Critical flicker fusion is the rate of flicker at which we perceive no flicker.The clinical evidence is not sufficient to permit conclusions on the health outcome effects of critical flicker fusion in the diagnosis of visual acuity.
In 2007, Shankar and Pesudovs, in continuing to develop a potential vision test based on the CFF phenomenon by using a brighter stimulus and optimizing its size, reported the results of a prospective nonrandomized study in which 225 participants were assigned to 1 of 4 groups: normal, media opacity only, retinal/neural disease only, and cataract plus retinal/neural disease. The CFF thresholds were measured for 3 stimulus sizes: 0.5 degree, 1.0 degree, and 1.5 degrees. Discrimination between groups was tested by analysis of variance and receiver operating characteristic analysis. The relationship between visual acuity and CFF in eyes without media opacity was determined by linear regression and used to predict visual outcomes in 23 eyes having cataract surgery. The mean age of the 225 participants was 71.4 years +/- 13.2 (SD); 134 (59.8%) were women. The normal group had 41 participants, and the other 3 groups had 61 participants each. Critical flicker fusion thresholds were reduced in retinal/neural disease but resistant to image degradation from media opacity. The 1.5-degree stimulus had 88% sensitivity and 90% specificity for discriminating groups. Visual acuity after cataract surgery was accurately predicted within +/-1 line in 43% of eyes, +/-2 lines in 83%, and +/-3 lines in 100%. All eyes with poor visual acuity (>0.50 logMAR) or dense cataract (>4.0 Lens Opacities Classification System III) were predicted within +/-2 lines. According to the authors, the CFF phenomenon effectively discriminated between subjects with and without retinal/neural disease and accurately predicted visual outcome after cataract surgery. The use of a brighter stimulus enhanced performance in cases of dense media opacity.
Vianya-EstopÃ and group, 2004, determined whether CFF thresholds fulfill the criteria for a potential vision test (PVT) by being unaffected by media opacity yet affected by retinal disease. CFF thresholds for three different stimulus sizes (0.5, 1.0, and 1.5 degrees ) were measured in 72 patients (mean age, 78.43 +/- 7.07 years) comprising 31 subjects with media opacity, 21 with macular disease, and 20 with pseudophakia. There were no statistically significant differences between CFF values from the media opacity and the pseudophakia groups for any target size (p > 0.10). However, CFF values were significantly lower in patients with macular disease for all the target sizes (p < 0.05). Analysis of a subset of six subjects with media opacity and seven subjects with macular disease and visual acuity of 20/200 or worse showed the media opacity group still had similar CFF values as the pseudophakia group and had significantly higher CFF than the macular disease group.
In 2006, Del Romo and colleagues reported the details of the clinical utility of CFF as a PVT (which attempts to predict the visual outcome that might be expected as a result of a cataract operation). CFF thresholds were determined in 31 subjects with age-related idiopathic cataract and no other eye disease, 19 subjects with macular disease (MD) and clear ocular media, and 24 age-matched control subjects. In addition, the CFF technique was administered before cataract surgery in 52 patients and compared with the information provided by presurgical case history and ocular examination alone (ophthalmological judgment [OJ]) and results from two commonly used PVTs (the retroilluminated pinhole and the potential acuity meter). CFF thresholds obtained in the nonsurgical cataract group were unrelated to cataract severity and were similar to those in the control group. In contrast, CFF scores were significantly related to visual acuity (VA) in the MD group. In the pre- and postsurgical studies, OJ predicted postoperative VA very well in patients with moderate cataract and normal fundi and better than all the PVTs. OJ performed less well in patients with comorbid eye disease and dense cataracts, when information from the PVTs would probably have been useful. CFF provided the most accurate predictions of postoperative VA in the small sample of patients with dense cataracts. The authors concluded, “CFF was unaffected by cataract, yet sensitive to MD, and provided useful information about the postoperative visual outcome beyond that obtained through history and ocular examination in patients with dense cataracts.”
Vianya-EstopÃ et al, 2006, evaluated the usefulness of a battery of PVTs including potential acuity meter (PAM), laser interferometer (LI), critical flicker/fusion frequency (CFF), superilluminated pinhole at distance (SPH(d)) and near (SPH(n)), and optimal reading speed (ORS) by their independence of the effects of cataracts and sensitivity to MD in 76 patients with age-related cataract and no other eye disease, 52 patients with MD and clear ocular media, and 28 patients with normal, healthy eyes. Potential vision tests were independent of the degrading effects of cataract up to a visual acuity (VA) level of 20/200 or worse (CFF), 20/125 (ORS and SPH), and 20/40 (PAM and LI). A high degree of association was found between PVT scores and distance VA in the MD group for SPH(d) (r2 = 0.93), SPH(n) (r2 = 0.89), and PAM (r2 = 0.71). A moderate correlation was found for LI (r2 = 0.55), CFF (r2 = 0.50), and ORS (r2 = 0.45). The authors concluded, “in regard to critical flicker/fusion frequency that while the test showed the greatest ability to bypass cataracts, its ability to predict VA in patients with early MD was limited.”
In a 2003 article, Trick writes that the temporal processing capability of the visual system has historically been evaluated by measuring sensitivity to flicker. The sensation of flicker occurs when light stimulation is intermittent rather than continuous. In general, as the flicker frequency increases, a point is reached at which the perception of flicker is lost and the light appears continuous or fused. This point is known as the critical flicker fusion (CFF) frequency. Trick wrote that CFF is one of many techniques and tools available to measure aspects of visual function that extend beyond visual acuity.