An individual with normal vision fixates an object or stimulus binocularly and visually responds to it as a single entity. Early in the history of visual science, it was assumed that the visual axes of a person intersect precisely at the point of regard of the viewed object or stimulus to result in a classic stimulation of corresponding retinal elements. For some time, however, investigators have known that the visual axes may be over or under converged and experimentation and observation has been carried out from as early as 1900 to detect the presence of this fixation disparity. In connection with these investigations, reference is made to the following publication:
I. Ogle, K. N., Researches in Binocular Vision, Hafner, New York, 1972. PA1 II. Mallett, R. F. J., A Fixation Disparity Test for Distance Use. The Optician, 1523927,1, July 8, 1966. PA1 III. Mallett, R. F. J., The Investigation of Heterphoria at Near and a New Fixation Disparity Technique, The Optician, 148, 3844-3845, December 1963. PA1 IV. Ogle, K. N., T. G. Martens, and J. A. Dyer, Oculomotor Imbalance in Binocular Vision and Fixation Disparity, Philadelphia, Lea and Febiger, 1967. PA1 V. Hebbard, F., Foveal fixation disparity measurements and their use in determining the relationship between accommadative convergence and accommodation, Am. J. Optom. Arch. Am. Acad. Optom., 37(1): 3-26, 1960.
Within certain small limits of such variations or misalignments, a sensory fusion of the monocular images as preceived by each eye still will occur. This small amount of misalignment which is physiologicallly allowed is due to the existence of Panum's fusional area and the angular subtense of the misalignment generally is described as a fixation disparity. Under the phenomenon, the image of the fixation point stimulates disparate retinal elements without diplopia resulting.
Past experimental techniques investigating fixation disparity have looked to an evaluation under both distance and near vision conditions, inasmuch as the oculomotor balance changes for stimuli situated at varying distances from the eye. Distant-vision measurments, for example at about 2.5 meters from a subject's eyes, involved the utilization of a lantern projected central fusable square of about 1.5.degree. visual angle within which short vertical bright lines were located. These lines or arrows were polarized, vertically displaced with respect to each other and movable horizontally at the square to define a variable mutual horizontal displacement. The subject being tested, perceiving the target through a phorometer to which had been attached a pair of Polaroid filters of mutually opposed polarization, observed the arrows at the target which were projected with complementary polarization, so that when viewed by the subject, one line would be seen by one eye and the other with the other eye. By mechanically adjusting the lantern, the displacement of the two arrows was read at that location at which the test subject indicated that the lines appeared to be vertically aligned. The actual physical separation of the lines represented the value of fixation disparity. Test instrumentation for near vision evaluation involved a similar geometry, however a mechanical device provided the two vertical lines, one movable with respect to the other by a mechanical adjustment. For further data concerning such prior test arrangements, reference is made to Publication I above.
Devices generally available in the marketplace for evaluating fixation disparity similarly utilize a polarization technique for effecting the visual isolation of each eye with respect to aligned targets. However, the test mechanisms do not provide an indication of fixation disparity, the test results only indicating the presence or absence of the phenomenon. For a more detailed discourse concerning such instrumentation, reference is made to the following publications:
Where an effective and precise evaluation of the amount of disparity is made available, the resultant data can be much more useful to the clinician in a diagnosis of binocular imbalance in the oculomotor system than an indication merely representing the presence or absence of the phenomenon. For fully effective diagnosis, the fixation disparity data must be of sufficient detail with respect to the precise angular extent of misalignment so as to be capable of combination with other clinical indicators or variables of binocular oculomotor balance. For example, an important such combination provides for the development of forced-vergence-fixation-disparity curves. See in this regard:
As indicated above, presently available devices for analyzing fixation disparity do not develop data suited for full clinical diagnosis, the subject being tested merely observing whether line images perceived in isolation by each eye appear aligned or not. Should the lines appear misaligned to the patient, then ophthalmic prisms are interposed in front of the eyes until the fixation disparity is eliminated. The measurement recorded by this current test is the amount of prism necessary to eliminate the fixation disparity.
Among the additional of the above-mentioned clinical indicators evaluated in the diagnosis of oculomoter balance are heterophoria and vergence. Heterophoria may be evaluated by the Von Graefe method of vertical disassociation. With this method, a target is viewed through a phorometer. With the arrangement, the eyes are disassociated vertically utilizing ophthalmic prisms whereupon double vision is observed. Following this initial procedure, lateral ophthalmic prisms are interposed in the phorometer in incremental amounts until the vertically displaced images appear to be vertically aligned. The resultant evaluation provides data concerning the position of the eyes when there is no stimulus for fusion. Vergence testing is carried out in similar fashion, however, no vertical disassociation is provided, convergent and divergent eye movements elicited by horizontal ophthalmic prisms in small increments to the points of image blur and image break or diplopia, the amount of prism inserted at each point being recorded. Vertical heterophorias and vergences can be similarly measured by changing the prism orientations by 90 degrees.
While the information made available from tests for phoria and vergence remain as valuable diagnostic inputs, considerable value would be promised with a provision of accurate fixation disparity data, in the analysis of oculomotor imbalances. The test instrumentation for carrying out the earlier experimentation required very fine movements of the target lines through the use of micrometers and the like, and, thus, has not found practical introduction into the clinical market as a practical testing procedure.
From the foregoing it may be observed that a need exists for a test method and apparatus which achieves an accurate measurement of the degree of misalignment evoked by a patient's fixation disparity. However, before such apparatus can be made available on a somewhat universal clinical basis, it must be fabricable as well as operable at relatively low cost.