Impact of Blur on Suprathreshold Scaling of Ocular Discomfort

Abstract

Purpose.: To examine the suprathreshold scaling of pneumatic stimuli, and the ratings of discomfort and intensity under clear and defocused visual conditions.

Methods.: Twenty-one participants rated sensory intensity and discomfort of a series of mechanical stimuli from a pneumatic esthesiometer, using a 0 to 100 numerical scale under clear and defocused visual conditions. Esthesiometry was performed on one eye while the fellow eye viewed a 3-m distant 6/60 target through a trial lens. For the clear visual condition, a +0.25DS lens was used over the subject's refractive correction, and for defocus, an additional +4.00DS was used. Central corneal mechanical thresholds were first estimated using ascending methods of limits. Then, stimuli that were 25%, 50%, 75%, and 100% above threshold were presented in random order in three sessions of clear and defocused vision, and subjective ratings were recorded. Power exponents that define the slope of the sensory transducer functions were derived for discomfort and intensity estimates.

Results.: No significant differences (P = 0.66) in mechanical thresholds, ratings of discomfort (P = 0.54), and intensity (P = 0.30) were observed between the visual conditions. Power exponents for discomfort showed significant differences (P = 0.05) between clear and defocus conditions, but not intensity (P = 0.22). Comparison between discomfort and intensity showed differences in exponents when vision was clear (P = 0.02) and defocused (P < 0.001).

Conclusions.: Scaling of suprathreshold pneumatic stimuli varies with viewing conditions. When vision was not clear, the exponent of the average transducer function for discomfort was steeper and this finding is the first demonstration of an association between ocular surface sensation and quality of vision.

The operation of any sensory system can be well understood when the stimulus and the resulting sensation is studied. The process of quantifying mental events, especially sensation and perception, and determining how the quantitative measures of mental events are related to the quantitative measures of physical stimuli is referred to as psychophysical scaling.¹ Magnitude estimation is one way of psychophysical scaling where subjects assign numbers to the sensation magnitude. A psychophysical relationship called the psychophysical magnitude (or transducer) function is established when the magnitudes of a sensory attribute are plotted against the corresponding physical values of the stimulus.² This is a power function,³ linear when plotted on a log scaled x- and y-axes. The value of the exponent in the power function determines the curvature of the function and has been characterized as “typical” for each sensory modality. Exponents have been derived for a number of perceptual continua to quantify the suprathreshold performance of these sensory systems.⁴ High exponents (more acceleration) indicate that the sensation grows more rapidly as the stimulus intensity is increased and for a sensory continuum with low exponents, the sensation growth is less rapid.⁵ 

The cornea is one of the most richly innervated structures in the body^6,7 that is primarily nociceptive in nature. Most of the corneal nerves have a sensory origin from the ophthalmic branch of the trigeminal nerve,⁸ and the nerve fibers terminate as nonspecialized nerve endings on the superficial layer of the cornea.^7,9 Based on the modality of stimuli that activates the nerve fibers and the quality of sensation evoked, the ocular sensory neurons can be functionally classified into mechano-nociceptors, polymodal nociceptors, and cold receptors.^10–12 Mechano-nociceptors are activated by mechanical forces and can cause acute sharp pain in response to any mechanical contact with the corneal surface. Their short lasting impulse discharge mainly serves to signal the presence and the change of the stimulus rather than its absolute intensity.^13,14 The polymodal nociceptors react to mechanical forces, chemical irritants, and noxious heat by producing an irregular continuous discharge roughly proportional to the intensity of stimulation.^10,15–19 The cold receptors are activated when the normal temperature of the cornea decreases below 33°C and they are sensitive to temperature decrements of 0.1° or less.^10,15 

Selective stimulation of the cornea with controlled mechanical, chemical, and thermal stimuli evoke sensations proportional to the magnitude of applied stimuli. Mechanical stimuli can cause a sensation of scratching²⁰ perhaps due to excitation of Aδ fibers, while sensations evoked by CO₂ are often described in terms of stinging and burning pain^10,20–22 perhaps due to the activation of C fibers. Moderate cold stimulation of the cornea can cause an innocuous sensation of cooling that becomes irritating with lower temperatures.¹⁰ 

