Inferior Olive and the Saccadic Neural Integrator

Abstract

Following a lesion of the inferior olive, spontaneous saccades are characterised by a backward postsaccadic drift. When performed in light, this drift has a time constant of 100– 150ms and after a few tens of ms, it gradually slows down to reach a steady level under a drive of the optokinetic reflex. In dark, this drift is followed by a slower drift due to the leakage of the neural integrator. When calculated on saccades of 10° of amplitude and ending near the midline the time constant is 0.9s. The amplitude of the postsaccadic drift (y) in light depends on the saccadic amplitude (x) and on the eccentricity (z), according to the equation y = 0.23 x + 0.24 z. This means that the gain of the pulse to step transformation is 0.77 at all saccadic amplitudes. Following flocculus-paraflocculus lesion the time constant of the neural integrator is 0.9s and the equation relating the amplitude of the postsaccadic drift to the saccadic amplitude and to the eccentricity is y = 0.21 x + 0.23 z. Thus, there is a striking similarity between the inferior olive and flocculus-paraflocculus lesion.