The minimum stimulus energy required to produce a cooling sensation in thehuman cornea

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 (N CCA) was analysed using a thermal imaging camera. Assuming that the cornea behaves as a near perfect black-body, the threshold for energy loss was cal culated by combining Stefan-Boltzmann's law and Kirchoff's law for imperfec tions in black-body radiation, taking into account the likely thermal condu ctivity 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 me an of 33.2-32.9 degreesC (i.e. 0.3 degreesC), over a 1 mm diameter circular area (0.785 mm(2)), 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. (C) 2001 The College of Optometrists. Published by Elsevier Science Ltd. All rights res erved.