Pupil size and critical flicker fusion threshold: A reevaluation

Abstract

A commonly used method of evaluating critical flicker fusion (CFF) threshold—the Leeds psychomotor tester—has been assessed with regard to the influence of pupil-size alteration on CFF measurements. Six healthy volunteers, (three males, three females) took part and were treated with saline, pilocarpine 2%, and homatropine 1% eye drops at intervals of one week according to a balanced crossover design. The regression of CFF threshold on pupil size, both measured up to 7 hr after treatment, was highly significant in all subjects. Increases of pupil size from 2 to 8 mm produced a rise in CFF threshold which varied between individuals but ranged from 2 to 9 Hz. In a separate study, the effect of clobazam 20 mg on binocular and monocular CFF, the latter both uncorrected and corrected for pupil size, was assessed over the 8-hr period after treatment. No significant changes in either measure of CFF were observed. On a separate study day the same four subjects submitted to mydriasis/miosis induced by homatropine followed by pilocarpine eye drops. This procedure showed that the use of an artificial pupil of 2 mm adequately corrected for pupil-size influences on CFF in all but one subject.