Lrrns define a visual circuit underlying brightness and contrast perception

Brightness and contrast are fundamental features of vision, crucial for object detection, environmental navigation, and feeding. Here, we identify a brightness-and contrast-processing circuit in the zebrafish visual system and uncover the role of Leucine-rich repeat neuronal (Lrrn) cell adhesion molecules (CAMs) in regulating its assembly. Deep-projecting retinal ganglion cells (RGCs) serve as the first synaptic relay to the brain requiring Lrrn2 and Lrrn3a for precise axonal targeting and connectivity within the optic tectum. Genetic targeting of these CAMs leads to circuit disorganization and impairments in contrast sensitivity, leading to deficits in visually guided behaviors. Additionally, ultrastructural circuit reconstruction and functional imaging analysis revealed their critical role in luminance processing. These studies define a fundamental visual processing pathway and establish Lrrn CAMs as essential molecular drivers of its assembly.