Critical flicker/fusion frequency (CFF) is the transition point for an intermittent light of increasing temporal frequency, where the flickering ceases and the light is perceived as continuous. If the light's modulation depth is high, then CFF is the temporal equivalent of a visual acuity measurement. CFF measurements for the diagnosis of eye diseases have been reported for more than one century.^1-7

Our CFF stimulus (Figure 1) consists of a red light-emitting diode (LED) with a peak wavelength of 625 nm, mean luminance of 750 cd/m², modulation depth of 98%, and an angular subtense of 1.5º at a distance of 38 cm. It is capable of emitting a frequency range from 1 to 86 Hz. To assess foveal CFF, a patient is asked to look directly at the LED, which is mounted at the center of a matte white rectangular screen of luminance 94 cd/m². Two red diagonal lines cross at the LED to help patients maintain central fixation on the LED target.

The patient is introduced to and made familiar with the flicker/fusion behavior of the LED target. This is followed by the CFF measurement, made by using a 2-second presentation of the CFF target at a fixed temporal frequency. The patient must decide whether the target is flickering or steady. To determine the thresholds, a method of limits collects three ascending and three descending frequency presentation staircase steps (with 2 Hz) in an alternating order. The fusion threshold is recorded as the mean of the three ascending runs, and the flicker threshold is the mean of the three descending runs. The mean of all six runs reveals the flicker/fusion threshold.

Patients find the test easy to perform. Measurements are made monocularly with natural pupil sizes and appear to be repeatable to within one staircase step.

The greatest advantage of CFF as a potential vision test is its robust resistance to the type of image degradation induced by a cataract,^1-4 even for visual acuities worse than 20/200.⁸ It appears to be sensitive to disorders causing visual impairment (eg, optic neuropathies, maculopathies).^2-9 The ability of CFF to predict visual acuity in patients with early macular degeneration may be limited, however.⁸

The measurement of temporal resolution—which is then used to predict spatial resolution—must be viewed with caution due to the relative independence of channels for temporal and spatial processing. The temporal resolution quantified by the CFF must also undergo a mathematical conversion if it is to be a predictor of visual acuity. Inevitably, the precision of the conversion will influence the prediction.

Despite this limitation, CFF scores accurately predict the postoperative visual acuity in pre- and postcataract surgery studies.^9-10 Del Romo et al⁹ compared the predictive ability of an ophthalmologic judgement, based on history and ocular examination, with the results of the Potential Acuity Meter (PAM; Haag-Streit, Köniz, Switzerland), the retro-illuminated pinhole, and a CFF device. In patients with moderate cataract and normal fundi, the ophthalmologic judgement accurately predicted postoperative visual acuity, and it performed better than the potential vision test. Ophthalmic judgement was less accurate in patients with comorbid eye disease and dense cataracts, where CFF provided a better visual acuity prediction than that obtained by ophthalmological judgement alone. Similarly, the CFF provided the best prediction of postoperative visual acuity in the presence of moderate-to-advanced cataract, both with and without comorbidity, when compared with the PAM, the laser interferometer, and optimal reading speed techniques.¹⁰

The ability of CFF to detect abnormality may be enhanced by placing the LED at various points in the visual field, however, this would increase the time of the examination. It must be noted that the patient task becomes increasingly difficult as the LED is moved away from fixation.

In summary, the CFF appears to offer promise as a potential vision test in moderate-to-dense cataract with and without comorbidity.

William A. Douthwaite, PhD, is from the Department of Optometry, University of Bradford, in the United Kingdom. Professor Douthwaite states that he has no financial interest in the products or companies mentioned. He may be reached at W.A.Douthwaite@bradford.ac.uk.

Marta Vianya-Estopa, PhD, is from the Department of Optometry, University of Bradford, in the United Kingdom. Dr. Vianya-Estopa states that she has no financial interest in the products or companies mentioned. She may be reached at M.Vianyaestopa@bradford.ac.uk.

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