Open Loop Optokinetic Responses of the Turtle

Turtle eye movements were measured during full-field horizontal optokinetic stimulation under closed and open loop conditions. Because these animals display unyoked slow-phase eye movement behavior, open loop stimulation could not be presented to a paralyzed eye, while monitoring the position of the contralateral eye. The turtle's optokinetic reflex loop was opened electronically by a continuous adjustment of the pattern's position that effectively canceled the effect of the movement of the recorded eye. The highest open loop gains (2-3) were observed at low speeds (< 1 deg/sec), demonstrating a more limited speed range and lower gain in turtle than in the mammalian optokinetic system. These results in the intact animal can be correlated with the visual response properties of the turtle's pretectum and accessory optic system recorded in vitro.