Investigations on the absolute thresholds of the cornea and conjunctiva,
14,16–19,31,32 and the ocular surface response to suprathreshold stimulus of different modalities demonstrate that the pneumatic esthesiometer can be effectively used to study the sensory processing on the ocular surface.
17,37–39 Our study in addition provided important insight about how the ocular surface detects
differences in stimulus intensities. The results of the study indicated that the difference thresholds of the central cornea can be successfully measured by using a pneumatic esthesiometer and there can be differences in how lens wearers and nonlens wearers appreciate stimulus changes. Understanding this functioning is vital to complete the characterization of ocular surface sensory processing. Detection and discrimination can be scientifically hypothesized to involve separate neural mechanisms inasmuch as the decision about a detection of stimulus is in the “absence” of a background or only in the presence of (usually) low levels of background noise, while a different decision is required to detect a stimulus when the background is more prominent. For example, the signal to noise ratio in the former (detection) condition is almost infinite, whereas with a substantial background, the signal to noise ratio is finite. On the other hand, detection and discrimination can be described as identical (detection of stimulus increment against physiological background or physiological plus additional stimulus background). Regardless of one's point of view about detection and discrimination, almost nothing in the world occurs
only at detection threshold levels. This truism exists for any sensory processing system
40 and therefore, a complete description of how the ocular surface sensory system performs cannot be done without understanding discrimination. Finally, since any treatment is presumably going to be developed to treat uncomfortable/painful conditions (e.g., dry eye), every sensory change (e.g., a decrement in symptoms) is a discrimination judgment (i.e., an improvement of existing symptom intensity), and understanding discrimination is imperative in order to quantify these potential therapeutic effects.
The Weber's fraction is a useful index of sensory discrimination for comparison across different modalities and stimulus conditions. In this experiment, the Weber's fraction (
ΔΦ/
Φ) approaches a constant value for high intensities and increases rapidly for low-stimulus intensities close to the absolute threshold. A similar phenomenon has also been observed in the discrimination of auditory tones
5 and tactile vibration.
10 In a study
5 that has investigated the intensity discrimination of auditory tones, the Weber's fraction rapidly decreases with an increase in stimulus intensity and the fraction gradually decreases without becoming a constant. This deviation from the Weber's law for low-intensity stimuli is known as the “near miss” of the Weber's law.
6 Discrimination experiments examining loudness and noise
7 also have demonstrated the Weber's constant to be higher with stimulus intensities closer to threshold that became a constant with increasing levels of intensity. The increase in Weber's constant toward the lower-intensity stimuli can be due to the presence of noise in the stimulus or sensory/neural noise, fluctuations in the activity of the neurons that carry signals from the ocular surface to the central nervous system,
1 which perturb the background sufficiently at low levels to interfere with discrimination thresholds and so elevate Weber's fraction. The background noise can also be present in the absence of the stimulus
41 and for a stimulus to be above
detection threshold, the ocular sensory system would need to respond strongly enough for the sensation to be distinguishable from the activity in the absence of the stimulus (sensory noise). This theoretical interpretation aside, it is clear from the data in our experiment that the ocular surface tissue is another place where Weber's law also holds.
The average difference thresholds were higher in lens wearers than nonlens wearers although there was no difference in absolute detection threshold between the two groups. Recent studies examining ocular surface sensitivity in silicone hydrogel contact lens wearers have demonstrated no changes in sensitivity
18 or an increased sensitivity
29 with lens wear. Contact lenses have been hypothesized to depress the sensitivity of the cornea by the mechanical effects of the lens against the ocular surface or cause a change in corneal physiology perhaps due to hypoxia,
28 and also because of lens wear effects on the tear film.
