No abstract

Results of three experiments related to the role of the paramedian pontine reticular formation (PPRF) in the control of gaze are described. (1) Chronic unit recording methods, used to study the on-directions of short-lead burst neurons in head-restrained monkeys, and (2) reversible inactivation techniques confirmed the traditional view of the importance of PPRF in the control of horizontal eye movements. Reversible inactivation of neurons in the vicinity of identified short-lead burst neurons produced dramatic reductions in the speed of saccades to horizontal target displacements. The reductions in velocity were largely compensated for by an increase in saccade duration. Only minor, if any, effects were observed upon the velocity, duration, and amplitude of saccades to upward target displacements. (3) Microstimulation was applied to omnipause neurons to gate activity of excitatory burst neurons that discharge during coordinated eye-head movements. The microstimulation failed to noticeably slow (prevent) head movements when stimulation was applied during (prior to onset of) gaze shifts, suggesting that signals relayed to motoneurons innervating the neck muscles are not inhibited by the omnipause neurons. In other words, the desired gaze signal is parsed into eye and head pathways upstream of the excitatory burst neurons.

To investigate the brain stem control of saccadic eye movements, the paramedian pontine reticular formation (PPRF) in rhesus monkeys was temporarily and partially inactivated with the local anesthetic lidocaine. The influence on ipsilesional, contralesional, and upward saccades was examined. While the effects of the inactivation on contralesional and upward saccades were inconsistent and small, consistent and marked modifications were observed for ipsilesional movements. For ipsilesional, horizontal saccades, all lidocaine injections caused a decrease in peak velocity and a proportional increase in duration, which substantially altered the shape of the velocity profile. The rise in duration usually fell short of preventing hypometric saccades at the peak of the effect. However, as the lidocaine effect dissipated, the amplitude often returned to control, even though the velocity and duration remained compromised. For ipsilesional, oblique saccades, the effect of lidocaine on the horizontal component was similar to that for horizontal saccades. The vertical component of oblique saccades was also influenced, albeit to a much lesser extent: the duration of the vertical component typically increased, while the vertical peak velocity either decreased or exhibited no significant change. These results were compared with simulations of three prominent models for cross-coupling oblique saccades. In general, these results of the temporary inactivation of PPRF are consistent with the predictions of local feedback models for saccadic control.

Constant frequency microstimulation of the paramedian pontine reticular formation (PPRF) in head-restrained monkeys evokes a constant velocity eye movement. Since the PPRF receives significant projections from structures that control coordinated eye-head movements, we asked whether stimulation of the pontine reticular formation in the head-unrestrained animal generates a combined eye-head movement or only an eye movement. Microstimulation of most sites yielded a constant-velocity gaze shift executed as a coordinated eye-head movement, although eye-only movements were evoked from some sites. The eye and head contributions to the stimulationevoked movements varied across stimulation sites and were drastically different from the lawful relationship observed for visually-guided gaze shifts. These results indicate that the microstimulation activated elements that issued movement commands to the extraocular and, for most sites, neck motoneurons. In addition, the stimulation-evoked changes in gaze were similar in the head-restrained and head-unrestrained conditions despite the assortment of eye and head contributions, suggesting that the vestibuloocular reflex (VOR) gain must be near unity during the coordinated eye-head movements evoked by stimulation of the PPRF. These findings contrast the attenuation of VOR gain associated with visually-guided gaze shifts and suggest that the vestibulo-ocular pathway processes volitional and PPRF stimulation-evoked gaze shifts differently.

Remembered saccades of rhesus monkeys are markedly influenced by starting eye position. Altering the initial position systematically affects the direction or amplitude of the movements to a striking degree. In general, changes in the horizontal or vertical starting position primarily produce changes in the horizontal or vertical component, respectively, regardless of whether the target displacement occurs in the horizontal or vertical direction. For some monkeys, a similar pattern of initial position influence on movement direction can be seen in the curvature of visually guided saccades. Starting position also modulates the upward offset in fixation, which monkeys display in the dark.