SUMMARY1. The effects of bilateral electrolytic lesions within and around the prepositus hypoglossi (p.h.) nucleus on horizontal saccades in the dark and on the horizontal sinusoidal vestibulo-ocular reflex (v.o.r.) in the dark were studied.2. After p.h. lesion, including its rostral part between P 7 and P 8, the v.o.r. showed a phase lead as much as about 90 deg at 0-10 Hz. A significant gain reduction paralleled that phase lead at lower frequencies. A large post-saccadic drift was also observed, the time constant of which ranged from 0 3 to 0-6 s.3. After p.h. lesion extending from P 8 to P 11 (but sparing the rostral part of the p.h.), no significant gain or phase lead change was observed. Post-saccadic drift was either missing or weak.4. A bilateral medial vestibular nucleus (m.v.n.) lesion from P 7 to P 11 produced a marked gain decrease, paralleled by a marked phase advance. A post-saccadic drift was observed (r = 0-6 s).5. A surgical mid-line lesion from P 7 to P 11 (depth: about 2 mm) was followed by no remarkable change in the gain and in the phase of the v.o.r. No post-saccadic drift was observed after such lesion.6. It was concluded that (i) both the horizontal v.o.r. integration processing, and the horizontal saccadic integration processing were destroyed when an electrolytic lesion was made 'in the region of' the rostral part of the p.h. nucleus, and that (ii) the posterior four-fifths of the p.h. was the location of neither the horizontal v.o.r. integrator nor the horizontal saccadic integrator.

No abstract