The Basics: Vergence is the simultaneous and opposite movement of both eyes to maintain binocular focus on objects. Convergence refers to eye movements toward both each other and the nose, and is used heavily for objects nearby. Divergenceis movement of the eyes away from one another, toward the goal of looking straight ahead, typically for objects off in the distance. Convergence vision has generally been clear for objects up to 10 centimeters from the nose, though this has recently been changing, with vergence accurate to even closer objects. Such improvement has been attributed to the small, closely-held screens found on smart phones, hand-held gaming devices, and tablets.



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Mechanisms


It is achieved through both the occulo-motor nerve (III) and the abducens nerve (VI). Malfunctions in nerve VI result in double vision (diplopia), since without troclear input, the eyes will pull toward the midline in a cross-eyed effect. This is most common in young children, in whom the vergence machanisms are not yet complete.
Both convergence and divergence have pathways based within the deep cerebellar nuclei and nucleus reticularis tegmenti pontis. Underlying the translation of sensorimotor information are pathways within the prearcuate region 8a in the Frontal Eye Feilds. Anterior and adjacent to Saccadic neurons (in the anterior portion of the arcuate sulcus) are a group of Saccadic Vergence Neurons, which trigger saccadic responses to vergence cues. However, saccadic cues will not trigger vergence responses and vergence responses that also trigger saccades or smooth pursuit do not otherwise differ from solely vergence responses. (Without the context of conjugate firing, one would not note the difference between solely vergence activity, and complex vergence/SP/Saccade activity). This suggests that vergence is more the response of motor activity than disparity on a retinal display. Sure enough, monocular testing confirmed this, with vergence being more of a motor activity than a retinal display reaction. Monocular preferring patients performed vergence in concert with a series of small saccades. Normal binoccular patients performed vergence without heavy saccadic input, particularly on a horizontal plane.
Smooth pursuit differs from saccades in that a portion of smooth pursuit neurons (9%-17% of them) respond only to pursuit and vergence linked tasks.
In the Midbrain, exclusively vergence neurons (no saccadic/pursuit implications) exist in the mesencephalic reticular formation, dorsolateral to the oculomotor nucleus.

Within vergence cells, three subtypes are identified, vergence tonic cells (positioning encoding cells), vergence burst cells (velocity encoding cells), and burst-tonic cells (phasic response units).


Vergence movements compared to Saccadic movements


Much of the activation between the two is overlapped, with peak activation occurring in the superior frontal sulcus (FEF), medial frontal gyrus (SEF) dorsolateral PFC, ventrolateral PFC, intraparietal sulcus (Brodmaan's area 40), the cuneus, pre cuneas, anterior/posterior singulates, and cerebellar vermis. However, within the FEF, vergence activity is higher anterior to saccadic activity. Within the midbrain, Vergence requires greater activation than does saccadic activity, which may help to explain it's slower time to completion.

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Speed


Vergence is notoriously slow, achieved at sluggish speeds of 20 degrees/second, as compared to 100 degrees per second for methods of smooth pursuit and up to 700 degrees/second for saccadic movements. Vergence (especially horizontal vergence) can be sped up by being performed in concert with saccades. This process also slows down saccadic movements, in proportion to the angle of vergence, but disregarding direction of conjugate gaze. Speed can be increased by the greatest degree when the saccade/vergence concert is purely horizontal, and decreases in speed as the movement becomes more vertical.
"Convergence burst cells display a discrete burst of activity just before and during convergence eye movements. For most of these cells, the profile of the burst is correlated with instantaneous vergence velocity and the number of spikes in the burst is correlated with the size of the vergence movement." (Mays, Gemlin, Porter)
Tonic firing rate (in addition to phasic firing rate) is positively correlated to conjugate angle for convergence gaze, but has not been observed in disconjigate gaze mechanisms, likely because disconjugate cells are much less abundant than their convergence counterparts. As such, they are more difficult to study. Velocity vergence cells can be found in mesencephalic reticular formation adjacent to and dorsolateral to the oculomotor nucleu, and convergence burst cells were also found in another more dorsal mesencephalic region, rostral to the superior colliculus. These burst cells largely define vergence speed. In this way, the vergence system mirrors the setup of the saccadic one.

