The stimulus AC/A ratio was measured with a gradient response method. With this method, the AC/A ratio was determined by first measuring the phoria at 40 cm with only the subject’s correction in place, and then with additional pairs of plus and minus lenses (+1.00, +2.00, −1.00, and −2.00 DS). The subjects’ phoria, both with and without the additional pairs of lenses, was measured with a flashed Maddox rod and tangent scale technique
19 27 to the nearest 0.25Δ. The five phoria measurements obtained were then plotted against the change in accommodative demand. A linear regression was then fitted to the plot, and the slope of the regression line used as a measure of the AC/A ratio. For all the phoria measurements taken with the flashed Maddox rod technique,
19 27 the exposure through the rod was limited to 0.25 second with the aid of a photographic shutter and shutter controller (model 05 ISC 850/04 IES 213; Melles Griot, Carlsbad, CA). The shutter was mounted on the refractor head in front of the right eye. The refractor head (Auto Optoteseter OT-7A; Nikon, Tokyo, Japan) housed the subject’s prescription and the Maddox rod. The refractor head was parallel to the plane of the tangent scale, and the visual axes through the refractor head were at the level of the central spot of the tangent scale. Before each phoria measurement, the subjects were allowed 15 seconds of binocular viewing on the test chart. Then the subject’s right eye was occluded for 15 seconds to allow fast vergence to decay.
23 One second before a phoria measurement (i.e., the 0.25-second exposure of the rod), the subject was notified by the use of a sound stimulus, which he or she reacted to by changing fixation from the test chart to the central spot of the tangent scale. The test chart was located at the same distance from the subject as the spot but 2° from the spot in a 45° angle, up and to the right. The test chart consisted of a vertical row of Snellen acuity letters subtending 7.5 minutes of arc, and the subject was instructed to fixate the letters except for the short periods when the phoria was being measured.
For the measurement of the stimulus CA/C ratio a method based on the technique developed by Tsuetaki and Schor
43 was used. The change in accommodation was measured using a refractometer. The PowerRefractor (MultiChannel Systems, Reutlingen, Germany, now manufactured by PlusOptiX, Nürnberg, Germany) is an objective refractometer based on eccentric infrared photograph refraction and is capable of continuous measurements of the refractive status. The instrument enables overrefraction through contact lenses or spectacles but needs a relatively low level of room lighting to operate. To ensure the proper light intensity, the room was darkened (105 lux), but at the same time the fixation target was illuminated with short-wavelength light (≤560 nm) to ensure stable fixation. This short-wavelength light does not interfere with measurements made by the refractor. For measuring the CA/C, a stimulus consisting of one cycle of a 0.2 cyc/deg DOG (difference of Gaussian) target (when viewed at 40 cm), was placed on the midsagittal plane 40 cm from the subject’s eyes and served as a binocular fixation target. The DOG target is relatively incapable of stimulating any reflex accommodation,
43 yet it is localized in space and has been found to be an ideal stimulus for disparity vergence, inasmuch as it is a nonrepeating target that avoids false fusion problems such as the wallpaper illusion. When the subject views the DOG target with natural pupils, it acts as a quasi open-loop condition for accommodation and so permits measurements of vergence accommodation.
43 During the measurements, the subject’s head was fixed in a head and chin rest, and the refractometer was positioned along the midsagittal plane 1.0 m from the subject’s eyes. During the measurement, the subject was instructed to keep the DOG target single. Before measurements were started, the subject viewed the DOG target for 5 minutes to allow accommodation to stabilize. The subject wore his or her refractive correction and the left eye was overrefracted with the refractometer. The value of overrefraction served as a baseline measure of the resting level of accommodation. An 8Δ base-out (BO) prism was then inserted over the subjects’ right eye, and the left eye was again overrefracted within 1 to 2 seconds of the subject reporting that that he or she had obtained fusion of the target through the prism. The CA/C was calculated by dividing the dioptric difference in overrefraction as obtained with and without the prism. All the subjects tested were able to fuse through the 8Δ prism and gave a clear indication that binocular vision had been restored through the prism. Each subject had the CA/C ratio measured twice.