Assessing Visual Performance with Progressive Addition Lenses

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ORIGINAL ARTICLE

Assessing Visual Performance with Progressive Addition Lenses

ARKADY SELENOW, OD, FAAO, ELIZABETH A. BAUER, PhD, STEVEN R. ALI, OD, L. WAYNE SPENCER, BS, and KENNETH J. CIUFFREDA, OD, PhD, FAAO

Manhattan Vision Associates, Institute for Vision Research, New York, New York (AS, EAB, SRA, LWS, KJC), Department of Vision Sciences, State University of New York, State College of Optometry, New York, New York (KJC, AS)

ABSTRACT:Purpose.As a result of lens design limitations, progressive addition lenses (PAL’s) present a limited field of view for tasks at intermediate distances, such as with computers. To assess whether this limitation results in diminished visual performance, PAL’s were compared with single-vision lenses in four different types of reading tasks in a computer workplace environment.Methods.Adult subjects performed four computer-based reading tasks using both single-vision lenses and PAL’s at an intermediate distance of 64 cm.Results.Single-vision lenses performed significantly better than PAL’s in one task, with a trend for better performance in another task. There was no difference in performance for the remaining two tasks.Conclusions.Visual performance tests that involved stimuli subtending the widest visual angles and demanded more fixational shifts were more sensitive in discerning performance differences between the lens designs. In general, PAL’s showed marginally diminished performance compared with single-vision lenses, presumably due to their restricted intermediate channel. (Optom Vis Sci 2002;79:502–505)

Key Words: progressive addition lenses, reading, presbyopia, multifocal lenses, optics

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daptation to and satisfaction with progressive addition lenses (PAL’s) have been hampered, in part, by the restricted optical zones that make reading at both intermediate and near distances problematic at times.^{1, 2} The narrower zones of clear vision with PAL’s produce an increase in frequency of compensatory head movements, which is thought to be a major contributor to their nonacceptance.^{1, 2} Previously, tasks used to measure visual performance were typically long-term dispensing studies (e.g., weeks or months) using questionnaires to measure subjective patient acceptance.^2–5Dispensing studies are, by defini- tion, poorly controlled and suffer from a lack of objective measure- ment.^{3, 6 – 8}Furthermore, none of these studies adequately assessed the narrow channel used for viewing at the intermediate distance.

Although dispensing studies allow subjects to use the lenses in all aspect’s of one’s daily living, there is also a need for carefully controlled in-office studies using objective responses of visual performance. Controlled, objective information would help to isolate factors that contribute to patient comfort as well as to aid in the future design of improved progressive addition lenses. Experi- ments should also take into consideration the typical task demands that many jobs place on workers, i.e., a need for clear vision avail- able through both the intermediate and near zones in absolute presbyopes. Such information would allow eye care professionals

to determine which lens is best for individuals and their specific needs.

The present research sought to define parameters for visual performance tasks that discriminated between single-vision (SV) lenses and PAL’s. Results from this study are being used in ongoing research to develop a range of naturalistic tasks that are sufficiently sensitive to discriminate between the various types of progressive addition lenses as well as their SV counterparts.

METHODS Subjects

Thirty subjects (mean age, 53.9 years; SD, 6.6 ; range, 43 to 67) were drawn from Manhattan Vision Associates (a large private optometric practice). All subjects received a comprehensive vision examination before participation. Each subject had corrected visual acuity of at least 20/25 in each eye, manifested a need for a near add at 64 cm as determined by their near symptoms and the fused cross-cylinder test,⁹had related presbyopic symptoms such as blur at near, and did not have any ocular disease or binocular dysfunc- tion. Of the 30 subjects, 17 were habitual PAL wearers (1 year or longer) and 13 were new PAL wearers (varying amounts⬍1 year).

All subjects signed a consent form and were compensated in the

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form of credit at Manhattan Vision Associates for participation in the study.

General Procedures

Subjects performed four computer-based reading tasks using both SV lenses corrected for intermediate distance and industry- standard PAL’s with full correction. All centrally positioned stimuli on the computer screen were located at an intermediate viewing distance of 64 cm. For all tasks, subjects were positioned so that the center of the computer screen was located about 15° below the line of site with the head straight and the eyes in primary position. This meant that the subject was looking through the lens 3.75 mm lower than the fitting cross. Most subjects found it necessary to adjust their heads slightly to find the clearest position through the lens, but this did not involve excessive backwards tilt of the head.