The presence of a contact lens on the ocular surface can stimulate the sensory nerves by the action of mechanical friction and changes in physiology, among various other reasons, giving rise to the sensation of discomfort.^23–25 Contact lens wearers primarily complain of discomfort and dryness^26–29 followed by difficulty with contact lens maintenance and poor vision.^26,27 While most studies report on the symptoms of dryness, grittiness, itching, and soreness in lens wearers,^30,31 there is anecdotal evidence that ocular comfort and vision might be related. One adumbration to this relationship is the covariation of vision and ocular discomfort toward the end of the day. One study (Papas B, et al. IOVS 2003;44:ARVO E-Abstract 3694) reported an interaction of vision and ocular surface comfort, with decreased comfort accompanying increasing level of blur. Similarly, a recent study³² on visual discomfort and blur demonstrated that a decrease in high spatial frequency contrast results in increased asthenopic (not specifically ocular surface) discomfort as well as perceived blur while viewing natural and artificial stimuli. Finally, in one of our previous experiments that aimed to investigate the influence of vision on ocular comfort ratings, a reduction in comfort was observed under conditions of spatial (image processed) blur and dioptric defocus.³³ To directly test the potential interaction between vision and ocular surface comfort, this study was conducted to examine how subjects responded to suprathreshold stimulation of the cornea when vision was clear and defocused. 

Methods

Subjects

Twenty-one healthy participants, both contact lens wearers and nonlens wearers of age ranging from 21 to 54 years were enrolled. There were 15 contact lens wearers and 6 were nonlens wearers. Because the study was not designed to uncover potential differences if any, of the ocular sensory transducer functions due to contact lens wear, the number of lens and nonlens wearers in the study were not balanced. Contact lens wearers were asked to not wear lenses the night before the experiment. 

The study was conducted in accordance to the guidelines of the Declaration of Helsinki and ethics clearance was obtained from the University of Waterloo, Office of Research Ethics (Waterloo, Ontario, Canada). Informed consent was obtained from each participant. 

Instruments

The computerized Waterloo Belmonte esthesiometer²⁰ was used to deliver mechanical stimuli to the central cornea. The temperature of the stimulus was set at 50°C, which translated to 33°C at the ocular surface.³⁴ The mechanical stimuli consisted of a series of air pulses with flow rate ranging from 0 to 200 mL/min. The 5-mm distance between the ocular surface and the tip of the esthesiometer probe was continuously monitored by a calibrated video camera. The psychophysical procedure, stimulus modality, and stimulus duration were automated using custom software and a button box was used to record the subjects' responses when measuring thresholds. 

Trial Lenses and Visual Conditions

Trial lenses were placed in a lens holder in front of the nontest eye to provide the necessary visual conditions for the study. For the clear visual condition, a +0.25DS trial lens was placed over the subject's distance refractive correction, and for the defocused condition an additional +4.00DS was used. 

Procedures

Measurements were conducted on one eye (randomly chosen), while the fellow eye viewed a 6/60 fixation target through a trial lens from a distance of 3 m. The mechanical threshold of the central cornea was determined using the ascending method of limits³⁵ and the absolute threshold was the average of three reports of stimulus detection. Then mechanical stimuli that were 25%, 50%, 75%, and 100% greater than the threshold were presented in random order. Participants rated the intensity of pain evoked by the pneumatic stimuli (henceforth referred as intensity) and the resultant sensation of discomfort (henceforth referred as discomfort) using a 0 to 100 numerical rating scale. For the intensity scale, 0 corresponded to ‘no pain' and 100 indicated ‘most intense imaginable,' while for the discomfort scale, a rating of 0 indicated ‘no discomfort' and 100 indicated ‘worst discomfort imaginable.' Suprathreshold stimuli were presented three times under clear and defocused visual conditions, with the order of condition being random. 

Data Analyses

Statistical analysis was performed using Statistica 9.0 (Statsoft Inc., Tulsa, OK, USA). Statistical significance was set at P less than or equal to 0.05 for all the tests. The numerical ratings of intensity and discomfort under the two visual conditions were compared using repeated measures ANOVA. 

Nonlinear regression using the method of least squares was used to estimate the relationship between the ratings of intensity, discomfort, and the respective pneumatic stimulus strengths. The exponent ‘n' and the scaling constant ‘b' were derived for each subject by the fitting the data to the power function y = bxⁿ (where y is the sensation magnitude, b is the scaling constant, x is the stimulus strength, and n is the power exponent). Paired sample t-tests were used to compare the absolute thresholds and the differences in the size of the power exponents between clear and defocused visual conditions. Unpaired t-tests were used to test for differences in the size of the exponents between lens and nonlens wearers. 

