Impact of Blur on Suprathreshold Scaling of Ocular Discomfort.

Purpose: To examine the suprathreshold scaling of pneumatic stimuli, and the ratings of discomfort and intensity under clear and defocused visual conditions.

Methods: Twenty-one participants rated sensory intensity and discomfort of a series of mechanical stimuli from a pneumatic esthesiometer, using a 0 to 100 numerical scale under clear and defocused visual conditions. Esthesiometry was performed on one eye while the fellow eye viewed a 3-m distant 6/60 target through a trial lens. For the clear visual condition, a +0.25DS lens was used over the subject's refractive correction, and for defocus, an additional +4.00DS was used. Central corneal mechanical thresholds were first estimated using ascending methods of limits. Then, stimuli that were 25%, 50%, 75%, and 100% above threshold were presented in random order in three sessions of clear and defocused vision, and subjective ratings were recorded. Power exponents that define the slope of the sensory transducer functions were derived for discomfort and intensity estimates.

Results: No significant differences (P = 0.66) in mechanical thresholds, ratings of discomfort (P = 0.54), and intensity (P = 0.30) were observed between the visual conditions. Power exponents for discomfort showed significant differences (P = 0.05) between clear and defocus conditions, but not intensity (P = 0.22). Comparison between discomfort and intensity showed differences in exponents when vision was clear (P = 0.02) and defocused (P < 0.001).

Conclusions: Scaling of suprathreshold pneumatic stimuli varies with viewing conditions. When vision was not clear, the exponent of the average transducer function for discomfort was steeper and this finding is the first demonstration of an association between ocular surface sensation and quality of vision.