Internuclear ophthalmoplegia: recovery and plasticity.

We studied refixational eye movements of a patient during the gradual resolution of an internuclear ophthalmoplegia (secondary to head trauma) in an attempt to determine the relative contributions of both medial longitudinal fasciculus (MLF) recovery and secondary central plastic changes. Adduction-refixational eye movements in the affected eye consisted of an initial fast (saccadic) portion followed by a slow drift toward the new intended eye position. The fast and slow components of the movements reflected, respectively, the pulse and step increases in neural innervation. Shortly after the traumatic insult, the affected eye exhibited low adduction gain (pulse gain 0.34; step gain 0.37) and slow saccades with peak velocities of 55% and durations of 278%, normalized for achieved, rather than intended, amplitudes. Several months later the pulse and step gains, peak velocities, and durations of the saccades improved to 0.81, 0.92, 87%, and 145%, respectively. The increased gains and faster velocity were accomplished by increases in the firing frequency of the pulse and step, reflecting recovery of MLF axons, rather than saccadic system plasticity, which would have resulted in increased duration of the saccadic pulse.