NORMAL RELATIONSHIP BETWEEN LUMINOUS THRESHOLD AND CRITICAL FLICKER FUSION FREQUENCY

JAVIER RODRÍGUEZ, MÓNICA GARCÍA, MARTA GONZÁLEZ-HERNÁNDEZ and MANUEL GONZÁLEZ DE LA ROSA

Hospital Universitario de Canarias, Universidad de La Laguna, Spain

Abstract

Purpose: To establish the relation between differential luminous thresholds (DLT) and the critical flicker fusion frequency (CFF). Methods: Twenty-eight eyes of 28 healthy subjects, with previous perimetric experience, mean age 34.7 years (SD = 14.1) and a refractive error lower than 3 D (spherical equivalent), were examined twice using the Octopus 1-2-3 perimeter; once for measuring CFF with flicker perimetry (background 31.4 asb, stimulus Goldmann III at 4000 asb and 1 sec long), and the other with conventional DLT perimetry. TOP strategy and grid 32 were used in both cases. Results: A relationship of 1 dB = 1.27

± 0.03 Hz was obtained at the 74 points examined. The authors’ previous estimation had some local variability, with average deviations of 1.25 Hz from the normal CFF values, the deviations being underestimated in the peripheral areas and mostly in the upper field. The relationship is more uniform in the present study. The deviation of the real local mean value from the estimated mean value is 0.6 Hz. The RMS error value between the CFF and DLT on the 20-2 examined points was 7.9 Hz. The CFF age loss (0.075 Hz/year) was similar to DLT age loss (0.059 dB/year), with a ratio between them of 1.27. Conclusions: The authors found a tight functional dependence between DLT and CFF: 1 dB = 1.27 Hz, with the Octopus 1-2-3 and for the kind of stimulus used. This relationship between both physiological functions is more constant than has been previously reported, using different samples of subjects.

Introduction

TOP flicker perimetry has previously been reported by our group as a promising test for the early diagnosis of glaucoma.1-4 This technique uses an adaptation of the TOP program for the Octopus 1-2-3 and evaluates the temporal processing characteristics of the visual system at several locations in the visual field of patients.

In a preliminary study,1 we compared the normal values for conventional perimetry, given by Octopus manufacturers (Interzeag AG, Schlieren-Zürich), with those published for flicker perimetry, using the Octopus 1-2-3.3 We found an almost perfect relationship between differential luminous thresholds (DLT) and the critical flicker fusion frequency (CFF), that is: 1 dB = 1.25 Hz. This value was called the ‘dB-flicker

Address for correspondence: Manuel González de la Rosa, C/. 25 de Julio, 34, 38004. Santa Cruz de Tenerife, Spain. Email: mgdelarosa@jet.es

Perimetry Update 2002/2003, pp. 341–344

Proceedings of the XVth International Perimetric Society Meeting, Stratford-upon-Avon, England, June 26–29, 2002

edited by David B. Henson and Michael Wall

© 2004 Kugler Publications, The Hague, The Netherlands

equivalent’, and was used to develop a TOP strategy for flickering stimuli.

In this paper, our purpose was to re-evaluate this relationship using the same group of patients.

Material and methods

Twenty-eight eyes of 28 healthy subjects (19 males and nine females), with previous perimetric experience, mean age 34.7 years (SD = 14.1) and a refractive error < 3 D (spherical equivalent) were examined twice using the Octopus 1-2-3 perimeter; once with CFF perimetry (background 31.4 asb, stimulus size Goldmann III at 4000 asb, 0- 50 Hz variable frequency and 1 sec long), and once with conventional DLT perimetry. TOP strategy and grid 32 were used in both cases together with a random order of stimulus locations.

All patients had normal ophthalmological examination results, visual acuity better than 15/20, did not suffer from any pathology, and had no other factors that were likely to affect the visual field.

The patients had previous experience with DLT perimetry and were given training with CFF prior to the collection of data. They were requested to respond only when the stimulus was seen as flickering, and not to respond when it was seen as still.

