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J. Chen, T. Simpson; Human Corneal Adaptation to Mechanical, Thermal and Chemical Pneumatic Stimuli. Invest. Ophthalmol. Vis. Sci. 2008;49(13):2562.
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Neural adaptation in somatosensory systems has been frequently described and has also been described in corneal neurons in cats and rabbits. However, there are no data on adaptation in human corneas and therefore, we designed this study to psychophysically investigate this after mechanical, thermal (cool) and chemical pneumatic stimuli (using a Belmonte esthesiometer).
20, 8 and 11 normal subjects were enrolled in the mechanical, cool and chemical experiments respectively. Mechanical pneumatic thresholds were estimated using an ascending method of limits, and chemical thresholds were estimated using the same psychophysical method when the flow rate was set at half of the mechanical thresholds. 3 intensities (sub-threshold, threshold and supra-threshold, in random order) were each presented at least 10 times and subjects scaled the intensity of the stimuli (0-4 [no stimulus - very intense stimulus]). Friedman non-parametric ANOVA was used to analyze the rating data.
There was measurable adaptation with both mechanical and cool stimuli. For both suprathreshold mechanical and cool stimuli, the earlier stimuli were rated more intense than subsequent ones (both p<0.05), although for each stimulus type, the average early intensity ratings were higher than those for subsequent stimuli. Paradoxically, for the chemical stimuli, we found adaptation for threshold stimuli (p=0.01), but no adaptation for subthreshold and suprathreshold stimuli (p=0.18 and 0.21 respectively).
Both mechanical (mechano-sensory or polymodal) and cold receptors on human corneas show adaptation to repeated suprathreshold stimuli with a reduction in perceived intensity after multiple exposures to the same physical stimulus intensity. This is in accord with the results found in electrophysiological and psychophysical experiments of somatosensation elsewhere in the body (and in other animals).The response to chemical stimuli is not as predictable as to mechanical and cool stimuli. This might indicate the existence of some silent nociceptive sensory afferents (subtype of polymodal nociceptors), which only can be activated by repeated suprathreshold chemical stimuli, and neutralize those classical polymodal nociceptors’ adaptation; or differences in the stimulus (the complex effects of the chemical stimulus in the tear film).
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