Studies comparing speeds of convergence against those of divergence have been contradictory. The variability of latency periods and overall speeds have fluctuated by 20ms depending on the study, which makes the difference between defining convergence of divergence as the speed limiting step. (One study defined the difference as slight as 49 deg/sec for convergence and 45 deg/sec for divergence) However, the phasic accepted proximal (defined as <40cm from face) convergence and divergence speeds are 69 and 53 degrees per second, respectively. Speeds are substantially slower for distance viewing, but divergence velocities far outspeed convergence ones. Convergence and divergence at 7 and 22 degrees per second, respectively.

Development


Development of vergence is slow: at 12 years of age, children still perform the task slower and with greater variability than their adult counterparts. (before 2 months of age, conjugate sensitivity is almost nonexistent) Tonic firing of vergence cells is also less significant the younger the child is, so phasic firing must hold a more important role. Overshooting and undershooting in this way are expressions of phasic variance, not tonic variance. Inducing tonic firing can mitigate against variability in vergence tasks. Often, due to weakness of the trochlear nerve, nasal bias dominates and children over abduct their gaze and the image blurrs (especially at close range). (This has been measured by tracking eye activity in the dark: once lights are shut off, gaze drifts nasalward in infants.) The cross eyed effect can be counteracted by tilting the head to the side, so the object of interest is to either the extreme left or right, and only one eye must adduct. Vergence demand in children is less than it is in adults die to decreased interpupillary distance; demand grows along with the child and settles in adolescence.


The Vergence Accommodation Conflict


3-D viewing often results in nausea, blurring, and headaches, since the visual display is often compelling enough to convince one into percieving depth, but not quite accurate enough to be true to reality. Such symptoms fade often within minutes for adults, but may linger for hours, especially for children. This results from a disparity between the distance of the display and the distance of the objects within the display, which typically require greater greater and faster flexibility that is typically produced. Such effects are greatly enhanced if the viewer of 3D films tilt their heads to the side, forcing vergence mechanisms to perform on a vertical plane. After leaving the screen, the real world can be severely disorienting, and exhausted young eyes may overreact to subtle depth changes, resulting in an almost vertigo-like experience. Studies exploring potential damaging developmental effects have shown no such danger, however. Even when young rhesus monkeys were kept solely viewing artificial 3-D for 3 months, their ocular development was not compromised.

Quiz (For answers scroll to bottom):


(ALL ARE TRUE FALSE)
1. Vergence mechanisms can exist independently of Saccadic/Pursuit mechanisms.
2. Vergence accuracy increases throughout the lifespan
3. Velocity of vergence is determined by specialized burst cells.
4. The pull of nasalward gaze is stronger than occular adduction.
5. Vergence can be performed at peak saccadic speeds if performed in concert with saccades.
6. Disconjugate gaze is more neuronally established than conjugate gaze
7. Speeds of vergence are greatest when moving across a horizontal plane.





References

Alkan, Y. Biswal, B. Alvarez, T. Differentiation Between Vergence and Saccadic Function Activity Within the Human Frontal Eye Fields and MidBrain Revealed Through fMRI

Babinsky, E. Candy, T. Is Tonic Firing Protective Against Stabismus during Development?

Gamlin, P. Neural Mechanisms for the Control of Vergence Eye Movements. 24 Jan 2006. Neurobiology of Eye Movements: From Molecules to Behavior

Gonzalez, M. Perez, R. Neural Mechanisms Underlying Stereoscopic Vision. 1998 Progress in Neurobiology

Hoffman, D. Ghirshick, A. Ackeley, K. Banks, M. Vergence Accomodation Conflicts Hinder Visual Performance and Cause Visual Fatigue. 2008

Lorenz, B. Brodsky, M. Pediatric Ophthamology, Neuro-ophthamology, Genetics 2010 (fee e-textbook!)

Mays, L., and Gamlin, P. A Neural Mechanism Subserving Saccade-Vergence Interactions. 1995 University of Alabama at Birmingham

Mays, L. Porter, J., Gamlin, P. Tello, C. Neural Control of Vergence Eye Movements: Neurons Encoding Vergence Velocity 1986 JN Physiology.

Wick, B. Bedell, H. Magnitude and Velocity of Proximal Vergence 1989 Investigative Ophthamology and Visual Science

Ven Leeuwen, AF, Collejewn, H. Erkelens, CJ. Dynamics of horizontal vergence movements: interaction with horizontal and vertical saccades and relation with monocular preferences. 1998 Erasmus University, The Netherlands.

Yang, Q. Bucci, M. Zappoula, Z. The Latency of Saccades, Vergence, and Combined Eye Movements in Children and in Adults
<http://www.iovs.org/content/43/9/2939.long>

Quiz Answers: T, F, T, T, F, F, T