The role of the medial longitudinal fasciculus in horizontal gaze: tests of current hypotheses for saccade-vergence interactions

Abstract

Rapid shifts of the point of visual fixation between equidistant targets require equal-sized saccades of each eye. The brainstem medial longitudinal fasciculus (MLF) plays a cardinal role in ensuring that horizontal saccades between equidistant targets are tightly yoked. Lesions of the MLF—internuclear ophthalmoparesis (INO)—cause horizontal saccades to become disjunctive: adducting saccades are slow, small, or absent. However, in INO, convergence movements may remain intact. We studied horizontal gaze shifts between equidistant targets and between far and near targets aligned on the visual axis of one eye (Müller test paradigm) in five cases of INO and five control subjects. We estimated the saccadic component of each movement by measuring peak velocity and peak acceleration. We tested whether the ratio of the saccadic component of the adducting/abducting eyes stayed constant or changed for the two types of saccades. For saccades made by control subjects between equidistant targets, the group mean ratio (±SD) of adducting/abducting peak velocity was 0.96 ± 0.07 and adducting/abducting peak acceleration was 0.94 ± 0.09. Corresponding ratios for INO cases were 0.45 ± 0.10 for peak velocity and 0.27 ± 0.11 for peak acceleration, reflecting reduced saccadic pulses for adduction. For control subjects, during the Müller paradigm, the adducting/abducting ratio was 1.25 ± 0.14 for peak velocity and 1.03 ± 0.12 for peak acceleration. Corresponding ratios for INO cases were 0.82 ± 0.18 for peak velocity and 0.48 ± 0.13 for peak acceleration. When adducting/abducting ratios during Müller versus equidistant targets paradigms were compared, INO cases showed larger relative increases for both peak velocity and peak acceleration compared with control subjects. Comparison of similar-sized movements during the two test paradigms indicated that whereas INO patients could decrease peak velocity of their abducting eye during the Müller paradigm, they were unable to modulate adducting velocity in response to viewing conditions. However, the initial component of each eye’s movement was similar in both cases, possibly reflecting activation of saccadic burst neurons. These findings support the hypothesis that horizontal saccades are governed by disjunctive signals, preceded by an initial, high-acceleration conjugate transient and followed by a slower vergence component.

Do brainstem omnipause neurons terminate saccades?

“…Classically, OPN were considered to function as a gate–with passive release of BN inhibition at saccade initiation and resumption of BN inhibition at saccade termination. A recent hypothesis suggests that, rather than functioning as a passive gate for saccade initiation, OPN induce a postinhibitory rebound discharge of BN mediated by low‐threshold calcium channels that assists with initial burst acceleration 38,46,47 . This, in combination with the baseline high‐gain properties of the saccadic neuronal control system and the coupling of IBN and EBN, makes the system prone to conjugate high‐frequency oscillations, especially under conditions when the OPN are silent but a saccadic motor command is no longer being provided to the BN 32,48 …”

Comment le cerveau contrôle-t-il les mouvements des yeux ? Les neurones moteurs et prémoteurs du tronc cérébral

“…Chen et al [72] examinè rent chez des patients souffrant d'ophtalmoparé sie internuclé aire et des volontaires sains (à titre de té moins) le pic de vitesse d'adduction/abduction et les ratios d'accé lé ration pendant les mouvements du regard entre des cibles é quidistantes et des cibles proche et lointaine aligné es sur l'axe visuel d'un oeil. Tandis que la composante initiale du mouvement de chaque oeil ne fut pas diffé rente entre les deux conditions, les patients ophtalmoparé siques internuclé aires n'adaptè rent pas, en regard des té moins, la vitesse adductrice en ré ponse aux conditions de vue, suggé rant que les saccades horizontales pourraient ê tre contrô lé es par des signaux disconjugué s pré cé dé s d'une phase transitoire initiale conjugué e, et suivies par une composante de vergence lente [72].…”

“…Tandis que la composante initiale du mouvement de chaque oeil ne fut pas diffé rente entre les deux conditions, les patients ophtalmoparé siques internuclé aires n'adaptè rent pas, en regard des té moins, la vitesse adductrice en ré ponse aux conditions de vue, suggé rant que les saccades horizontales pourraient ê tre contrô lé es par des signaux disconjugué s pré cé dé s d'une phase transitoire initiale conjugué e, et suivies par une composante de vergence lente [72].…”

Improvement of internuclear ophthalmoparesis in multiple sclerosis with dalfampridine

“…4 Results. We used interactive programs written in MATLAB (The MathWorks, Inc., Natick, MA) to identify saccades using a 20 deg/s velocity threshold.…”