T. C. A. Jenkins

Binocular vision, age and symptoms

Changes with age in heterophoria, associated heterophoria, fixation disparity and stereopsis for near vision were investigated in a sample of 187 subjects (age range from 10 to 65 years) divided into six age groups. With increasing age, heterophoria, associated heterophoria and fixation disparity increased in the exo‐direction. No change in stereopsis was found.

Criteria for decompensation in binocular vision

Two groups of patients, one under 40 years of age and the other 40 years and over, were divided into subgroups according to whether they had symptoms associated with near vision. A method developed in ‘signal detection theory’ was used to determine whether a value could be found for heterophoria, or associated heterophoria, which would predict which patients had symptoms due to decompensated heterophoria in normal routine investigation. Receiver operator characteristic curves (ROC curves) were constructed to look for these cut‐off values between patients with, and without, symptoms.

Binocular summation in visually evoked responses and visual acuity

Monocular and binocular transient visually evoked responses (VER) were recorded on 50 adult subjects using a pattern reversal stimulus (check size 5.5 min arc). The peak‐to‐trough amplitude of the VER wave was measured and compared with the subjective visual acuity (Landolt C). The binocular amplitude enhancement over the monocular amplitude was around 26% across the acuity range studied, whereas the mean binocular subjective visual acuity improvement was around 11.3%, but this varied from 1% at the high acuity end to 17% at the low acuity end of the range.

Distance and near readings of associated heterophoria taken on 500 patients.

Distance and near readings of associated heterophoria were taken, using standard Mallett units, on 500 patients passing consecutively through an optometric practice and any symptoms, connected with distance and near vision, were noted. Complete sets of results were obtained on 383 people. Distance readings, either in size or direction, did not appear related to symptoms and such readings did not appear helpful in patient management. For near vision a fair degree of association existed between readings of associated phoria and symptoms and it was possible in the under 40 years and 40–59 years age groups to express clinically useful rules regarding the relationship. Over 60 years of age such a relationship was complicated by the fact that high degrees of exophoria readings existed in both symptomatic and asymptomatic patients and it is suggested that this may be related to the quality of binocular vision.

THE VER AS A PREDICTOR OF NORMAL VISUAL ACUITY IN THE ADULT HUMAN EYE

Abstract— The ability of the evoked potential to predict conventional visual acuity, as measured with a Landolt C test, was investigated for subjects with normal acuities. Both peak‐to‐trough and Fourier‐analysed amplitudes were used. The extrapolation technique assumed curvilinear regression and produced correlations no better than did a method employing amplitude measurements of a single check size of 5.5 min arc (r= 0.67). Assessment of Landolt C acuity by subjective use of the evoked potential stimulus was unable to produce a higher correlation.

The effect on fixation disparity and associated heterophoria of reading at an abnormally close distance

Abstract— It is well established that prism stress on binocular vision produces fixation disparity. It is therefore suggested that fixation disparity is a sign of stress on binocular vision. The authors have previously looked at the stress caused by reading in reduced levels of illumination, and found that the average fixation disparity and associated heterophoria become more exophoric. In this paper the effect on fixation disparity and associated heterophoria of reading at an abnormally close distance is reported. It was found that in 50 young adults fixation disparity and associated heterophoria increased to a greater degree of exo‐disparity. An analysis of the visual symptoms indicated that there was also an increase in the visual discomfort and other symptoms.

Fixation disparity and near visual acuity.

Twenty patients having associated heterophoria values of 2 delta and greater, as recorded with a Mallett unit, had their near monocular and binocular visual acuities measured. In the latter case, measurements were taken both with the associated heterophoria corrected by prisms and without prism correction. Bradford Near Vision charts, specially designed to measure visual acuity at near in seconds of arc, were used in the acuity measurement. The improvement of binocular acuity compared with monocular acuity was less than would occur in normal subjects without associated heterophorias. When the associated heterophorias were corrected with prisms the improvement in binocular over monocular acuity was similar to that found in normal subjects, with zero associated heterophoria readings.

FIXATION DISPARITY IN BINOCULAR STRESS

Abstract—Fixation disparity has been taken as a sign of stress on binocular vision because it is established that prism stress creates fixation disparity. This paper looks at the effect on fixation disparity of the stress caused by requiring subjects to read in inadequate illumination. It is found that the reduction in illumination does not in itself immediately change the magnitude of the fixation disparity. There is, however, an increase in the mean slope of the central part of the fixation disparity curve which suggests that when the effect of reduced illumination is added to prism stress, fixation disparity is increased. The stress created by asking subjects to read in reduced illumination for half an hour resulted in the mean associated heterophoria being increased, and over half the subjects reported symptoms of stress. It is concluded that fixation disparity is changed by this type of visual stress in some subjects, and in near vision is increased to a more marked degree of exo‐disparity. Most of this increase occurs in the first ten minutes.

Effect of induced fixation disparity on binocular visual acuity

Fixation disparities were artificially created for distance vision by prisms, and the monocular and binocular visual acuities were measured. The normal approximate 10% improvement in binocular visual acuity compared to monocular visual acuity, deteriorated in proportion to the amount of fixation disparity created by the prisms. This was true in both eso‐ and exo‐disparity. although not to the same extent.

Effect of attention on the VEP in binocular and monocular conditions.

To study the effect of voluntary concentration on the visual evokedotential (VEP) with monocular and binocular rivalry stimulation, attern reversal VEPs were recorded using 5.5 min arc and 50 min arc check sizes. Subjects viewed the VEP stimulus with the left eye and a light emitting diode (LED) with the right eye. They were asked to concentrate on either the VEP stimulus or the LED. The two targets were arranged so that the left eye remained accurately focused on, and directed to. the centre of the VEP stimulus throughout the recordings. The binocular VEP wave became attenuated when the subject concentrated on the LED. The wave was reduced lo around two–thirds of the amplituderesent when concentrating on the VEP stimulus. This reduction in the VEP amplitude is considered lo be due to a binocular interaction because there was a lack of amplitude attenuation when the experiment was repealed with monocular conditions. The results also suggest that the wave is smaller when the non–dominant eye receives the VEP stimulus compared with that when the dominant eye is stimulated by the VEP target.