Fig. 2 The ocular motor control system is composed of the dual-mode version and the vergence subsystems. The output of the pons sums with that of the vergence neural pulse generator at the ocular motor nuclei (OMN) to produce the three basic types of eye movements: fast (FEM), slow (SEM), and vergence (VEM). OKN, optokinetic nystagmus. (Modified from Dell'Osso LF, Daroff RB: Functional organization of the ocular motor system. Aerospace Med 45:873, 1974)