Emergent properties of the optic tectum revealed by population analysis of direction and orientation selectivity.

How local circuits within the brain process visual information has classically been addressed at the single neuron level. Such reductionist approaches, however, struggle to capture the full scope of functional properties associated with even "simple" brain nuclei. Using population functional calcium imaging, we aim to describe how local circuits within the zebrafish optic tectum process visual information. Specifically, how are previously identified direction-selective (DS) and orientation-selective (OS) retinal ganglion cell (RGC) inputs (Nikolaou et al., 2012) represented in tectal cells? First, we identify an emergent population of DS tectal cell with a direction preference not explicitly present in any one of the RGC inputs. Second, this is associated with a striking shift from a tiled and triangular representation of directional space (RGC inputs) into an overlapping cardinal representation by tectal cell populations. Third, and in contrast, we find that orientation space is represented similarly in both the RGC input and tectal cell populations illustrating feature-dependent differences in how tectal circuits process their inputs. Finally, we identify OS and two populations of DS cells at the superficial border of the tectal neuropil, one of which is an emergent population. This study, together with our previous one (Nikolaou et al., 2012), demonstrate that direction-selectivity is established in both the retina and tectum.