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The minimum stimulus energy required to produce a cooling sensation in the human cornea
P J Murphy1, S Patel, P B Morgan
1Department of Vision Sciences, Glasgow Caledonian University, Scotland, UK. p.murphy@gcal.ac.uk
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.