Forty-five Hong Kong Chinese children aged 6 to 12 years participated in the study. Thirty-five myopes (21 boys, 14 girls) of median age 8.0 years (range, 6–12), mean spherical equivalent refractive error (MSE) −3.41 ± 0.99 D (range, −1.63 to −5.63 D), were recruited. All were habitual single-vision spectacle wearers, but had refraction fully corrected with hydrophilic daily disposable contact lenses for the purposes of this experiment (2-hydroxyethyl methacrylate [HEMA] lenses, 58% water content; Acuvue Dailies, Vistakon, Johnson & Johnson Vision Care, Jacksonville, FL). Owing to the high prevalence of myopia in Hong Kong Chinese children older than 8 years, the control emmetropic sample was smaller in number (five boys, five girls; MSE +0.00 ± 0.23 D; range, +0.50 to −0.25 D) and slightly younger (median age, 7.0 years; range, 6–10) than the myopic group. Subjects were not taking any medication and were prescreened to ensure that they had no ocular disease or binocular vision abnormalities. Corrected visual acuity ranged from −0.04 to 0.02 log minimum angle of resolution (logMAR; average, 0.00 ± 0.01), and astigmatism was less than 1.25 D. Informed consent was obtained from a subject’s parent after explanation of the nature and possible consequences of the study. The research adhered to the tenets of the Declaration of Helsinki and was approved by institutional human experimentation committee of The Hong Kong Polytechnic University.
The infra-red (IR) autorefractor (SRW-5000; Shin-Nippon Ophthalmic Instruments, Tokyo, Japan) was used to measure the accommodative state of the subject’s right eye throughout the experimental trials. The instrument provides an open field of view and quantifies accommodation by image analysis of an IR ring of light, reflected from the retina. Previous studies have shown that this system provides measures of high validity and repeatability in both adult
24 and juvenile
25 myopes. The autorefractor was modified to allow continuous recording of accommodation, with a resolution of less than 0.01 D.
26 Pupil size was always greater than 3 mm, resulting in an approximately constant depth of focus.
27
The visual axis of the infrared autorefractor was aligned with the right eye, with the left eye occluded. The subject viewed a row of Arabic letters (>90% contrast), subtending 1 minute of arc (equivalent to 0.00 logMAR), and was prompted at regular intervals to maintain fixation, focus, and attention on the letters. The letters were viewed along the visual axis through a +5.00-D Badal system, and the accommodative demand was changed instantaneously (i.e., in <100 ms) using a solenoid stepper motor. The Badal system ensured the same size and contrast of the target, regardless of the accommodative demand. The subject initially viewed the letter target, of luminance 20.0 cd/m
2, located 20 cm behind the Badal lens (i.e., imaged at optical infinity), and concurrent measures of the autorefractor IR measurement ring were recorded dynamically, using a computer program (Labview
; National Instruments, Austin, TX), and statically, using the autorefractor’s preset internal calibration. All data are presented in relation to the subject’s baseline distance accommodative state (0.0 D). The lead of accommodation, indicated by the static autorefractor readings when the subject viewed the distant target, is shown on the
y axis of
Figures 2 3 4 .
Initially, accommodation was monitored dynamically while the subject viewed a 0.1 cyc/deg difference-of-gaussian target (DoG) at 25 cd/m
2 for 5 minutes placed at optical infinity to determine the tonic accommodative level.
28 Three trials were randomly allocated to each subject, with the test letters viewed for 5 minutes at one distance, after which the accommodative demand of the target was instantaneously changed to its new value and the target viewed for a further 3 minutes: a near task at 5.0 D, followed by 3 minutes at 0.0 D; a near task at 2.5 D, followed by 3 minutes at 0.0 D; and a distance task at 0.0 D, followed by 3 minutes at 5.0 D. The online analysis system monitored both the position of the target and the corresponding accommodative response, facilitating assessment of response latency characteristics. Demographic data and details of family history of refractive error were also collected for each subject.
Split-plot analysis of variance (StatView; SAS, Cary, NC)
29 was used to examine the differences in posttask NITM between the refractive groups over time. Pearson’s product moment coefficient and stepwise analysis were used to test the relationship between NITM, demographic data, and family history.