Results

The absolute thresholds of the central cornea for mechanical stimuli were not statistically significantly different between clear and defocused conditions (P = 0.66). The mean difference between the absolute thresholds was 0.55 mL/min, 95% confidence interval (CI), −3.22 to 2.11. As expected, there was a significant main effect of stimulus strength on the ratings (P < 0.001); a progressive increase in rating score was observed with an increase in stimulus strength. However, there was no statistically significant main effect of the visual condition on the ratings of discomfort (P = 0.54) and intensity (P = 0.30). Comparison between intensity and discomfort showed the mean ratings of intensity to be higher than the mean ratings of discomfort under conditions of both clear vision (P < 0.001) and defocus (P < 0.001). 

When ratings of discomfort and intensity were analyzed separately (Figs. 1, 2), there was no statistical difference between clear and defocused conditions across all suprathreshold stimulus strengths for both discomfort (P = 0.10, Fig. 1) and intensity (P = 0.075, Fig. 2) ratings. Similarly, when clear vision and defocus conditions were examined separately (Figs. 3, 4), the relationship between rating type (discomfort/intensity) and suprathreshold stimulus strengths was not statistically significantly different under clear vision (P = 0.85, Fig. 3), and defocus (P = 0.70, Fig. 4). 

Figure 1

 

Mean ratings of discomfort for different pneumatic stimulus strengths under conditions of clear and defocused vision. Vertical bars denote 95% CI of mean ratings.

Figure 2

 

Mean ratings of intensity for different pneumatic stimulus strengths under conditions of clear and defocused vision. Vertical bars denote 95% CI of mean ratings.

Figure 3

 

The relationship between systematically increasing strengths of pneumatic stimuli and mean ratings of discomfort and intensity under clear visual condition. Vertical bars denote 95% CI of mean ratings.

Figure 4

 

The relationship between systematically increasing strengths of pneumatic stimuli and mean ratings of discomfort and intensity under defocused condition. Vertical bars denote 95% CI of mean ratings.

The relationship between the suprathreshold stimulus strengths and their subjective ratings was monotonic and nonlinear regression was used to estimate the exponents defined by Stevens' power law. The mean power exponents obtained for clear and defocused conditions are presented in Table 1. There was a statistically significant difference between clear and defocused condition in the derived exponents for discomfort (P = 0.05; mean difference = −0.12; 95% CI, −0.20 to 0), while no statistical difference was observed for intensity (P = 0.22; mean difference = −0.08; 95% CI, −0.23 to 0.05). Comparison between the exponents for discomfort and intensity showed significant differences between the two under both clear (P = 0.02, mean difference = −0.14, 95% CI, −0.24 to −0.04) and defocused conditions (P < 0.001; mean difference = −0.18; 95% CI, −0.26 to −0.08). 

Table 1.

 

Mean Power Exponents for Clear and Defocused Visual Conditions

 

There were no significant differences between lens and nonlens wearers in both clear and defocused visual conditions when the size of the exponents were compared. Tables 2 and 3 show the mean exponents of discomfort and intensity for lens and nonlens wearers under conditions of clear and defocused vision. 

Table 2.

 

Mean Power Exponents of Discomfort in Lens and Nonlens Wearers Under Clear and Defocused Vision

 

Table 3.

 

Mean Power Exponents of Intensity in Lens and Nonlens Wearers Under Clear and Defocused Visual Conditions

 

Discussion

There have been reports that when vision is not clear, discomfort occurs³² (Papas B, et al. IOVS 2003;44:ARVO E-Abstract 3694). This study explored the association between vision and ocular discomfort by evaluating how people scale suprathreshold pneumatic stimuli when vision is clear and defocused. We and others^22,36,37 have shown that suprathreshold scaling using mechanical, chemical, and thermal stimuli can evoke intensity and/or unpleasant (affective) stimulus attributes on the ocular surface. Generally, the estimated magnitude of sensation is proportional to the magnitude of the stimulus applied^{16,21,36,38,39}; as stimulus intensity increases, so reports of perceived intensity or perceived discomfort increases systematically as a power function (Stevens' power law³). Though previous studies derived the power exponents of the psychophysical magnitude (transducer) functions for the ocular surface, the perception of sensations when vision is defocused was not examined. Our study demonstrated that the transducer functions for discomfort are different when vision is clear and defocused, despite statistically nonsignificant differences in ratings of discomfort and intensity. The implications of the differences in exponents (though statistically just significant) point to the influence of affective pain mechanisms that are distinct from the processing of simple sensory ‘pain intensity,’ and a possible higher order integration of pain and vision. Interestingly, another study on heat pain suggests that sensory-discriminative (pain intensity) and affective-motivational (discomfort) components of pain can have transducer functions with different exponents.⁴⁰ 

