Twenty emmetropic non–contact lens wearers aged 24 to 51 years were enrolled. Participants were in good health.
The right eye was the test eye, whereas the fellow eye was occluded with a dark patch. There were three randomly ordered experimental viewing conditions: clear, spatially blurred, and dioptrically defocused condition. A flowchart of the experimental conditions is provided in Fig. 1. During each condition, participants viewed targets (from a distance of 3 m) projected on a clear white wall by a digital projector.
The visual targets consisted of a collection of 10 natural pictures placed on a PowerPoint presentation (Microsoft Corporation, Redmond, WA) in random order. There were 10 slides, and each slide was displayed for 30 s. Unfiltered pictures were viewed during the clear and dioptrically defocused conditions. For the spatial blur trials, the pictures were spatially filtered (unclear) to a nominal +6.00DS equivalent defocus (as defined by VOL-CT software; Sarver Associates, Chicago, IL). Sample pictures of unfiltered (clear) and filtered (unclear) visual targets are presented in Fig. 2, A and B, respectively.
Dioptric defocus was induced using Focus Dailies (CIBA Vision, Duluth, GA) contact lenses of +6.00DS refractive power. The base curve of the lens was 8.6 mm with a diameter of 13.8 mm. The lens was allowed to settle for 15 min after insertion, followed by slitlamp biomicroscopy examination to assess the lens centration, movement, and lens lag.
Ratings of vision, comfort, and other ocular sensations (burning, itching, and warmth) were reported in random order at baseline and toward the end of each experimental viewing condition using magnitude estimation. For the comfort scale, 0 indicated “no discomfort” and 100 indicated “worst discomfort imaginable.” For the other ocular sensations, 0 indicated “no burning/itching/warmth” and 100 indicated “worst imaginable burning/itching/warmth.” Vision was also rated from 0 to 100, where 0 represented “clear vision” and 100 indicated “vision as bad as it can be.”
Fifteen participants from part 1 of the study were enrolled in part 2. Their ages ranged from 24 to 51 years. The experimental conditions were similar to part 1, with the following two differences. First, contact lenses were used while viewing clear and spatially filtered (unclear) targets and also to induce dioptric defocus. For the clear viewing condition and spatial blur, a +0.25DS lens was used, whereas for dioptric defocus, a +6.00DS lens was used. Second, participants rated vision, comfort, and other ocular sensations in two additional experimental conditions: 1) when viewing an illuminated ganzfeld and 2) when occluded by a black patch, each of which provided (different) conditions of the absence of visual structure. The experimental conditions (illustrated in Fig. 3) therefore were randomly ordered conditions of clear vision, spatial blur, dioptric defocus, ganzfeld viewing, and occlusion.
Statistical analyses were performed using Statistica 7 (Statsoft Inc., Tulsa, OK), and p ≤ 0.05 was considered to be statistically significant. Repeated-measures analysis of variance and pairwise post hoc analysis with Tukey HSD tests were used to compare the ratings of vision, comfort, and other ocular sensations under the different experimental conditions. Pearson correlation coefficients were estimated to examine linear associations between vision and comfort ratings.
There was a significant difference in the ratings of vision compared with the ratings of comfort (p = 0.003); ratings of vision were higher than comfort ratings. The viewing conditions had a significant main effect (p < 0.001) on the ratings of vision and comfort. There were no differences between the sensations of burning, itching, and warmth (p = 0.75), and the viewing conditions did not have a significant main effect on these ratings (p = 0.89).
Interactions between Ratings of Vision, Comfort, and Viewing Conditions
There were significant interaction effects between the viewing conditions and the ratings of vision and comfort (p < 0.001), as illustrated in Fig. 4. Tables 1 and 2 summarize all of the pairs of conditions with statistically significant differences in the ratings of vision and comfort.
