Purpose: To estimate the minimum stimulus energy required to stimulate the corneal nerves, using a thermally cooling stimulus.
Methods: The localised temperature change produced in the pre-corneal tear film by the air-pulse stimulus of the Non-Contact Corneal Aesthesiometer (NCCA) was analysed using a thermal imaging camera. Assuming that the cornea behaves as a near perfect black-body, the threshold for energy loss was calculated by combining Stefan-Boltzmann's law and Kirchoff's law for imperfections in black-body radiation, taking into account the likely thermal conductivity of the human cornea.
Results: The average normal subject has a threshold for detecting a cooling effect when the local corneal tear film surface temperature drops from a mean of 33.2-32.9 degrees C (i.e. 0.3 degrees C), over a 1 mm diameter circular area (0.785 mm2), and a stimulus time duration of 0.9 s. This gives rise to a sensation threshold for perceiving a loss of heat energy of 7.0 ergs, or 7.0 x 10(-7) joules.
Conclusion: The cornea possesses stimulus specific receptors, which have an exquisite sensitivity to a rapid, thermally cooling stimulus.