Year : 1972 | Volume
: 20 | Issue : 4 | Page : 149--152
Voluntary convergent squint
S A Gokhale
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Gokhale S A. Voluntary convergent squint.Indian J Ophthalmol 1972;20:149-152
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Gokhale S A. Voluntary convergent squint. Indian J Ophthalmol [serial online] 1972 [cited 2021 Apr 11 ];20:149-152
Available from: https://www.ijo.in/text.asp?1972/20/4/149/34647
According to LYLE AND WYBAR  , the term purposive squint is applied to a convergent squint which may be produced voluntarily in persons with normal binocular function, and it usually represents merely the exercise of an excessive degree of voluntary convergence so that both eyes develop an equal degree of convergent deviation. DUKE-ELDER  has similarly stated that the faculty o producing an internal strabismum is not uncommon by an act of voluntary convergence affecting always both eyes.
We were surprised and inspired to study the subject when we saw an actress showing a convergent squint whenever she wanted by moving only one eye inwards, the other eye looking straight ahead. With some practice, every normal person may be able to imitate a convergent squint by moving only one eye, while the other eye keeps on looking straight ahead.
The method of practicing is shown in [Figure 1],[Figure 2],[Figure 3],[Figure 4]. With both eyes open, the gaze is directed to a distant object (A in [Figure 1]). Now [Figure 2] the left eye is covered and another small transparent object (B) is held in front of the right eye so that it falls exactly in the visual axis of that eye, which is looking at the object A. Now [Figure 3] the left eye is uncovered and the gaze is fixed at A. The gaze is now rapidly changed and directed towards B. [Figure 4] The right eye momentarily moves but again assumes the previous position. The left eye markedly turns to the right. If the gaze is alternatively directed towards A and B, it will appear that only the left eye is moving. After practicing like this several times the object B may be removed and its position may be imagined, so that the eyes are alternately directed towards A and towards the point where B was situated.
It would appear that Hering's law does not apply in these circumstances. Several experiments have been done to prove that it does apply. The law states that in all voluntary movements of the eyes an equal and simultaneous innervation flows from the oculogyric centres to the muscles of both eyes concerned in the desired direction of gaze.
After doing electromyographic studies, WESTHEIMER AND MITCHELL (the movement occurs as shown in [Figure 5],[Figure 6],[Figure 7] state that there is first a saccadic movement of the two eyes to a new direction of conjugate lateral gaze such that the bisector of the angle of convergence actually includes the new fixation point. A convergence movement then takes place, bringing the lines of sight to intersect in the new fixation point. The line of sight of the right eye first swings out in unison with that of the other eye and then is brought in again during the response to binocular convergence. It finally assumes the same direction it had at the outset. The two components of the adductive movement of the other eye, are clearly distinguishable by their different characteristics of time. Similar findings are reported by TAMLER, JAMPOLSKI AND MARG. 
BREININ  , on the other hand, did not find the two separate innervations electromyographically, and the movement during asymmetric convergence was confined to one eye. He stated that, in the stationary eye, for each unit of increased excitation of the medial rectus induced by convergence there is opposed an equal inhibition induced by the version. For each unit of inhibition of the lateral rectus induced by convergence there is opposed an equal excitation induced by the version. The resultant is the exact level of innervation for both muscles that existed before.
ALPERN AND WOLTER  agree with WESTHEIMER AND MITCHELL  that both eyes move during asymmetric convergence, and their record shows this movement.
ALPERN AND ELLEN  found no movement of the eye on the side of the fixation object when the other eye was occluded, but, both eyes showed movement similar to Westheimer's findings when fixation was carried out binocularly. This can be explained by considering the pattern of fixation. When both eyes are open, the object B is seen by the temporal retina of the left eye. So there is a version movement with Hering's law. Then the image of B falls on the temporal retina of both eyes, hence there is convergence, with the result that the right eye assumes the previous position.
Another view is that Hering's law applies only to voluntary movements and not to reflex movements, in this case a fusional convergence.
It is to be noted that in voluntary convergent squint, the eyes are in a state of accommodation for near vision, and that the pupils are constricted. If a person with voluntary convergent squint wants to concentrate on a distant object, he has to correct his voluntary deviation.
With some practice, every normal person may be able to show a voluntary convergent squint whenever desired, by moving only one eye at a time. The method of practicing is simple. There is some difference of opinion regarding the application of Hering's law to this movement.
|1||Alpern, M. and Elisn, P.: A Quantitative Analysis of the Horizontal Movements of the Eyes in the Experiments of Johannes Mueller, Am. J. Ophth., 42: 289 (1956).|
|2||Alpern, M. and Wolter, J.: `The Relation of Horizontal Saccadic and Vergence Movements', A. M. A. Arch. Ophth. 56: 685, (1956).|
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