The postural eye
by René GENTAZ
Posturology service of the APAS surgical center, Paris
This study aims at showing that both eyes do not have the same value in the control of the postural tonic activity in the standing position. In the normal subject, one eye is more performant than the other - we call it the "postural eye". In order to understand the interest of that postural laterality, it is essential to remind than man stands up thanks to the data given by three sensors: the eye, the vestibule and the foot. The sensors' messages are transmitted to a control which sends the motor system the impulses that accordingly adapt the tonicity of the erector muscles.
The eye that is under discussion here gives double information: through sight on one hand, and through the oculomotor system on the other hand. We have tried to reduce the role of that second system to its absolute minimum, in order to focus our study on the sight element. It is essential - the mere fact of closing one's eyes diminishes the accuracy of the postural system in a very significant way (200 to 300%). The visual mechanism can be schematized as follows: a standing man continually sways in all directions and the image of a vertical displaces itself during all those movements: that shift of the image on the retina is provoked at the subject's slightest displacement, and it is on the basis of the retinal data that the equilibrium system constantly adapts the body axis to the vertical of the place (fig. 1).
FIG. 1 - Diagram of the oculo-postural system
MATERIAL AND METHODS
The tested population is composed by 60 men and women in full professional activity. The mean age is of 30 years, the women form the greatest part (hospital nurses). Simple ocular tests compelled us to eliminate from that series about 20 persons that showed important refraction or oculomotricity disorders.
The subject, barefoot, is set standing on the statokinesimetry platform and is invited to stare at a vertical, in fact a simple plumb line. One of the eyes is masked and the vertical appears strongly lighted during 4 seconds. A recording comprises six light stimulations. Each stimulation is followed by a seven-second rest interval in darkness, which helps attenuate the persistent image of the vertical. The subject's displacements are recorded and analyzed. The mean recording appears under the form of two curves: one represents the right-left movements (R-L) and the other the forward-backward movements (f-b). The area of each curve is the chosen parameter. That area is representative of the energy spent by the subject during the stimulations. It is expressed in conventional units (CU).
We also searched for links with other lateralities: manual, podal or ocular. For the ocular laterality, we only looked for the "aiming" eye, i.e. the preferential eye, spontaneously chosen during the use of monocular optical instruments.
The oculo-postural system, stimulated by the vertical visible to one eye, checks the lateral sway more than the sagittal movements. We only use the numbers of the R-L displacements to evaluate the postural activity according to the open eye. A mere look at those recordings reveals a lot and is enough to tell immediately which eye achieves the best performance, i. e. the eye that induces the subject's smallest displacements or the least energy expenditure during the light stimulations (fig. 2).
FIG. 2 - Curves of the lateral displacements recorded on a normalised platform and averaged on six stimulations.
Above: mean area of a subject's sway with his postural eye open and the other closed (A = 0.408 conventional units (CU)).
Below: mean area of the displacements of the same subject, with his postural eye closed and the other one open (A = 0.846 CU).
For the 40 subjects, the mean value of the areas of the postural eye is of 377.1 ± 289 mm˛, and that of the nonpostural eye of 747.7 ± 347 mm˛. In the paired Student's test, that difference is highly significant: p<0.0005.
The postural eye is on the right side in 55% of the cases (and therefore on the left side in 45%). The comparison with the manual or podal lateralities shows that there is no link between the side of the postural eye and the other lateralities. The results appear identical whatever the normal subject's sex, size, weight, age or ethnic group.
The term "postural eye" needs clarification: both eyes are only mere sensors whose functions are identical in the normal subject. What differenciates them is the oculo-postural system that appears more performant on one side. That laterality is similar to that of the vestibulospinal system, also lateralised. The interconnections between those two systems are well known to make one big oculo-vestibulo-postural system.
That "postural eye" truly corresponds to a reality: the same results can be reproduced at any moment in the same subject on condition that we keep an identical examination protocol, but it is sometimes difficult to avoid all the technical traps. For instance, if we mask an eye thanks to a mask held at some distance of the subject, the peripheral sight of the masked eye remains active and interferes in the control of the postural system. Oculomotricity represents another trap: experience teaches us that it is impossible to keep the eye constantly staring at that target - when the light stimulation occurs, the eye searches for the vertical and more or less explores the close environment of the target. We have not managed to eliminate that "palpation of the look" - therefore, oculomotricity is always more or less present in that study.
Besides, we have tried to sensitize the method by setting an elastic cushion under the feet of a few subjects. The results obtained are very different and seem to be linked to other factors than sight: that technique studies another phenomenon because the postural system uses, in that case, a different strategy to maintain the subject's equilibrium.
The parallel study of the twenty subjects that showed various ocular disorders brings elements that can be used in current ophtalmological practice. We have equipped the subjects showing convergence disorders with prisms - those prisms were more active when set in front of the postural eye. The knowledge of that laterality therefore seems essential to set up a prismatic prosthesis, but it should also be included in the preoperative ophtalmological check-up of the squinters.
Finally, two subjects who showed an almost nil macular sight on one side, kept the postural eye on that same side, which confirms the importance of peripheral sight for posture. We can also deduce from it that the postural eye is part of our genotype, that it does not depend on an acquisition as the manual laterality sometimes does.
That work was presented during the "Sight and Posture - Paris 26-27 of February, 1988" meeting and published in the review "Agressologie" 1988, 29, 10: 685-686. It is reproduced on the Internet with the authorization of SPEI médical, Paris.
Gagey P.M. (1973) Examen clinique et posturographie. Revue de Médecine du Travail, 2: 237-241.