Saccade-like Behaviour in the Fast-Phases of Optokinetic Nystagmus: An Illustration of the Emergence of Volitional Actions from Automatic Reflexes

As a potential exemplar for understanding how volitional actions emerged from reflexes, we studied the relationship between an ancient reflexive gaze stabilisation mechanism (optokinetic nystagmus) and purposeful eye movements (saccades) that target an object. Traditionally these have been considered distinct (except in the kinematics of their execution) and studied independently. We find that the fast-phases of optokinetic nystagmus (OKN) clearly show properties associated with saccade planning: (1) They are characteristically delayed by irrelevant distractors in an indistinguishable way to saccades (the saccadic inhibition effect); (2) horizontal OKN fast-phases produce curvature in vertical targeting saccades, just like a competing saccade plan. Thus we argue that the saccade planning network plays a role in the production of OKN fast-phases, and question the need for a strict distinction between eye movements that appear to be automatic or volitional. We discuss whether our understanding might benefit from shifting perspective and considering the entire ‘saccade’ system to have developed from an increasingly sophisticated OKN system.