CELLULAR AND CIRCUIT MECHANISMS UNDERLYING SENSORY SEGREGATION IN THE DEVELOPING SUPERIOR COLLICULUS

The superior colliculus is a laminated midbrain structure in which superficial layers primarily process visual information, while deeper layers integrate somatosensory and auditory cues. In mice, during embryonic development, the superior colliculus is intrinsically multimodal, such that somatosensory stimulation can elicit visual responses. Around birth, spontaneous retinal activity drives the refinement of this circuitry, leading to the emergence of modality-specific pathways. Notably, visual deprivation either before or at birth prolongs this multimodal state until the end of the first postnatal week, revealing a critical window of sensory plasticity (Guillamón-Vivancos et al., 2022). Here, to dissect the subcortical circuits underlying sensory specification and plasticity, we characterized distinct neuronal populations across superior colliculus layers and mapped their connectivity with the retina and with each other. Using a combination of in vivo calcium imaging, electrophysiological recordings, and optogenetic manipulations, we found that in the superficial layers, both GABAergic neurons and Rorβ-expressing neurons participate in multimodal responses. Remarkably, Rorβ-expressing neurons establish functional projections to the visual thalamus as early as embryonic day 18. In the deeper layers, we focused on trigeminorecipient Pitx2-expressing neurons and uncovered exuberant projections to superficial layers, together with multimodal-like response properties. Collectively, these findings identify specific cell types and circuit motifs that support early multimodality and its subsequent refinement, providing a cellular framework for understanding how sensory circuit segregation emerges in the developing superior colliculus.