42–44 Any or a combination of these factors might alter the equilibrium of the ocular surface, causing a change in sensory input in contact lens wearers, possibly increasing the neural noise in lens wearers and thereby giving rise to increased Weber's constants. The shift in neural activity levels between lens and nonlens wearers can also be observed in the bilinear fit of the pooled data as illustrated in
Figures 3a and
3b. The relatively fewer data points for stimulus intensities higher than 100 mL/min in lens wearers might have been because only 5 DLs were measured for each participant or perhaps, lens wearers were able to judge intensity discrimination between closely spaced stimulus intensities, whereas nonlens wearers required a higher change in intensity to appreciate a difference. Despite this apparent limit, there still were two linear components to the bilinear fits as illustrated in
Figure 3b. Another potential explanation for the observed difference in lens wearers is that the discrimination of pneumatic stimuli itself may be processed differently in the nervous system of lens and nonlens wearers owing to the differences in ocular surface physiology.
Suprathreshold sensory processing on the ocular surface has been shown to be different for mechanical and chemical stimulus modalities,
17,31,37 and with varying locations (Situ P, et al.
IOVS 2007;48:ARVO E-Abstract 5387) but the influence of contact lens wear on suprathreshold stimuli is yet to be studied. At suprathreshold levels, as in discrimination of pneumatic intensities, the stimulus comparisons are based on the differences in the sensation magnitude of two stimuli, whereas absolute threshold involves the difference between no sensation and presence of a sensation (i.e., the detection of stimulus from the background), and therefore it is not unreasonable that discrimination and detection could be affected differently in lens wearers.
Not finding a statistical difference in absolute thresholds between lens- and nonlens-wearing groups might be because there are no differences. Pneumatic esthesiometry has been used to examine detection thresholds and the results are equivocal, with some experiments finding no difference and others finding differences in one direction or the other.
20–22,25,27 No difference uncovered could also be an error, not revealed because we did not have statistical power in the experiment to uncover this effect, partly owing to the sample sizes. We did not use detection thresholds to estimate sample size since this outcome variable is not integral to our experiment and hypotheses. We measured it in order to scale suprathreshold discrimination stimuli appropriately and we tested detection threshold differences to determine if we were able to demonstrate differences statistically, but it is an ancillary outcome. In other sensory dimensions,
11,45 dissociation between detection and discrimination thresholds has been reported. For example, difference thresholds have been found to be similar even though there were detection threshold differences. Therefore, even if the absence of a statistical difference between absolute thresholds is actually a type II error, it is not self-evident that the difference in discrimination thresholds is also an error arising from differences in stimulus detectability. In addition, the increased variance in detection thresholds in nonlens wearers might also have contributed to a smaller effect size (and therefore nonsignificant difference in detection thresholds between the two groups), while indeed there were actual differences existing between them (i.e., a type II error). The origin of this variance difference between groups is unclear and could itself perhaps be simply due to sampling differences rather than being meaningful physiologically.
The clinical consequences of differences in discrimination thresholds is by no means clear, but it would suggest that contact lens–adapted subjects would in general be less able to tell differences apart than those not wearing lenses or require greater change in stimuli than those not wearing lenses to detect differences. For example, as many as half of lens wearers report discomfort toward the end of the day
46,47 and the data in this study suggest that they perhaps have “adapted” to be more tolerant of uncomfortable stimuli so that greater shifts in uncomfortable stimuli would be required for them to report change. This reduced ability to detect change is also complicated by the phenomenon reported in symptomatic lens wearers who show less suprathreshold adaptation than asymptomatic lens wearers.
48 Also, lens wearers might be less able to differentiate comfort (or other sensory effects) when wearing different lens types.
In summary, we determined the difference thresholds of the ocular surface by using a pneumatic esthesiometer. In addition, ocular surface sensory processing is altered by contact lens wear, as exhibited by the differences in discrimination thresholds for pneumatic stimuli, with lens wearers demonstrating lower discrimination sensitivity. The importance of this difference needs to be established in larger groups of subjects and in those with underlying differences other than controls and lens wearers, but these results do point to a novel neural metric that might be used when trying to understand ocular surface sensory processing in a fuller context.