The role of compensatory eye and head movements for gaze stabilization in the unrestrained frog

Compensatory eye, head and gaze movements of unrestrained frogs were recorded simultaneously in response to table movements in the light. Passive displacement was compensated with a gain between 0.55 and 0.85, depending on stimulus amplitude. At small stimulus amplitudes gaze was stabilized exclusively by compensatory eye movements. At larger stimulus amplitudes compensatory head movements contributed up to 80% gaze stabilization. The contribution of compensatory eye movements became increasingly more restricted to those brief transient periods, at which head velocity changed only slowly in response to a change in stimulus direction or velocity. The wave forms of both eye and head movements exhibited characteristic and complementary distortions. Their combination, the gaze wave form compensated almost exactly in phase for the imposed passive displacement in space. Head saccades of small amplitude were rather well compensated by fast eye movements in the opposite direction, with the result that the combined gaze movement was smooth. The occurrence of these compensatory fast eye movements depended neither upon the function of the labyrinthine organs nor upon retinal image slip.