A new population of neurons has identified that control binocular eye movements in 3D space

Researchers at the University of Alabama in Birmingham have discovered a previously undescribed population of neurons that help control our eyes as they look into three-dimensional space.

During normal vision, we direct our eyes between objects in three-dimensional space many times per minute. With each change, the left and right eyes rotate, generally in the same direction, but mostly with different degrees of rotation. These unequal movements are known as disjunctive saccades.

Disjunctive saccades differ from two other eye movements: one, called conjugate saccades, in which the eyes rotate in unison, and one called symmetrical vergence eye movements, in which the eyes rotate in equal but opposite directions. The mechanism behind the disjunctive saccades is not known.

Different eye movement models have predicted the existence of a population of neurons called burst saccade-vergence neurons, or SVBNs, which would produce an explosion of activity exclusively during the disjunctive saccades, while not activating during the other two types of movements. eyepieces.

UAB researchers, led by Julie Quinet, Ph.D., looked for these putative neurons in a region of the midbrain located near the oculomotor nucleus called the central mesencephalic reticular formation or cMRF. Recent anatomical studies had suggested that cMRF may contain premotor neurons involved in the neural control of disjunctive saccades.

Using brain recordings from trained rhesus monkeys, Quinet and colleagues found and recorded 18 SVBNs in the cMRF. “As far as we know,” said Quinet, researcher V in the UAB’s Department of Ophthalmology and Visual Sciences, “no cell class of this type has been reported in previous registration studies.”

This new SVBN population exhibited three unique features that had been predicted by the models: 1) neurons fired when animals performed a disjunctive saccade; 2) Neurons remained silent during unison eye movement called conjugate saccades and also during eye movement when eyes rotate in equal but opposite directions, called symmetrical vergence eye movement, and; 3) Neurons burst regardless of the direction – right or left – of the disjunctive saccade. Furthermore, the bursts of spikes during the disjunctive saccades were highly correlated with the vergence velocity, the rate at which the eyes approach or move away from each other.

Curiously, half of the recorded cells increased the discharge rate for the convergence disjunctive saccades, while half increased the firing rate for the divergence disjunctive saccades.

Quinet and colleagues say that further studies of disjunctive saccades in areas of the brain that can provide input to SVBNs can help explain and advance solutions for treating strabismus, a condition in which the eyes do not align properly with each other while they look at an object.

Findings from this study and previous studies elsewhere and at the UAB, Quinet says, suggest that SVBNs could play a role in all components of the neighboring triad responses: lens accommodation, pupillary constriction, and vergence.

Co-authors with Quinet in the study, “Neural control of rapid binocular eye movements: Saccade-vergence burst neurons”, published in Proceedings of the National Academy of Sciences, are Kevin Schultz and Paul D. Gamlin, UAB Department of Ophthalmology and Visual Sciences; and Paul J. May, Department of Neurobiology and Anatomical Sciences, University of Mississippi Medical Center, Jackson, Mississippi.