A hypothetical fixation system capable of extending foveation in congenital nystagmus.

Good visual acuity in the presence of the high-velocity oscillations of pendular nystagmus requires that the ocular motor system be capable of reliable and repeat­ able foveation of the target. The ability of the ocular motor system to acquire a tar­ get, despite these ongoing oscillations, requires seamless interaction between the smooth pursuit (SP) and saccadic subsystems. In a previous modell we proposed that this cooperation is mediated by the "internal monitor" (1M), a hypothetical grouping of functions that calculates the necessary control signals based on efference copy of position and velocity commands issued by the ocular motor system (OMS). Here we extend this model, adding the ability to extend foveation duration beyond the period of low-velocity motion that occurs naturally at the reversal of eye direction during pegdular waveforms,