Pontine Lesions Cause Deficits of Vergence to Step but not to Ramp Targets

The pontine nuclei are part of the cortico-ponto-cerebellar loop of conjugate eye movements, i.e., saccades and smooth pursuit. There is some evidence that the pontine nuclei may also be important in disconjugate eye movements (vergence). As conjugate eye movements, vergence can be separated in vergence to ramp targets (slow vergence) or step targets (fast vergence). It is unknown so far whether pontine nuclei selectively impair slow or fast vergence. To test this hypothesis binocular eye movements were recorded with the scleral search coil system in two patients with median pontine lesions and in 10 healthy controls. As stimuli step targets (7 deg vergence angle), ramp targets (1.5 deg/s vergence velocity), and sinusoidal targets (0.01, 0.15, 0.3Hz; peak velocity 1.5 deg/s) were used. All targets were presented on a horizontal plane immediately below the eye level in a dark surrounding. Conjugate smooth pursuit and saccades were additionally tested on a tangent screen in 145cm distance to the eyes. High resolution MRI was performed in all patients. While conjugate saccades were not different from healthy controls, conjugate smooth pursuit eye movements had a reduced gain in horizontal and vertical directions in all patients. In both patients fast but not slow vergence was impaired. The lesion involved the medial pontine nuclei and the rostral tip of the nucleus reticularis pontis (NRTP), but spared the medial longitudinal fascicle (MLF). We conclude that neural control of vergence to step and ramp targets might be anatomically different in the pontine nuclei.