The PAL had an intermediate zone width of 6.5 mm, which al- lowed for a clearly visible area of 16.6 cm (14.0°) in horizontal extent on the test screen. (Note: To obtain the channel width, we measured between 0.75 D of unwanted astigmatism at a point in the corridor where 50% of the add power is achieved for a plano

⫹2.00 add lens.)

The order of lens testing was randomized for each task. There were two forms of reading stimuli that were matched for difficulty for each task, with one to be used for each lens.

Apparatus

Stimuli were created using a Hewlett Packard Pentium II Pro- cessor computer. Computerized stimuli were displayed on a Hita- chi SuperScan 753 high-resolution 19-inch monitor; off-screen stimuli were printed using a Hewlett Packard DeskJet 697C. Lu- minance was measured using the Minolta Spotmeter F with a 1.0°

field of view. Luminance of the computer screen was 72 cd/m²and a hard copy display located next to the screen was 25 cd/m²; all off-screen stimuli were high-contrast, laser-printed words.

Task 1: Alternating Sentences

Stimuli. Sentences taken from standardized adult-level Visa- graph¹⁰selections were presented both on a computer screen centered on the subject’s midline and on a freestanding display located to the left of the computer screen. Text was presented in 12-point font, which is about 20/47 Snellen letter size at the test distance.

On-screen sentences were, on average, 15-cm wide (13.2°), and the overall paragraph was 5.5-cm high (4.9°). Sentences located to the left of the screen were about 13-cm wide (11.5°) with an overall paragraph height of 5.0 cm (4.5°). The distance from the center of the screen to the center of the laterally placed freestanding sentences was 35 cm (29°).

Procedure. In this task, subjects read aloud a series of sentences that alternated in location between the computer screen and the freestanding display. The sentences were numbered; those with odd numbers were located on the computer screen, and those with even numbers were located on the display stand. Sentences were read in numerical order in an alternating fashion between the screen and the display. This forced the subject to shift their gaze

(and head) across a wide visual field. Task completion time was recorded in seconds.

Task 2: Reading Rate

Stimuli. An adult-level selection from the Nelson-Denny Reading Test¹¹ was displayed on the computer screen centered along the subject’s midline. The font size of these selections was 9 point, which is about 20/33 Snellen letter size at the test distance.

The overall text of the selection was 12.5-cm wide (11.1°) and 21.5-cm high (18.6°).

Procedure. Per the standardized test instructions, subjects read the selection aloud for a period of 1 min. The number of words read from the passage at the end of the 1-min test interval was recorded.

Task 3: Reading Comprehension

Stimuli. A different adult-level selection from the Nelson- Denny Reading Test¹¹was presented on a computer screen centered on the subject’s midline. The text was presented in 12-point font (about 20/47 Snellen letter size), subtended 18.2° across the screen, and was 10.0-cm high (8.9°). Five multiple-choice questions placed to the left of the computer screen subtended 11.5°

wide and 14.9° high. The visual angle from the center of the screen to the center of the questions subtended about 29°.

Procedure. Subjects were timed while they read the selection silently. They were then timed while they verbally responded to the questions located on the display to the left of the computer. Sub- jects were free to gaze back and forth between the computer and the questions while they responded. Time to answer the questions was recorded, as was the number of correct answers.

Task 4: Spreadsheet Task

Stimuli. Alphanumeric information was presented in a spreadsheet format on a computer screen centered on the subject’s midline. The text was 16-cm wide (14°) and 6.5-cm high (5.8°).

Multiple-choice questions subtending 11.5° wide and 10.2° high were located on a freestanding display to the left of the computer screen; distance from the center of the screen to the center of the questions subtended about 29°.

Procedure. Subjects viewed the data that were displayed on the computer screen and verbally responded to questions that were located on the display to the left of the screen. Time to completion and percentage correct were recorded.

