Abstract
The superior colliculus (SC) is critical for saccade generation. Recent work has shown that, despite bursting at times other than saccades, SC population activity at the time of saccade motor bursts is more temporally aligned than for visual bursts (Jagadisan & Gandhi, 2022). Similarly, population activity in motor bursts resides in different subspaces to visual bursts (Baumann et al., 2023), and even the sensory signal embedded in SC motor bursts (Baumann et al., 2023) is transformed relative to visual bursts, such that the same individual neurons “prefer” different visual features in the two bursting epochs. However, how might such a transformation from a visual regime to a motor regime be realized? Here we first show that when a planned saccade is finally released with a go signal (removal of a fixation spot), peripheral SC neurons (representing the saccade target location) exhibit a robust, short-latency pause in spiking, before the motor bursts eventually erupt. This pause starts within ~50 ms from the go signal, and it is stimulus-dependent (e.g. having a stronger firing rate dip for a salient peripheral stimulus). Additionally, this pause still occurs, to a weaker extent, with saccades to a small spot or blank. When we then recorded from foveal SC neurons in similar tasks, we found that these neurons actually burst after the go signal, rather than paused. Remarkably, these foveal bursts occurred (and peaked) several milliseconds earlier than the pauses in the peripheral neurons, and they were not explained by offset responses to the removal of the fixation spot. Foveal bursts also occurred when releasing memory-guided saccades (with no peripheral visual targets), and they were not sensitive to peripheral target appearance. Thus, we found a transient foveal SC signal jumpstarting peripheral saccadic orienting, likely facilitating a necessary representational transformation needed for saccade motor bursts to occur.