocular dominance and orientation preference, which in many higher-order mammals have a complex spatial structure. In contrast, in the olfactory system, widely dispersed olfactory sensory neurons expressing the same odorant receptor project axons which converge on the same glomerulus in the optic bulb. A key feature of the neural mapping field has been an unusually tight interplay between experimental and theoretical work. Beginning with pioneering mathematical work by David Willshaw, Christoph Von der Malsburg, and others in the 1970s, productive theoretical paradigms have been established which have made important contributions to interpreting and guiding experimental work across many areas of neural map formation. One of the key goals of the meeting was to bring people together who have shown an interest at combining theoretical and experimental techniques. Experimental and theoretical work spanning many of the different model mapping systems is presented in this Special Issue. The reviews by Kita et al. (2015a) and Hunter et al. (2015) emphasize the importance of zebrafish as a model system for studying neural maps. Due to their ease of manipulation, and their accessibility and optical transparency during early development, they offer many opportunities for probing mechanisms of map formation. (Kita et al., 2015a) introduce in a zebrafish context some of the basic principles that are then developed in the other contributions, most importantly the role of both molecular and activitydependent cues in driving map formation. The main focus of (Hunter et al., 2015) is the potential of zebrafish for large-scale brain mapping, including gene expression mapping, understanding the functional classes of retinal ganglion cells, and the retinal projectome, and understanding the relationships between neural activity, neural circuits and behavior. They also discuss some of the big data problems involved, and possible approaches to resolving those. A variety of molecular cues are critical for the initial formation of maps in many different neural systems. Missaire and Hindges (2015) review the critical role of cell adhesion molecules (CAMs) in several different aspects of map formation in the visual system. CAMs are initially important for promoting neurite outgrowth, both by cytoskeletal remodeling and modulation of gene activation. They then play a key role in target selection. This includes the decision of retinal axons whether or not to cross the midline, and topographic targeting in the tectum, where CAMs can act in concert with graded tectal cues such as the ephrins. CAMs also play an important role in dendrite self-avoidance, helping ensure the even spread of neural resources. While the role of ephrin gradients in guiding map formation is well established, it has been unclear whether they affect the targeting of different types of retinal ganglion cells equally. In the research contribution of (Sweeney et al., 2015) the authors study the laminar organization of targeting in the superior colliculus in ephrin-A2/A5 double mutant mice. Intriguingly they find a misalignment between the 2015 Wiley Periodicals, Inc. Published online in Wiley Online Library (wileyonlinelibrary. com). DOI 10.1002/dneu.22270