Neurophysiology of Disjunctive Eye Movements

Brainstem reticular neuron encodes monocular eye movements

Although both eyes often move together, they may also move differentially as when one attempts to look at something near (e.g., one's nose). Such movements are disjunctive. Recent work in the King Laboratory and elsewhere suggest that each eye may be independently controlled to a greater extent than once thought. For example, many neurons in the brainstem reticular formation encode signals specifically related to the movement of one or the other eye (monocular eye movement). Furthermore, neurons that encode monocular eye movements are common in premotor structures such as the reticular formation, medulla, vestibular nuclei and cerebellum. Surprisingly, however, monocular eye movement commands related to both eyes converge on individual ocular motoneurons that innervate muscles in only one eye. The implications of this neural organization for eye movement control are an active research interest of the laboratory.

  1. Conjugate saccade. Left (green trace) and right (red trace) eye position shifts rapidly to the left (10 deg saccadic eye movement)
  2. Monocular saccade. Left eye position is relatively constant whilst right eye position moves about 10 deg to the left. Conjugate eye position (gray trace) is the average of left and right eye position
  3. Disjunctive convergent eye movement. Left eye rotates rightward and right eye rotates leftward. Conjugate gaze shift to the right

Each record is a superposition of 5 trials and each trace shows eye position as a function of time. Beneath the traces are rasters displaying the action potentials of a reticular neuron recorded simultaneously with the eye movements. Each tick in the raster marks the occurrence of an action potential and each line of a raster corresponds to an eye movement trial. This neuron only fires during saccades when the eyes move to the left. The number of spikes emitted is proportional to the amplitude of the eye movement and the interspike frequency is related to the speed of the eye movement. Note that in B, the neuron emits about the same number of spikes as in A even though the conjugate component of the eye movement is much smaller. In C, the neuron continues to discharge even though the conjugate component of the eye movement is directed to the right. We conclude from analyses of data such as these, that this and similar neurons encode monocular eye movement rather than conjugate eye movement.