The power exponent in a transducer function describes the relationship between the sensation magnitude (ratings) and the stimulus magnitude (pneumatic stimulus strengths).² Exponents greater than 1.0 suggest an accelerating function while exponents less than 1.0 are decelerating functions. The exponent for discomfort as observed in this study is 1.08 for defocused condition, while it is 0.96 for clear visual condition. Although these are both close to 1.0 (a linear relationship between stimulus and intensity), this difference in exponents might reflect the subtle changes in the operating characteristics of the sensory system when the visual conditions are changed from clear to defocused. 

Pain is multidimensional, consisting of sensory, affective, and cognitive dimensions.⁴¹ The sensory aspects of pain deal with intensity, locus, duration, and quality of pain, while affective pain reflects unpleasantness (discomfort), emotions, and motivational variables.⁴² In our study, in addition to the sensory input by the pneumatic stimuli, there can be potential contributions from affective and cognitive factors such as attentional focus, stimulus context, and expectation of pain that can cause discomfort with defocused vision. The probable influence of these factors on the sensory transducer functions are discussed below based on the mechanisms of pain processing and pain perception. 

When multiple stimuli are presented to a sensory system, selective attention occurs to filter out unwanted information and focus attention on a particular stimulus attribute.⁴³ For example, in the visual system, when a cluttered natural scene is presented to the eye, selective attention mechanisms helps to resolve the competition among the multiple objects in the scene in favor of the object that is relevant for the current task.⁴⁴ Furthermore, identification of a single object/attribute yields better performance than a task of judging two or multiple attributes. In an experiment where subjects were presented with two different objects, and asked to identify two different attributes (color of one, orientation of other) at the same time, the subjects' performance was worse than the task if performed with only a single object.^45,46 In our experiment, when two sensory conditions (pneumatic stimulation and defocus) are presented together to the subject, the neuronal activity may compete for representation in the brain, and the pattern of activity might have been different than when only one primary stimulus (pneumatic stimulation and clear vision) is present. These differences in neural activity levels may be reflected through the differences in sensory transducer functions. In contrast, distraction, a process of attending to information unrelated to the painful stimulus can also alter the perceived intensity or emotional reaction to a painful stimulus.⁴⁷ Studies^48–50 designed to investigate pain while manipulating attention suggest that subjects rate pain lower when they direct their attention away from the painful stimulus. Similarly, measurement of intensity and unpleasantness of thermal pain under different conditions of attention, suggests that pain is perceived to be more intense and unpleasant when subjects focused attention on noxious heat stimuli than when attention is diverted from the stimuli.⁵⁰ In our experiment, it is possible that differences in the exponents of the power functions under clear and defocused conditions arose for similar reasons: subjects' attention to the fixation targets were subtly different when vision was clear or defocused. 

The context in which a stimulus is presented can alter the experience of pain through warning signals (temporal context),^51–54 subject's visual attention,^55,56 and knowledge of tissue damaging properties of the stimulus (evaluative context).⁵⁷ It is known that the warning of noxious signals through cues, and the expectation of pain that occurs due it can amplify pain perception. For example, when a stimulus is preceded by a cue that denotes high intensity, the noxious input is found to hurt more than when it is preceded by a stimulus that denotes low intensity.⁵⁴ In our study, defocus perhaps produced a nocebo like effect and acted as cue for increased discomfort. The nocebo effect is a phenomenon where anticipation and expectation of a negative outcome induces worsening of a symptom.⁵⁸ Neuroimaging studies have shown the neural mechanisms responsible for the expectations of pain interact with the regions involved in afferent nociceptive processing and change the perception of pain.^54,59,60 Activation of the anterior cingulate cortex (ACC), the prefrontal cortex (PFC), and the posterior insula (PI) occur during anticipation of pain,^51,61 and these areas are also involved in the processing of afferent sensory information.⁶² To complement the findings by neuroimaging techniques, there is pharmacological evidence^63,64 that anticipatory anxiety can activate the cholecystokinin A and B receptors that facilitate pain transmission. These neurobiological findings provide potential mechanisms for how defocus can influence affective pain mechanisms and alter the perception of discomfort. 

