OKN is a series of involuntary ocular movements elicited by visual stimulus across the visual field. It involves the macula, the lateral geniculate body, the occipital lobe, the cerebellar flocculus, the paramedian pontine reticular formation, and the ocular motor neuron. Many investigators have reported the feasibility of the OKN test for measuring visual function.
10–12 However, one of the pitfalls in interpreting the OKN response as a parameter of visual function is that the OKN stimulus is usually presented at near, whereas conventional visual acuity and the requirement criteria for visual impairment are usually measured at distance.
In earlier work, we developed a computer-assisted OKN vision test system to evaluate objective visual acuity at near.
4 In this study, we modified the previous system to measure objective visual acuity at distance. A new distance OKN vision test system introduced a 127-in. screen, instead of a 17-in. monitor, to project OKN stimuli onto the wide range of visual fields.
13 An adequate test distance was calculated to get the same amount of visual angle as the OKN stimulus from the 17-in. monitor screen at near. We also developed a computer program to generate OKN stimuli and analyze OKN responses from the captured motion pictures. As shown in
Figure 2, this software provides real-time captured video from the infrared oculography, which allows the physician to monitor the status of pupil acquisition, fixation, and accommodation and to determine whether the patient is cooperating or malingering. This monitoring window would be especially useful in uncooperative or noncommunicative patients who may lose visual fixation of the target, intentionally or unintentionally, during the test.
Our results showed that there was a significant correlation between objective visual acuity by two OKN methods and subjective visual acuity at distance. Objective visual acuity measurement with OKN methods also had good reproducibility. Linear regression showed that the subjective visual acuity had a higher correlation with the suppression method than with the induction method. This difference may be partly due to the similar nature of visual acuity, which is recognition visual acuity, measured by the suppression method and with a Snellen chart.
We used only one direction (right to left) for OKN stripes—that is, the temporonasal direction for the right eye and the nasotemporal direction for the left eye. Although monocular OKN is known to be nearly symmetrical for stimulus in the temporal-to-nasal and nasal-to-temporal directions in adults without any history of binocular disruption in early life,
14 all subjects in this study underwent a history and pretest ocular examination to exclude any ocular disease, including strabismus or congenital cataract.
In the induction method, both the central and peripheral retina would contribute to the OKN response. However, in the suppression method, the central retina gains input to suppress OKN, whereas the peripheral retina still has input to generate OKN. Abadi and Pascal
15 have reported that although the peripheral retina also provides input to generate OKN, the OKN response is strongly influenced by the central retina. Thus, this central dominance of OKN could explain the suppression of OKN with two opposite stimuli to those for the central and peripheral retina.
Although mean subjective visual acuity differs among certain objective visual acuity points, we also found thatthere was a wide range of subjective visual acuities at each objective visual acuity point. This range of acuities would make it difficult to predict precise subjective visual acuity on the basis of the measured objective acuity in the individual patient. In practice, objective visual function could be assessed more effectively by combining the induction and suppression methods. In the induction method, 87.8% (36/41) of the patients whose vision was 20/200 or better responded to the smallest stimulus. In the suppression method, on the contrary, the OKN response was not suppressed with the largest suppression stimulus in 91.3% (21/23) of the patients whose vision was 20/800 or worse. This result indicates that the induction method would be helpful in the patients whose visual acuity is expected to be very poor (20/800 or worse), whereas the suppression method would be more feasible in the patients whose visual acuity is expected to be 20/200 or better.
Another limitation of this study is that each test method measured a different type of visual acuity. The induction method evaluated the minimum resolvable acuity, since the OKN response is elicited only when the patient perceives the separation of the stimulus stripes. Meanwhile, the suppression method evaluated the minimum visual acuity that the patient must have to recognize the presence of a suppression stimulus. Subjective visual acuity testing measured the recognition visual acuity (letter) and resolution visual acuity (Landolt). We assumed that each type of visual acuity is not independent from the others. Still, certain types of visual acuity could be more greatly affected in patients with ocular disease.
We tested cooperative volunteers. Although the subjective visual acuity test is based on the involuntary OKN response and equipped with the monitoring device to surveil the status of gaze during the test, it still requires the patient's ability or intention to communicate or cooperate.
In conclusion, objective distance visual acuity measured with a computerized OKN system correlated well with subjective visual acuity, with good reproducibility. Further studies to enhance the effectiveness and efficiency of the test, along with development of the various patterns of induction and suppression OKN stimuli, are warranted.
Supported by Grant A080299 from the Korea Health 21 R&D Project, Ministry of Health, Welfare, and Family Affairs, Republic of Korea.