Post hoc Tukey analysis showed statistically significant differences in ratings of vision under the clear viewing condition and the blurred conditions (p < 0.001 for both spatial blur and dioptric defocus). Vision ratings were higher (worse vision) under dioptric defocus than spatial blur (p < 0.001). There were no differences between the vision ratings at baseline and the clear viewing condition (p = 0.08).
Comparison of comfort ratings showed significant differences in comfort between the clear viewing condition and blurred conditions (p = 0.002 for spatial blur and p < 0.001 for dioptric defocus). However, there was no statistically significant difference in the ratings of comfort (p = 0.97) between spatially blurred and dioptrically defocused conditions. Comfort ratings at baseline and in the clear viewing condition were also not statistically significantly different (p = 0.74).
Pearson Product Moment Correlation of the Ratings of Vision and Comfort
Vision and comfort ratings were statistically significantly correlated in the clear viewing (r = 0.62) and dioptrically defocused conditions (r = 0.60), and there were no significant correlations for any of the other pairs of experimental conditions.
Other Ocular Sensations
The ratings of burning, itching, and warmth showed no statistically significant differences between them across all viewing conditions (p = 0.07).
The results of this part of the study were similar to part 1 inasmuch as the ratings of vision were significantly different from the ratings of comfort (p < 0.001); ratings of vision were higher (worse vision) than comfort. The viewing conditions had a significant main effect on the ratings of vision and comfort (p < 0.001). There were no differences between the ratings of burning, itching, and warmth (p = 0.75), and the viewing conditions did not have a statistically significant main effect on these ratings (p = 0.90).
Interactions between the Ratings of Vision, Comfort, and the Viewing Conditions
There were significant interaction effects between the viewing conditions and the ratings of vision and comfort (p < 0.001), as illustrated in Fig. 5. Tables 3 and 4 summarize all of the pairs of conditions with statistically significant differences in the ratings of vision and comfort.
Post hoc Tukey analysis showed that vision rating under the clear viewing condition was significantly different from spatially blurred and dioptrically defocused conditions (both p < 0.001). Vision when stimulated dioptrically defocused had higher ratings (worse vision) than when stimuli were spatially blurred (p < 0.001). There were no statistically significant differences in vision ratings between the clear and ganzfeld viewing conditions (p = 0.21), whereas the ratings of vision when the eye was occluded were significantly different (p < 0.001) from all other viewing conditions.
Comparison of comfort ratings using post hoc Tukey analysis showed no statistically significant difference in comfort between the clear viewing and spatially blurred condition (p = 0.79) but a significant difference between the clear viewing and dioptrically defocused condition (p = 0.003). However, comfort ratings under spatially blurred and dioptrically defocused conditions showed no statistically significant difference (p = 0.28). There were also no significant differences in ratings of comfort under the conditions of clear viewing and absence of visual structure, ganzfeld viewing, and occlusion (all p values at least 0.99). However, significant differences in comfort were observed between the conditions of dioptric defocus and absence of visual structure (for ganzfeld viewing, p = 0.008; for occlusion, p = 0.002).
Pearson Product Moment Correlation of the Ratings of Vision and Comfort
Vision and comfort ratings were significantly correlated only in the dioptrically defocused condition (r = 0.76), and there were no statistically significant correlations for any of the other pairs of experimental conditions.
Other Ocular Sensations
The ratings of burning, itching, and warmth were not statistically different across all viewing conditions (p = 0.07).
This study attempts to clarify the reports of the influence of vision on ocular discomfort. The results of both part 1 and part 2 experiments suggest that comfort and vision show higher ratings (worse vision/comfort) when participants viewed spatially blurred and dioptrically defocused stimuli. Vision was worse when targets were dioptrically defocused than when they were spatially blurred. However, there were no differences in comfort observed between the two conditions, suggesting that the change in the ratings of vision does not parallel the change in comfort ratings. In addition, when visual structure was absent during occlusion or ganzfeld viewing, comfort was unaffected and appeared similar to clear viewing conditions. This phenomenon of noncovariation of vision and comfort suggests that the decrements in comfort with the induction of blur might perhaps be caused by more complex psychological influences such as Hawthorne or nocebo effects and not because of a simple influence of vision on comfort.
