Ocular counterroll modulates the preferred direction of saccade-related pontine burst neurons in the monkey

Saccade-related burst neurons in the paramedian pontine reticular formation (PPRF) of the head-restrained monkey provide a phasic velocity signal to e xtraocular motoneurons for the generation of rapid eye movements. In the su perior colliculus (SC), which directly projects to the PPRF, the motor comm and for conjugate saccades with the head restrained in a roll position is r epresented in a reference frame in between oculocentric and space-fixed coo rdinates with a clear bias toward gravity. Here we studied the preferred di rection of premotor burst neurons in the PPRF during static head roll to ch aracterize their frame of reference with respect to head and eye position. In 59 neurons (short-lead, burst-tonic, and long-lead burst neurons), we fo und that the preferred direction of eye displacement of these neurons chang ed, relative to head-fixed landmarks, in the horizontal-vertical plane duri ng static head roll. For the short-lead burst neurons and the burst-tonic g roup, the change was about one-fourth of the amount of ocular counterroll ( OCR) and significantly different from a head-centered representation. In th e long-lead burst neurons, the rotation of the preferred direction showed a larger trend of about one-half of OCR. During microelectrical stimulation of the PPRF (9 sites in 2 monkeys), the elicited eye movements rotated with about one-half the amount of OCR. In a simple pulley model of the oculomot or plant, the noncraniocentric reference frame of the PPRF output neurons c ould be reproduced for recently measured pulley positions, if the pulleys w ere assumed to rotate as a function of OCR with a gain of 0.5. We conclude that the saccadic displacement signal is transformed from a representation in the SC with a clear bias to gravity to a representation in the PPRF that is closely craniocentric, but rotates with OCR, consistent with current co ncepts of the oculomotor plant.