Results

A conventional regression analysis found that DLT = 17.2 + (0.67 x CFF), with a correlation coefficient of r = 0.4. Forcing the regression analysis to pass through the center of the coordinates axis altered the relationship to DLT = 1.27 x CFF, with a correlation coefficient of r = 0.2. However, the similarity of the RMS errors (3.1 and 3.4 Hz, respectively) justifies the use of the more simple relationship.

The mean values for CFF and DLT, in the same patient, are shown in Figure 1. Only isolated peripheral points in the inferior nasal field showed significantly different values.

Deviation of the actual data from the estimated values, using our method to calculate the mean value of CFF from the DLT mean for a given age (applying the relationship described previously), is 0.6 Hz, resulting in r = 0.85.

In our study, the CFF age loss (0.075 Hz/year) was similar to the DLT age loss (0.059 dB/year), with a ratio of 1.27.

Discussion

Several studies have described the influence of different stimulus factors (luminance, spectral composition, temporal waveform, duration etc) on CFF. Ferry-Porter’s law describes a CFF increase with stimulus luminance and the logarithm of retinal illumination. Using this relationship, Rodríguez and Méndez described a different stimulus processing time for different illuminations.5 Granit-Harper’s law describes how CFF depends on the logarithm of the stimulated retinal area and its eccentricity. For a constant modulation, it is easier to identify a large flickering stimulus in peripheral

Fig. 1. CFF and DLT normal mean values in a 43.7-year-old patient, and mean local relationship between both physiological thresholds. Areas with CFF/DLT ratios of more than p < 0.05 away from the mean value (1.27) are shown in black.

CFFF / DLT

1.131.16 1.12

1.191.15 1.15

1.31X

Fig. 2. Areas with CFF/DLT ratios more than p = 0.05 away from the mean value (1.25) in the previous study.1

areas, and smaller ones in the central retina (<1º). At photopic levels, CFF is not dependent upon spectral composition, but at scotopic levels, CFF increases for short wavelengths.

Therefore, the relationship described in our study could be modified if test conditions were changed (stimulus and background luminance, size, frequency, duration, etc.).

Our previous estimation, using two different groups, had some local variability (underestimation in the peripheral areas, mostly in the upper field). In several peripheral points, and more in the inferior nasal visual field, the relationship is smaller than 1.120. The results are highest in the inferior and nasal fields, but are smaller than 1.346 (Fig. 2).

In our previous study, calculating the mean value of CFF by applying the relationships to the mean value of DLT for a given patient’s age, the deviation from estimated values was 1.2 Hz. This relationship is more uniform in the present study (0.6 Hz). There is a tight functional dependence between DLT and CFF (1 dB = 1.27 Hz) for the kind of stimulus used with the Octopus 1-2-3. This relationship between both physiological functions seems to be more constant than what has been previously estimated using a different sample of subjects.

References

1.González de la Rosa M, Rodríguez J, Rodríguez M: Flicker-TOP perimetry in normals, patients with ocular hypertension and early glaucoma. In: Wall M, Wild J (eds) Perimetry Update 1998/1999, pp 59-66. The Hague: Kugler Publ 1999

2.Rodríguez J, Cordovés L, Abreu A, González de la Rosa M: TOP flicker fluctuation in ocular hypertension. In: Wall M, Wild J (eds) Perimetry Update 2000/2001, pp 149-153. The Hague: Kugler Publ 2001

3.Matsumoto, Uyama K, Okuyama S, Uyama R, Otori T: Automated flicker perimetry using the Octopus 1-2-3. In: Mills (ed) Perimetry Update 1992/1993, pp 435-440. Amsterdam: Kugler Publ 1993

4.Matsumoto C, Okuyama S, Uyama K, Iwagaky A, Otori T: Automated flicker perimetry in glaucoma. In: Wall M, Wild J (eds) Perimetry Update 1994/1995, pp 141-146. Amsterdam: Kugler Publ 1995

5.Rodríguez M, Méndez E: Quantal processing of visual information in the brain. Neuroscience, 84:641-

644.1998