The pathway for the transmission of ocular surface pain (that presumably is responsible for the discomfort experienced in our experiment) and the visual pathways are anatomically distinct, with separate receptors, pathways, and cortical structures. The visual information from the environment is processed by the receptors in the retina that transduce the optical input to neuronal signals that is transmitted via the optic nerves of each eye to the primary visual cortex in the occipital lobe of the brain.12 The pathway for ocular surface pain involves the transmission of nociceptive signals arising from the corneal/conjunctival receptors to the somatosensory cortex via trigeminal brainstem components, reticular formation in the midbrain and the thalamus (among other places).12,13 The processing of vision and ocular surface pain occurs in different areas of the brain and whether integration between them occurs (in the form, for example, of direct neuronal connection) is yet to be fully understood. There are reports of functional relations between the visual cortex and the trigeminal nociceptive system to explain the visual discomfort caused by excessive light (photophobia) in healthy humans.14,15 However, the generalizability of this linkage to ocular nociception and blur is unclear.
Other senses such as olfaction and audition are each coupled in some way to vision. For example, visual objects can modulate the neural processing of odors and the orbitofrontal cortex can be a potential substrate for the integration of visual and olfactory stimuli.16,17 Odors can affect visual processing by attracting attention to the odor source18 and the visual cortex is found to be activated during pure olfactory tasks, suggesting that vision might be influenced by nonvisual sensory processes in the brain.17,19 The presence of multisensory neurons and the integration of visual, auditory, and somatosensory signals have been observed in the superior colliculus.20 However, the impact of multisensory mechanisms (in the superior colliculus and many other subcortical structures) on conscious processes and how the integration of information within the brain influences visual perception are unknown.
As there was not a direct influence of vision on discomfort and it even appears that there is a double dissociation (i.e., changing vision under many circumstances does not affect comfort, and similarly changing comfort with and without contact lenses does not affect vision), we need to invoke less linear/direct relationships and perhaps “higher-order” accounts for our data. Although mild ocular surface pain or ocular discomfort is being studied, inferences from the mechanisms of pain processing and pain perception can help us understand the possible means of interaction between vision and ocular comfort. The changing visual conditions in the study, the awareness of being in an experiment, and expecting some experimental outcome/difference might have induced affective pain, subtly influencing the participants to anticipate some change in comfort when vision was experimentally manipulated. One example of, perhaps, a subtle influence is the nocebo effect, where the symptoms can worsen or be reported to be worse because of anticipation and expectation of a negative outcome.21
In pain studies, the nocebo effect has been investigated using neuroimaging techniques where the brain regions involved in the expectation of pain are found to interact with those involved in the processing of afferent nociceptive information and can alter the subjective experience of pain.22,23 The neuroimaging findings are further complemented by pharmacological evidence24 that anticipatory anxiety can activate the cholecystokinin A and B receptor systems that facilitate pain transmission. The blurred visual conditions in our study perhaps created a nocebo-like effect, causing an experience of reduced comfort under blur.
The awareness of participation in an experiment, the Hawthorne effect, can also be one of the contributing reasons for the reduced comfort under blur. The Hawthorne effect is a potential problem that occurs in experiments in which subjects’ knowledge that they are in an experiment modifies their behavior from what it would have been without the knowledge.25 The first Hawthorne effect was reported in an industrial study, where an increase in productivity of the workers was hypothesized to be caused by the attention they received, despite worse working conditions.26 Although there are controversies around the first Hawthorne study,27–29 the term “the Hawthorne effect” has survived to describe the phenomenon as an increase in productivity or other outcome caused by the participation in the study.30 A similar scenario would have been possible in our study in that the changing visual conditions may have created a Hawthorne effect affecting the subjects’ response to the experimental conditions.
