Measurement of difference thresholds on the ocular surface.

Purpose: To establish difference thresholds of the central cornea and compare thresholds between contact lens wearers and noncontact lens wearers.

Methods: Mechanical sensitivity of the central cornea was determined in 12 lens wearers and 12 nonlens wearers using a modified Belmonte pneumatic esthesiometer and method of limits. Then, a series of systematically increasing stimuli were presented, with the first stimulus being 25% less than threshold. Subjects compared intensity of each stimulus with the preceding one and reported if any difference in intensity was detectable. Intensities at which an increase was perceived from the prior stimulus were recorded and the difference between the intensities was the difference threshold (DL). Five DLs were measured for each subject. Weber's constants that relate the size of difference threshold to stimulus intensity were derived for each DL level. Repeated-measures ANOVA was used to compare Weber's constants between lens wearers and nonlens wearers.

Results: A significant main effect of DL level on Weber's constant (P < 0.001) was observed, with the first DL being higher than following DLs. Lens wearers had higher Weber's constants than nonlens wearers (P = 0.02) However, no interaction was found between DL level and group type on Weber's constants (P = 0.38).

Conclusions: Differential sensitivity of the ocular surface can be successfully measured with a pneumatic esthesiometer and it appears that Weber's law holds for corneal nociceptive sensory processing. There are subtle differences in mechanical difference thresholds between lens wearers and nonlens wearers, suggesting the possibility of different neural activity levels in the two groups.