Stimulation of the corneal surface was performed with a gas esthesiometer.
3 The general procedure has been described elsewhere.
4 Briefly, gas jets of 3 seconds’ duration, of different flow, temperature, and composition were applied to the center of the cornea, separated by 2-minute pauses. For mechanical stimulation, nine pulses of air of variable flow between 0 and 264 mL/min (0, 33, 58, 83, 110, 138, 170, 208, and 264 mL/min) and heated to +50°C at the tip of the stimulus probe (to reach the corneal surface at a neutral temperature of 33–34°C), were applied. For chemical stimulation, eight pulses of air containing variable concentrations of CO
2 (0%, 10%, 20%, 30%, 40%, 50%, 60%, and 70% CO
2) also heated to +50°C at the probe tip and at flow values 10 mL/min below mechanical threshold were used. Thermal stimulation was obtained with 11 pulses of air at subthreshold flow rate and temperatures between −4.5°C and +85°C at the tip of the probe, that changed the basal temperature of the corneal surface (33–34°C) between −4.5° and +3°C (−4.5°C, −4°C, −3.5°C, −2°C, −1°C, 0°C, +0.1°C, +1°C, +1.75°C, +2.5°C, and +3°C).
4 In all modalities of stimulation, pulses of different magnitude were applied at random.