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Y. Feng, T.L. Simpson; Interaction of Corneal Thermal, Mechanical and Chemical Stimuli at Threshold Measured using a Belmonte Esthesiometer . Invest. Ophthalmol. Vis. Sci. 2003;44(13):3710.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:To investigate the interaction of three modes of stimuli on human corneal thresholds (and the induced sensations). Methods:Five healthy subjects (30-38 years) participated in this experiment. A computer controlled Belmonte pneumatic esthesiometer presented mechanical and chemical stimuli at different temperatures. Threshold was estimated from a logistic psychometric function fitted to data collected using the method of constant stimuli. These stimuli (air with different proportions of added CO2) were presented at 20 (room temperature), 30, 40, 50°C. The mean flow rates were 36, 46, 56, 66 and 76ml/min and the mean CO2 proportions were 20, 30, 40, 50 and 60%. Results:Without added CO2, the mechanical thresholds (±SE) at the four temperatures were 49.9±6.0, 57.3±5.7, 68.0±8.0 and 73.3±9.4ml/min (p=0.001). Post-hoc testing showed 20° thresholds were different from 40°C and 50°C (p<0.05) and 30°C different from 50°C (p<0.05). The sensation of "cooling" or "cold" reported by subjects at low temperature (20°C) gradually changed to irritation at higher temperatures. There were no significant differences in chemical thresholds (CO2) from 20 to 50°C (p=0.620). There were strong interactions when chemical and mechanical stimuli were added. Chemical thresholds were different when the flow rate increased (p=0.002) and mechanical thresholds were altered as the % added CO2 increased (p=0.000). Post-hoc tests showed generally that chemical thresholds were reduced as flow rate went up and (similarly) that mechanical (flow) thresholds were reduced when % added CO2 increased. Conclusions:This study suggests that without CO2, stimuli at different temperatures excite different receptors and/or psychophysical channels, specifically, non-noxious cold (cold receptor mediated) at room temperature and mechanical nociceptive channels when the stimulus temperature was higher. The chemical (CO2) stimulus also excites a nociceptive (chemical receptor mediated) channel. The linearity of the effect of flow rate on chemical sensitivity and, similarly, chemical composition on mechanical sensitivity points to chemical and mechanical stimuli not being independent.
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