Accommodative Gain in Relation to Perceived Target Clarity

Abstract

Purpose: The purpose of this study is to determine the relationship between target clarity and the magnitude of accommodative lag using the metric accommodative gain (AG).

Methods: Monocular accommodative responses were measured with Grand Seiko autorefraction using both proximal and minus lens techniques in 139 subjects aged 5 to 35 years. Subjects viewed a 1.5-mm letter at 13 discrete distances (range, 40 to 3.33 cm) for the proximal technique and fixed at 33 cm through minus lenses of increasing power for the lens technique. Subjects were instructed to keep the target clear and report when it blurred. The AG was calculated (accommodative response/accommodative demand) for the four greatest consecutive demands perceived clear (termed conditions 1 to 4) and the first demand perceived blurry (termed condition 5).

Results: Multivariate planned contrast, including age as a predictor, revealed that mean AG was significantly larger when the target was clear (range, 0.71 to 0.77 for conditions 1 to 4 across techniques) versus blurry (0.59 and 0.68 for condition 5 across techniques) (p < 0.001 for proximal and p < 0.036 for lens). Age was only a contributing factor for the proximal technique, with the youngest subjects having the largest decrease in AG when the target changed from clear to blurry (p = 0.017).

Conclusions: These data suggest that across age and technique, the AG is relatively constant when the target is perceived clear but drops below approximately 70%, on average, once the target is perceived as blurry for subjects aged 5 to 35 years. The AG may be a useful metric to compare accommodative responses across a range of demands and to identify accommodative responses that may not be sufficient to perceive a clear target.

Figure 1

Theoretical accommodative stimulus response curve representing possible ways to describe the accommodative response over multiple accommodative demands using accommodative lag, accommodative gain (AG), slope (gradient) of the linear portion of the response, and the accommodative error index (AEI).

Figure 2

Custom target mounts for proximal accommodative testing. (A) Target positioned on the investigator side of the beam splitter for demands from 2.5 to 10.5 D. (B) Target positioned on the subject side of the beam splitter for demands from 12.5 to 30 D. (C) Subject view of the printed letter Target for demands from 12 to 30 D. A color version of this figure is available online at www.optvissci.com. Reproduced with permission from Anderson HA, Stuebing KK. Subjective versus objective accommodative amplitude: preschool to presbyopia. Optom Vis Sci 2014;91:1290-301. ©The American Academy of Optometry 2014.

Figure 3

Representative accommodative responses (mean ± standard deviation) for an 8 year-old (diamonds) and a 34 year-old (squares) subject tested with the proximal technique. Numbers indicate the termed conditions for analysis with 1–4 representing a clear target perception and condition 5 representing a blurry target perception.

Figure 4

Accommodative gains (mean ± standard deviation) for each condition for both the proximal and minus lens techniques. The solid brackets represent the comparison of the mean of conditions 1–4 to condition 5 (Proximal technique: n = 132, Lens Technique: n = 130). The dashed lines represent the comparison of neighboring conditions (condition 1 to 2, 2 to 3, 3 to 4, 4 to 5 and 5 to 6; Proximal technique n = 131, Lens technique n = 125). The asterisks represent p < 0.05.

Figure 5

Accommodative gains (mean ± standard deviation) grouped by age for both techniques (proximal & minus lens) to illustrate that the AG was similar across age with the exception that the AG decreased by a greater amount for the youngest age group when comparing conditions 4 and 5 for the proximal condition (asterisk represents p = 0.013) and the older age groups for the lens condition when comparing conditions 5 and 6 (asterisk represents p = 0.013). (Conditions 1–5 included subjects that had data for all Conditions from 1–5 (Proximal technique n = 132, Lens technique n = 130), whereas Condition 6 included fewer subjects since not all subjects had data for condition 6 (Proximal technique n = 131, Lens technique n = 125)).

Figure 6

Comparison of the mean ± standard deviation accommodative gains for Conditions 1 through 5 between the proximal and lens techniques. The AGs for the two techniques were not found to be significantly different from one another for conditions 1 – 4 (p > 0.05). The AG for the proximal technique was significantly less than the AG for the lens technique for Condition 5, once the target was perceived as being blurry (asterisk represents p = 0.004).