OCULAR LIMIT-CYCLES INDUCED BY DELAYED RETINAL FEEDBACK

Lisberger's and Robinson's models of smooth pursuit predict very diffe rent results from altering retinal feedback delay. We have therefore i nvestigated the effects of increasing the retinal feedback time delay in three normal human subjects by means of an artificial feedback para digm. When additional delays were incorporated into the retinal feedba ck path a threshold was reached beyond which the eye exhibited sustain ed self-excited oscillations or ''limit cycles''. The oscillation peri od increased linearly (as the added delay was increased) with slopes r anging from 1.41 to 1.6 with zero-delay intercepts of between 212 and 306 ms. Contrary to our experimental findings the Robinson and Lisberg er models predict that the plot of oscillation period against added de lay should have a slope of 3.4 and 2.7 and an intercept of 479 and 554 ms, respectively. Neither model produced comparable limit cycles, bot h being unstable at delays greater than 280 ms. Our results imply that the models of smooth pursuit need to incorporate predictive control.