|Year : 1997 | Volume
| Issue : 3 | Page : 169-171
Randot stereoacuity at various binocular combinations of snellen acuity
V Menon, A Bansal, P Prakash
Dr. Rajendra Prasad Centre for Ophthalmic Sciences, AIIMS, Ansari Nagar, New Delhi, India
Dr. Rajendra Prasad Centre for Ophthalmic Sciences, AIIMS, Ansari Nagar, New Delhi
Source of Support: None, Conflict of Interest: None
Binocular combinations of Snellen acuity were varied from 6/60 to 6/6 in standard steps in twenty normal adults and Randot stereoacuity was measured at each level. Using the 560 unique stereoacuity scores thus obtained, a nomogram was constructed which provides expected stereoacuity scores at all unique binocular combinations of Snellen acuity. It was seen that there is a linear correlation between binocular isoacuity (at level 6/36 or better) and Randot stereoacuity.
Keywords: Randot stereoacuity, Snellen acuity, binocular isoacuity, nomogram
|How to cite this article:|
Menon V, Bansal A, Prakash P. Randot stereoacuity at various binocular combinations of snellen acuity. Indian J Ophthalmol 1997;45:169-71
Various disease processes are known to affect stereoacuity (SA). Whether this impairement of SA is secondary only to Snellen visual acuity (VA) impairment or is dependent on other effects of the disease process, can be known only by compairing the effect of the disease process on SA and that of similar amount of VA imapirment without altering other visual functions. For this comparison it is necessary to know the expected SA for all binocular combinations of VA in otherwise normal eyes.
Donzis et al have studied the effect of binocular variation of VA and SA using the Titmus Stereotest (TST). However, TST is known to be capable of indicating artifactual stereocapability. Donzis et al in his study has shown that SA improves with uniocular improvement of VA (despite the fact that VA disparity between the two eye is increasing and as such should worsen the SA). This clearly indicates the TST allows significant monocular cues.
The present study determined the variation of SA with binocular VA using Randot test which has been shown to be a more reliable test of stereoscopic vision. Thus, it would provide more accurate data.
| Materials and Methods|| |
Twenty healthy adult subjects with an average age of 29.1 years (range 14-51 years) were studied. Fourteen subjects were males and six females.
Only patients with VA correctable to 6/6 or better in both eyes and with low refractive errors (between -3.0 D and +3.0 D) without significant anisometropia (±0.75 D) were included. None of the patients had history of ocular disease or systemic disease likely to affect visual functions.
Detailed ophthalmic examination including VA for distance (6 m) and near (0.33 m), without and with correction, refraction under cycloplegia followed by a post mydriatic test, and a detailed fundus examination to rule out any abnormalities, were done.
Stereoacuity testing was done using Randot test circles which provided a range of 20-400 seconds of arc. The response was considered positive if the subject could identify the circle depth cues with respect of other circles. Testing was done under standard conditions in all subjects. SA was measured at 0.4 m from root of the nose. All subjects were tested in similar light conditions. Lighting was provided by ceiling fluorescent light (40 W) and an incandescent lamp (100 W) over the subjects' left shoulder and at a distance of 1 m from the target. All patients were made to understand the test completely and none of them had difficulty with test compliance. All SA testing was done by the same observer (author-AB).
The subjects was given the best correction for distance (6 m). One eye was occluded and a +6.0 D lens was added to the best correction for distance to blur the vision. The power of additional convex lens was reduced in 0.25 D steps and lenses required for each standard line of distance VA was noted. This procedure was repeated in the other eye. To allow the same level of VA at 0.4 m a +2.50 D addition was given. It was confirmed in all patients with Snellen chart for near that VA at 0.4 m was Snellen equivalent of VA at 6m after addition of +2.5 D lens.
One eye selected randomly was corrected to equivalent of 6/60 at 0.4 m by additional convex lenses, and the fellow eye was improved one line at a time from VA equivalent from 6/60 to 6/6 at 0.4 m. The Randot response was noted at each combination of binocular VA. Following this the VA at 0.4 m in the randomly selected eye was improved one line at a time while sequentially improving the VA of fellow eye at each step. Randot scores were recorded for each combination of binocular VA from 6/60 or 6/6 so that a total of 28 Randot scores were recorded for each subject.
| Results|| |
The 560 unique Randot scores thus obtained (28 in each of the 20 subjects) were used to calculate mean and standard deviation of Randot SA at each binocular combination of VA as shown in [Table - 1].
A nomogram [Figure - 1] was constructed from mean Randot scores. The Isostereoacuity points were plotted by a method of linear interpolation and joined to give Isostereoacuity curves which demarcated the Isostereoacuity zones. Each point in the nomogram corresponds to a unique combination of binocular VA between 6/60 and 6/6 and the Randot score isozone into which it falls represents the mean expected score for normal subjects.
A simplified version of this nomogram [Table - 2] allows easy determination of minimum expected SA at commonly used VA levels. This is determined by considering the lower limit of the isostereoacuity zone as normal for a given visual acuity combination.
| Discussion|| |
This study has shown that SA improves with improvement o binocular isoacuity as also reported by Donzis et al. However, this present study demonstrates a near linear relation between binocular isoacuity (6/36 or better) and SA [Figure - 2], whereas Donzis et al found a logarithmic relationship. One of the reasons for this difference in the two studies could be because of a different type of test for SA used by Donzis et al, namely Titmus stereotest which because of presence of inherent defect of having monocular cues may have indicated a stereoperformance more than actual, thus leading to a different type of curve.
According to our study SA depends on the VA of the worse eye and improvement of VA in the better eye to a level more than that of the worse eye VA does not improve SA. This is because if one eye is having poor VA it cannot coordinate effectively with other eye howsoever good the VA of that eye may be. In fact improving the better eye VA increases the image disparity between the two eyes which is a known hindrance to effective binocular coordination. However, the observations in this study assume improtance because previous work by Donzis et al had found dissimiliar results. In their study the mean SA improves with improvement of better eye VA despite the increase in difference of VA of the two eyes. This disparity may again be because of using TST which is known to indicate artifactual stereocapability when none actually exists. Thus the present study also demonstrates that Randot test is relatively free of monocular cues as stereoacuity is not improved by improving vision of one eye.
The nomogram [Figure - 1] provides a reliable and ready reference for expected SA at any binocular combination of VA and thus can be used for studying actual effect of any ocular pathology on SA independent of its effect on VA.
| References|| |
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[Figure - 1], [Figure - 2]
[Table - 1], [Table - 2]