Our visual environment contains both stationary and moving objects. The oculomotor system allows us to rapidly shift the visual axis between different positions by means of saccades and to follow moving objects by means of smooth pursuit eye movements. The brainstem pathways subserving these different kinds of eye movements were thought to be relatively independent. In particular, a group of midline cells in the pons called omnipause neurons (OPNs) were considered to be part of only the saccadic system. Recently, we have described activity modulation of these brainstem premotor neurons during both kinds of eye movements. The results suggest that the omnipause neurons form a common inhibitory mechanism that maintains fixation and that must be released before either saccades or smooth pursuit can be made. The quantitative details of the modulation in OPNs are somewhat different for the two oculomotor behaviors just as the dynamic function of the two are different. Current models of both systems will have to be revised as a result of our new findings concerning these premotor brainstem neurons. The figure below shows the discharge of a typical OPN during a series of saccadic movements on the left and during a series of pursuit movements on the right. Note that the neuron is completely inhibited for saccades, but only partially inhibited for pursuit.

In a second study we attempted to find the cells that provide the inhibitory signal that turns down the activity of OPNs during pursuit eye movements. We have recorded from a particular type of long lead burst neuron in the nearby pontine reticular formation that seems to have the appropriate behavior to be the potential source of inhibition to OPNs. We hypothesize that these saccade/pursuit neurons are the "latch neurons" found in many system models of the saccadic system.