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Characteristics of Human Corneal Psychophysical Channels

From the Centre for Contact Lens Research, School of Optometry, University of Waterloo, Waterloo, Ontario, Canada.

PURPOSE. To characterize human corneal psychophysical channels.

METHODS. Twenty subjects participated in this study. A Belmonte pneumatic esthesiometer was used to deliver stimuli, and the ascending method of limits and the method of constant stimuli were used to estimate thresholds. Sensation was characterized for different stimuli. Corneal mechanical and chemical thresholds were measured at different temperatures.

RESULTS. The qualities of the sensations induced by stimuli with different temperatures were different, and the corresponding detection thresholds of the pneumatic stimuli at four temperatures gradually increased (repeated measures ANOVA (F_(3,12) = 10.326, P = 0.000). There were no temperature effects on chemical thresholds (repeated measures ANOVA F_(3,12) = 0.235, P = 0.870) or mechanical discomfort thresholds from 20°C to 50°C (paired t₍₁₄₎= –0.233, P = 0.818). There were strong interactions when chemical and mechanical stimuli were added. Chemical thresholds were progressively lower when the flow rate increased and mechanical thresholds went down as the percentage of added CO₂ increased (repeated measures ANOVA F_(3,12) = 6.407, P = 0.007, F_(4,16) = 19.904, P = 0.000).

CONCLUSIONS. This study suggests that humans sense corneal non-noxious cold and noxious mechanical and chemical stimuli, and that the sensitivity of some submodalities can be modulated by others. There are at least five psychophysical channels (non-noxious cold, noxious mechanical, noxious chemical-H⁺, noxious heat, and itching) processing corneal sensory information. Both decreased corneal chemical thresholds at high flow rates and decreased mechanical thresholds with an added chemical stimulation demonstrate that corneal psychophysical channels are not independent.

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