RESULTS

A multivariate, repeated measures analysis of variance demon- strated that there was a significant difference in completion time between SV lenses and PAL’s in the Alternating Sentences Task (F_1,29⫽12.62, p⫽0.001) (Fig. 1). The analysis also showed a trend toward significance in completion time for the Spreadsheet Task (F_1,28⫽ 3.05, p⫽ 0.09). In both cases, performance was better with the SV lenses. No significant differences (p⬎ 0.05) were found between lenses for the Reading Rate or Reading Com- prehension Tasks. The results are summarized in Fig. 1.

Visual Performance with Progressive Addition Lenses—Selenow et al. 503

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DISCUSSION

Tasks containing stimuli that subtended a relatively wide visual angle and therefore fell outside of the PAL intermediate channel width required more concurrent head movements and were more sensitive to differences between PAL and SV lenses. A modest decrease in performance was found with the PAL in some tasks, presumably because the area through which the stimuli are viewed, i.e., the intermediate zone, is much narrower than that of a SV lens.

This appears to explain why no differences in performance were found in the Reading Rate task. Here the reading stimulus was sufficiently narrow such that it fell within the clear field of view of the intermediate channel and thus did not force subjects to move their head much to see clearly. Although the stimuli used in the Reading Comprehension Task subtended a relatively wide visual angle, the subject was free to concentrate on relatively smaller sections for longer periods of time rather than shifting back and forth quickly as was necessary in both the Alternating Sentences and Spreadsheet Tasks.

For many typical intermediate office tasks, the PAL lens pro- vides performance equivalent to SV lenses. This is similar to results found in a study by Levy-Schoen and O’Regan.¹²In their study, which investigated the effects of defocus on reading, they found that global reading performance was not affected by blur until the degree of defocus was very large (i.e.,⬎2 D). They attributed this to the fact that reading is a highly practiced skill, and contextual cues as well as clarity can be used to maintain acceptable performance. Similarly, PAL wearers appear able to adapt reasonably quickly to the restricted zone of the lens for many tasks. A primary consideration when considering the efficacy of PAL’s seems to be the configuration of the task at hand. If the material for any given

task is presented entirely on a computer screen and within the field of view of the intermediate channel, performance is unlikely to be markedly affected. However, in tasks that require both on- and off-screen gaze changes, with shifts that force the material to be outside of the intermediate channel of clear focus, there is a greater chance of performance deficits.

Although it was found that objective global measures of reading did not show significant differences between SV and PAL under all conditions, it is possible that qualitative measurements (i.e., subjective patient response) would reveal a preference. Anecdotally, some subjects expressed a strong preference for one lens over another, despite similarity of reading completion time. Additionally, subjective responses using rating scale measures may help to discriminate between PAL’s that differ in certain key lens design parameters, such as intermediate zone width and lens distortion.

It could also be argued that reading aloud was the limiting factor regarding completion time and therefore might have contributed to smaller differences between lenses. However, this factor was constant for both PAL and SV lenses and, hence, should affect both conditions equally. Moreover, it is required per the standardized instructions of the Nelson-Denny Test.

Further experiments comparing PAL’s among themselves, as well as to SV lenses, will involve subjective ratings of visual comfort simultaneously with objective measures of reading and comprehension. It is possible that objective performance may be similar between lenses, whereas subjective measures will demonstrate a preference. Ongoing research in this area involves using similar tasks that contain a wider viewing field, lower contrast stimuli, and demand more frequent fixational shifts (coincident with head movements) to increase the sensitivity of the task.

ACKNOWLEDGMENTS

Supported by The Spectacle Lens Group of Johnson & Johnson Vision Care.

An earlier version of these data was presented at Association for Research in Vision and Ophthalmology, Fort Lauderdale, Florida, 2000.

Received October 17, 2000; revision received May 16, 2002.

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FIGURE 1.

Completion times for all tasks. Note that there are two measures of completion time for the Reading Comprehension Task. The filled bars show completion times for single-vision lenses, and the open bars show completion times for progressive addition lenses. Data shown are mean⫹ 1.0 SEM. AS, Alternating Sentences; RR, Reading Rate; RCR, Reading Comprehension Rate; RCAR, Reading Comprehension Answer Rate; ST, Spreadsheet Task.

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Arkady Selenow Manhattan Vision Associates Institute for Vision Research 160 East 56th Street New York, New York 10022 e-mail: [email protected] Visual Performance with Progressive Addition Lenses—Selenow et al. 505