Influence of spectral distribution on accommodation—vergence and reading performance

Introduction

The process of which the crystalline lens varies its focal length in response to changes in the vergence of incident light is the definition of accommodation (1). Accommodation is also associated with pupil constriction (miosis) and convergence during near fixation which are termed as the near vision complex or near triad (2). In 1855, Herman von Helmholtz described the changes taking place in accommodation as constriction of pupil, moving forward of pupillary margin of the iris and the anterior surface of the lens. The anterior surface of lens becomes more convex while the posterior surface becomes slightly more convex (1). Most of the studies proved the influence of accommodation on electronic devices and addressed one of the rationales for visual fatigue during reading. Hue et al. reported that reading soft copy material from electronic devices cause change in accommodation compare to hardcopy printed material (3). In addition viewing angle and gaze from screen also influences the accommodation (4). Simonson et al. in his study stated that visual performance is reduced due to accommodation fatigue which was influenced by the variety of room illumination (5). However, Hennessy et al. proves that during accommodation pupil size is reduced, the amplitude of accommodation is diminished and approaching a fixed intermediate resting focus corresponding to approximate distance of the visual task (6). Miller et al. postulated that a 3-hour exposure to visual task did not produce any significant changes in dark focus eventhough it is a demanding visual task. He also quoted that accommodation remains same at dark focus. So, the fatigue can be observed when accommodative effort is required (7). Individual differences such as age, race, gender and refractive error might influence accommodation parameters (8). However, Yavas et al. Showed that gender has insignificant influence on amplitude of accommodation (9). Reading performance is one of the important issue that need to be addressed while discussing about reading at near. Carver et al. suggested that measuring reading rate in standard words per minute is due to compensate for changes in difficulty level across reading material, nature of reading task level and purpose of reading (10). Huey et al. study explains oral reading rate is 50% slower than silent reading (11). Later on Carver et al. proved that silent speech is mostly used to aid memory during reading hence it is not active during skim and scan of reading (10). Owens DA et al. studied convergence in relation to the low illumination and explained that convergence is most important distance cue than accommodation in low lighting conditions. So, convergence is more affected than accommodation (12). Current paper tried to conduct an experiment on different spectral distribution of light, to show their influence on reading rate, reading speed, accommodation and convergence.

Methods

Participants

This study includes forty subjects irrespective of their gender and race. The subjects were within an age group of 18–21 years pursuing their bachelor degree. A written informed consent was obtained from all the participants and an approval (UH/IEC/2015/134) to conduct the study was obtained from the Ethical Committee of the School of Medical Sciences, University of Hyderabad, India. Participants are the student of University of Hyderabad and a randomized sampling was used with equal opportunity and equity. No monetary reward was offered to the participants in this experiment. Subjects with emmetropia (visual acuity 6/6, 20/20) and devoid of any ocular pathology were included in this study.

Materials & experimental setup

This experimental quantitative study design was divided into two phases: preliminary examination and experimental phase. Preliminary examination includes comprehensive eye examination and experimental phase includes calculating intensity of light [compact fluorescent light (CFL) 12 watt, fluorescent light 20 watt, tungsten 100 watt, light emitting diode (LED) 8 watt] with the help of digital photometer model-(HS1010). Near point of accommodation (NPA) and near point of convergence (NPC) were measured with the help of royal air force (RAF) ruler (Zeiss clement Clarke model). To assess the reading speed and reading rate, readability score was checked. Equal readability score passages were created with the help of “Free Readability Text Consensus Tool” (13) and then from the readability score the reading speed and reading rate was calculated [reading speed = (total no. of words/time taken to complete the passage) × time in seconds; reading rate = CWPM − errors read). Text passages with equal readability score was printed on a nonglossy white paper with hundred percent contrast can see in Figure 1. Reading time was measured with the help of stop watch (Kadio model KD-2004). Reading pad of 5×5 feet was used to hold reading material at 40 cm from the subject along with an overhead illumination placed at a distance of 1 meter.

Figure 1 Readability passage used for near visual task.

Experimental procedure

All participants were instructed about their job throughout the experiment, prior to the experiment. All tests were carried out under four lighting conditions (LED, CFL, fluorescent and tungsten) within an interval of 15 minutes. An interval of 15 minutes was allotted to overcome the light adaptation effect. Before starting the experiment, a randomized sequence was generated with the help of computer software and each individual has to follow that sequence for measuring reading rate, reading speed, NPC and NPA. Throughout the experiment a constant intensity of light (400 lux) was maintained for all four illuminations with the help of digital photometer.

