Human corneal adaptation to mechanical, cooling, and chemical stimuli

To psychophysically investigate adaptation in human corneas using the Belmonte pneumatic esthesiometer.

Twenty, 8, and 20 healthy subjects were enrolled in the mechanical, cool, and chemical experiments, respectively. Thresholds were estimated using an ascending method of limits and three intensities (subthreshold, threshold, and suprathreshold, in random order) were each presented 10 or 20 times, and subjects scaled the intensity of the stimuli (0-4 [no stimulus to very intense stimulus]). Friedman nonparametric 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 intensely than subsequent stimuli (both P < 0.05). However, this was not the case for subthreshold and threshold mechanical and cool stimuli (all P > 0.05). Paradoxically, for the chemical stimuli, there was adaptation to threshold stimuli (P = 0.03) but no adaptation for subthreshold and suprathreshold stimuli (P = 0.19 and 0.11, respectively).

Both mechanical (mechanosensory 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 was different, and this might reflect proximal and distal neural or stimulus-specific effects.