We do not have neurophysiological or anatomical evidence that vision and ocular surface somatosensation can interact. Vision and pain have separate receptors and pathways, and different regions of the brain are responsible for the perception of vision and pain.⁶⁵ In addition, the lack of electrophysiological (or other) evidence about the sensory integration of vision and pain highlights the difficulty in meaningfully ascribing a physiological mechanism to relate pain/discomfort and vision in neural terms. However, studies on olfaction^66–68 and audition^69–71 show integration between these senses and vision, and the visual cortex may be activated during a purely olfactory task suggesting the possibility that a sensory process may be influenced by processing of other unrelated sensory information in the brain.^67,72 

The human brain can process incoming sensory information through modality specific channels from which a unified and coherent perceptual experience is normally derived. This synergy or interaction among the senses and the fusion of their information, ‘multisensory integration,' occurs through the convergence of information from different sensory systems on a common group of neurons. Sensory inputs from the visual, auditory, and somatosensory systems transmitted through the ascending sensory pathways and descending projections from the cortex are found to converge in various combinations on the superior colliculus (SC) neurons of the midbrain.^73–75 The ability of the SC to integrate multisensory information further depends on higher order processes such as the inputs from a small area of association cortex, the anterior ectosylvian sulcus that determines the final unified experience. The extent to which these multisensory mechanisms can be generalized to the results in our experiment, and the integration between vision and somatosensation is yet to be understood. However, recent work on synesthesia does provide evidence that disparate senses (previously believed to be not connected in any meaningful physiological or perceptual way) can interact in some subjects,^76,77 suggesting at least that potential visual-somatosensory interactions are not totally ludicrous. 

Another interesting finding in this study was that the average ratings of intensity and discomfort were dissimilar, and pain intensity was rated higher than discomfort. Intensity and pain unpleasantness are two distinct dimensions of pain^50,78 and they can have different relationships to the nociceptive stimulus. Other investigators^79–81 have also demonstrated that pain descriptor scales can provide independent measures of the pain dimensions and it is possible to alter one dimension and leave the other unchanged. 

The power exponents obtained by suprathreshold scaling were different under varying visual conditions, in contrast to the results of the repeated measures ANOVA that suggested that the ratings of discomfort and intensity do not change with differing conditions. The inability of repeated measures ANOVA to detect differences may be because defocus does not alter discomfort or intensity ratings, or perhaps the sample size was too small to uncover potential differences in ratings that may exist when the visual conditions are changed. The power functions characterized each subject's sensation at different stimulus intensities, whereas the repeated measures ANOVA assessed the mean ratings under the two viewing conditions at different (nonquantitative) stimulus levels. The power exponents were derived for each individual participant and a paired t-test was performed to investigate the differences in exponents between clear and defocused conditions, and the details in the subjects' transducer functions appear to provide more sensitive information for this experiment on the processing of ocular surface sensations than just the comparison of means. 

Studies that examine the impact of contact lens wear on ocular surface sensitivity show equivocal results, with some reporting a difference in thresholds, while some report no differences.^82–88 These studies primarily report sensory detection thresholds with contact lens wear and there are no reports of how contact lens wearers respond to suprathreshold stimulation of the ocular surface. However, it has been demonstrated that contact lens wearers discriminate suprathreshold pneumatic stimuli differently than nonlens wearers.⁸⁹ We do not know how these discrimination differences can be related to the scaling of pneumatic stimuli using magnitude estimation techniques, but it can point to probable differences in operating characteristics that can exist between lens and nonlens wearers. In our study, no statistical differences in sensory transducer functions were found between lens and nonlens wearers. This finding should be interpreted with caution, as the sample size was not balanced between the lens- and nonlens-wearing groups and there could be potential effects uncovered due to the lack of sample size. 

In summary, the results of the study indicate that the operating characteristics of the ocular surface transducer functions are subtly different under clear and defocused visual conditions, suggesting that defocus may alter the experience of ocular comfort. The differences in the perception of discomfort does not appear to be attributable to the differences in detection threshold or sensory intensity, but from the influences of affective pain mechanisms or a probable higher order sensory integration between vision and pain. Ratings of pain intensity and discomfort have different relationships to the magnitude of pneumatic stimuli, reflecting the different dimensions of pain mechanisms. 

Acknowledgments

Supported by an operating grant from Natural Sciences and Engineering Research Council of Canada (Ottawa, Ontario, Canada). 

Disclosure: S. Basuthkar Sundar Rao, None; T.L. Simpson, None