Data preparation and analysis

All data was entered in Microsoft Excel version 2010 and statistical analysis was carried out with the help of IBM-SPSS 21.0 software. G-power software was used to calculate the sample size with a medium effect size. Normality of the data was assessed by Shapiro-Wilk test. Based on the normality data, Friedman test was chosen to compare the reading rate, reading speed, NPA and NPC with four different spectral distribution of light. Post hoc analysis was done within the groups by using Wilcoxon signed rank test. A P value of <0.05 was considered significant.

Results

This study includes 40 participants with an age group of 18–21 years, out of which 50% male and 50% female. A statically significant result was obtained when Reading rate, Reading speed NPC, NPA was compared among different spectral distribution of light (P<0.001) as shown in Tables 1-4 respectively. Post hoc analysis showed a significant difference (P<0.001) when tungsten was compared with fluorescent light (FLOU), CFL, and LED for reading rate reading speed and NPC. But there is no significant difference exist when fluorescent was compared with CFL for reading rate (P=0.530), reading speed (P=0.595) as shown in Tables 5 and 6 respectively. Similarly, LED showed no significant difference when compared with CFL (P=0.682) and fluorescent (P=0.490) for NPC as shown in Table 7.When NPA was assessed within the group LED showed no significant difference with CFL (P=0.205) and fluorescent (P=0.275) similar like fluorescent and tungsten (P=0.482) as shown in Table 8.

Discussion

Though considerable amount of research is going on around the world on spectral distribution and eye, we still need to have a good understanding about its influence on our visual system. Our study not only aimed to assess the influence of different spectral energy distribution of lighting on accommodation and vergence but also the reading performance (reading rate, reading speed). This study showed a significant effect (P<0.001) of spectral distribution of light on reading performance which is supported by Lin et al. In 2014, Lin et al. concluded that light intensity (P=0.002) and background color (P=0.003) having significant influence on reading performance (14). Although both the studies supporting each other but still there are few discrepancies exists in terms of methodology. This study not only kept intensity and background color fixed but also checked readability score of each passage which was not measured in Lin et al. study. Apart from that Succar TA et al. also emphasized extra lighting during near task for low vision patients than normal lighting which will be more beneficial for their reading performance (15). In another study by Simonson et al., reported that changes in optimum level of illumination causes visual fatigue and deterioration of performance (16). Current study also showed that reading rate and reading speed was affected more in tungsten followed by LED. This effect leads to visual fatigue due to the focusing changes with different spectral distribution of light.

This study showed that NPC (P<0.001) is significantly changing with different spectral distribution of light. Although it was observed that NPC with tungsten light is better than fluorescent, CFL and LED, but subjects experience more comfort with fluorescent, CFL and LED while doing near task. This study result is supported by Owens DA et al. where he has mentioned convergence is the most important distance cue than accommodation in low lighting conditions and chances of affecting convergence is more than accommodation (12).

Moreover this study also proved, NPA (P=0.43) has no significant effect on different spectral distribution of light. But post hoc analysis showed a significant difference between tungsten—CFL (P=0.002) and tungsten—LED (P=0.023). Aggarwala et al. in his study concluded that accommodation increases and lag decreases when a stimulus of spectral bandwidth is varied which contradict current study findings. The discrepancy between the two studies is because methodology difference (17).

Conclusions

This study concluded that reading performance (reading rate and reading speed) and near point of convergence significantly changes with different spectral distribution of light. However, accommodation has not much significant effect when changing the spectral distribution of light. Tungsten spectral energy influencing the convergence which can also shows an impact on reading and near visual tasks.

Acknowledgments

Funding: This research was supported by (India vision institute optometry student research grant, 2015) and we also acknowledge Dr. Supriya Bhardwaj for inputs in this experiment and to all the participants for their patience in the experiment.

Footnote

Conflicts of Interest: The authors have completed the ICMJE uniform disclosure form (available at http://dx.doi.org/10.21037/aes.2017.06.01). SRM reports grants from INDIA VISION INSTITUTE-HYDERABAD, an Non profit NGO working towards eye care, during the conduct of the study; grants from INDIA VISION INSTITUTE-HYDERABAD, outside the submitted work. The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). A written informed consent was obtained from all the participants and an approval (UH/IEC/2015/134) to conduct the study was obtained from the Ethical Committee of the School of Medical Sciences, University of Hyderabad, India.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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