LAUSR

Previous studies have revealed unexpected relationships between the firing rates of horizontally-acting motoneurons and vergence. During a vergence task, for example, antidromically-identified abducens internuclear neurons show a negative correlation between vergence angle and firing rate, which is the opposite of the modulation displayed by the medial rectus motoneurons to which they project. For a given horizontal eye position, medial rectus motoneurons discharge at a higher rate if the eyes are converged than if the same eye position is reached during a task that requires version; paradoxically, however, the horizontal rectus eye muscles show co-relaxation during vergence. These complex and unexpected relationships inspired the present author to investigate whether the tonic firing rates of vertically-acting motoneurons in oculomotor nucleus are correlated with vergence angle. Monkeys were trained to fixate a single, randomly selected, visual target amongst an array of 60 red plus-shaped LEDs, arranged at 12 different distances in 3-dimensional space. The targets were arranged to permit dissociation of vertical eye position and vergence angle. Here I report, for the first time, that most vertically-acting motoneurons in oculomotor nucleus show a significant negative correlation between tonic firing rate and vergence angle. This suggests the possibility that there may be a general co-relaxation of extraocular muscles during vergence.