The minimum stimulus energy required to produce a cooling sensation in the human cornea.
OBJECTIVE
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.


CONCLUSIONS
The cornea possesses stimulus specific receptors, which have an exquisite sensitivity to a rapid, thermally cooling stimulus.
Murphy
P J
PJ
Department of Vision Sciences, Glasgow Caledonian University, Scotland, UK. p.murphy@gcal.ac.uk
Patel
S
S
Morgan
P B
PB
Marshall
J
J
D003080:Cold Temperature; D003315:Cornea / Q000294:innervation*; D006801:Humans; D008954:Models, Biological; D010812:Physical Stimulation / Q000379:methods; D012684:Sensory Thresholds; D013697:Thermosensing