The superior colliculus (SC) projects to other brain centres through multiple
pathways, which are thought to be important in rapid, visually guided
behaviours. Projections to the visual thalamus and the periaqueductal gray
have been hypothesised to be important for visually-guided defensive
behaviour, however, the extent to which these projections are anatomically
and functionally distinct remains poorly understood. Further still, our
understanding of the influence of these pathways on visually-guided
defensive responses is incomplete. We tested the anatomical localisation
and segregation of the SC cells projecting to each target using dual
retrograde tracing. Through injecting CTB protein conjugated to different
fluorophores into the visual thalamus and periaqueductal gray, we found
intermingled yet separate SC populations projecting to each target. We took
advantage of the intermingled yet separate nature of the two SC output
pathways to measure their functional properties. Injecting retrogradelytransported cre virus into either the visual thalamus or periaqueductal gray,
we gated expression of GCaMP7s in the SC. We implanted a lens above the
SC and measured the global, calcium-dependent fluorescent signal while
presenting visual stimuli to head-fixed mice free to run on a treadmill.
Employing different visual stimuli, we observed that while there was shared
visual tuning between the two SC output pathways, they exhibited different
dynamics and responses to the animal’s own movement. Finally, we
performed behavioural experiments to better understand the roles each
pathway might play in defensive behavioural responses. Our results inform
theories about the neural pathways that allow mice to produce defensive
responses to visual threats.