, 2009). Equipped with this new reporter and a confocal microscope, Nikolaou et al. (2012) could monitor activity across the population of synapses providing the visual input to the optic tectum and relate their response properties to their location. The main recipient of visual input to the optic tectum is a band of neuropil called the stratum fibrosum et griseum superficile (SFGS). Although the SFGS has at least six laminae, individual ganglion cells send their axons to one only. This structural organization recalls the preceding www.selleckchem.com/products/Nutlin-3.html stage of synaptic transmission in the visual system, where bipolar cells send a variety

of signals into six different strata of the inner plexiform layer. It might therefore also be expected that different kinds of information are transmitted in different layers of the SFGS. But what information? Nikolaou et al. (2012) concentrated on probing how motion was encoded by providing stimuli consisting of light and dark bars of different orientations moving in different directions. They found that specific areas of the tectum received inputs from synapses sensitive to the direction of motion, and these were distinct from synapses that responded preferentially see more to either vertically or horizontally oriented

bars. This is the first work to demonstrate that different laminae of those the tectum receive different kinds of visual information. Directionally selective inputs were clustered around three distinct angles, with those signaling tail-to-head motion being the most numerous. The synapses displaying this tail-to-head tuning were restricted to the most superficial lamina of the SFGS, whereas those with preferred sensitivity at the other two angles were located immediately below. Synapses

sensitive to the orientation of the bars tended to avoid these directionally selective layers, instead targeting layers deeper in the SFGS. Thus, while the population activity of synaptic inputs can encode any angle of object approach, a particularly large fraction are concerned with detecting objects that approach from behind, and these project to a specific zone in the tectum. This conclusion highlights the strength of the systematic approach allowed by genetic targeting of a reporter to a particular class of neuron combined with imaging through a defined structure in the brain, and it yields an important insight: of the many messages that the fish’s eye sends to the fish’s brain, one of the loudest is “look out—he’s behind you! Finding the locations of direction- and orientation-selective inputs across different laminae of the optic tectum is only the start of the exploration.