Ramp-to-threshold dynamics in a hindbrain population controls the timing of spontaneous saccades

Alexandro D Ramirez,Emre R F Aksay

doi:10.1038/s41467-021-24336-w

Abstract

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 perform calcium imaging in the larval zebrafish to discover a range of responses surrounding spontaneous saccades, from cells that display tonic discharge only during fixations to neurons whose activity rises in advance of saccades by multiple seconds. When we lesion cells in these populations we find that ablation of neurons with pre-saccadic rise delays saccade initiation. We analyze spontaneous saccade initiation using a ramp-to-threshold model and are 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.

Highlights

Organisms have the capacity to make decisions based solely on internal drives
We generated a comprehensive map of neuronal activity patterns underlying spontaneous saccades and subsequent fixations—these maps are generated while animals are in the dark, ensuring that the signals are internally generated, and with a nuclear-localized calcium sensor, eliminating neuropil contamination
In the first part of the results, we present an analysis of spontaneous eye movements in the dark in larval zebrafish (Fig. 1) and a map of the temporal dynamics of neurons that were active during this behavior (Figs. 2 and 3)

Summary

Introduction

Organisms have the capacity to make decisions based solely on internal drives. it is unclear how neural circuits form decisions in the absence of sensory stimuli. Work in animal models suggests that this macroscopic activity is reflective of neuronal firing patterns where rates increase ahead of spontaneous movement in a ramp-like manner[2,3,4] Such studies have shown trial-to-trial variation in the timing of the movement is associated with variation in the ramp rate, with a slower rise coupled to longer rise time, suggesting that action is initiated when neural activity ramps to a cell-specific threshold[3,5]. These findings provide insights into the mechanisms controlling the choice of when to shift gaze, and establish a new model system for understanding the neuronal processes underlying spontaneous, self-initiated actions

Methods

Results

Conclusion