An alternative explanation to the observed discrepancy between the ratings of vision and comfort can be attributed to the differences in how subjects used the scales to rate vision and comfort. The mean ratings for comfort tended to lie in the lower end of the rating scale (mean discomfort rating is 25), whereas a wide range of the rating scale was used to rate vision. It is possible that the subjects being emmetropic may not have had a very clear idea about how bad the vision could have been, whereas they might have had a better idea about how discomfort may feel like. Hence, the subjects would have restricted the comfort ratings to the lower end of the scale as the amount of discomfort experienced would have been minor. It is possible that a narrowly anchored rating scale would have elicited a higher range for ratings of discomfort. This logic however does rest on the separation of understanding blur and discomfort and, because our participants were not contact lens wearers and had a normal ocular history, it is possible that their understanding/experience of blurriness and ocular discomfort was similar.
The reason that emmetropes were chosen as subjects was to minimize the variability in the effects of defocus, especially in the dioptric defocus condition. There is no biological reason to believe that emmetropes and ametropes would differ in their response to the stimuli used in this experiment, and so there are no reasons to limit the generalizability of these results from emmetropes to the population in general.
It is unlikely that the reduction in comfort could have been caused by contact lenses because the results were similar (reduced comfort under blurred conditions) in both parts 1 and 2 of the study despite the presence/absence of lenses. Because the lenses were worn for a short period, the physiological changes that might occur because of the interaction of the contact lens with the ocular surface31,32 may not be the causative factors for the decline in comfort observed in this study. Mechanical effects such as friction between the contact lens and the lid margin33 and poor fitting pattern of the contact lens34 can lead to ocular discomfort; however, the lack of difference in the comfort ratings between spatial blur (condition with no contact lens) and dioptric defocus (condition with contact lens) in part 1 of the study shows that the possibility of contact lens fitting and friction to affect the results is minimal.
Blur/defocus of +6D is worse than the kind of blur a contact lens wearer would experience from his or her lenses because of minor error in refractive correction, tear film break-up, poor wetting, or toric lens rotation. The rationale for choosing +6D was to produce an appreciable amount of degradation in vision under both spatial blur and dioptric defocus conditions. The amount of blur/defocus experienced by the subjects under the two conditions may not be equal because an addition of +6D contact lens would make the eye myopic under dioptric defocus conditions, changing the total optical power of the eye and a shift in far point, whereas under spatial blur, the optical power of the eye remains unchanged (emmetropic). The amount, though, is less relevant than the simple experimental tests examined: Does poor vision unambiguously influence discomfort reported? It is not a question about subtle vision changes influencing discomfort. Unless an additional inverse dose-response relationship was invoked to suggest that higher amounts of defocus/blur did not produce effects on discomfort whereas lower amounts did, it would appear that Occam’s Razor35 imposes on us a more simple/direct interpretation of the results rather than a more complex list of possibilities to account for the outcome of the experiments.
In summary, it appears that, under some circumstances, reported comfort is altered when vision is not clear. The reduction in comfort reported during the conditions of spatial blur and dioptric defocus may not be caused by a simple putative influence of vision but perhaps caused by subtle influences of the affective aspect of pain and/or a higher-order sensory integration of vision and ocular surface pain processes.
Subam Basuthkar Sundar Rao
210 Crittenden Blvd.
Rochester, NY 14623
Supported by a grant from the Natural Sciences and Engineering Research Council of Canada.
Part 1 of the study was presented as a poster at the Academy Annual Meeting 2008, Anaheim, California. Part 2 of the study was presented as a paper at the Academy Annual Meeting 2010, San Francisco, California.
Received April 3, 2015; accepted September 25, 2015.
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Keywords:© 2016 American Academy of Optometry
ocular comfort; vision; contact lenses; spatial blur; dioptric defocus