Optokinetic and vestibular stimulation determines the spatial orientation of negative optokinetic afternystagmus in the rabbit

Prolonged binocular optokinetic stimulation (OKS) in the rabbit induces a h igh-velocity negative optokinetic afternystagmus (OKAN II) that persists fo r several hours. We have taken advantage of this uniform nystagmus to study how changes in static head orientation in the pitch plane might influence the orientation of the nystagmus. After horizontal OKS, the rotation axis o f the OKAN II remained almost constant in space as it was kept aligned with the gravity vector when the head was pitched by as much as 80 degrees up a nd 35 degrees down. Moreover, during reorientation, slow-phase eye velocity decreased according to the head pitch angle. Thereafter, we analyzed the space orientation of OKAN II after optokinetic stimulation during which the head and/or the OKS were pitched upward and do wnward. The rotation axis of OKAN II did not remain aligned with an earth v ertical axis nor a head vertical axis, but it tended to be aligned with tha t of the OKS respace. The slow-phase eye velocity of OKAN II was also affec ted by the head pitch angle during OKS, because maximal OKAN II velocity oc curred at the same head pitch angle as that during optokinetic stimulation. We suggest that OKAN II is coded in gravity-centered rather than in head-ce ntered coordinates, but that this coordinate system may be influenced by op tokinetic and vestibular stimulation. Moreover, the velocity attenuation of OKAN II seems to depend on the mismatch between the space-centered nystagm us rotation axis orientation and that of the "remembered" head-centered opt okinetic pathway activated by OKS.