Ramp-to-Threshold Dynamics in a Hindbrain Population Controls the Timing of Spontaneous Saccades

Organisms have the capacity to make decisions based solely on internal drives. However, it is unclear how neural circuits form decisions in the absence of sensory stimuli. Here we provide a comprehensive map of the activity patterns underlying the generation of saccades made in the absence of visual stimuli. We performed calcium imaging in the larval zebrafish to discover a range of responses surrounding spontaneous saccades, from cells that displayed tonic discharge only during fixations to neurons whose activity rose in advance of saccades by multiple seconds. We lesioned cells in these populations and found that ablation of neurons with pre-saccadic rise delayed saccade initiation. We analyzed spontaneous saccade initiation using a ramp-to-threshold model and were able to predict the times of upcoming saccades using pre-saccadic activity. These findings suggest that ramping of neuronal activity to a bound is a critical component of self-initiated saccadic movements.