Influence of flicker frequency on flicker-induced changes of retinal vessel diameter.

PURPOSE To investigate the effect of diffuse luminance flicker of different frequencies on retinal vessel diameter in the human eye. METHODS Nine healthy subjects participated in the study. A retinal vessel analyzer (RVA; Zeiss, Jena, Germany) was modified to allow for continuous measurement of vessel diameter during flicker stimulation. With this technique, the light used for measurement of vessel diameter consists of wavelengths between 567 and 587 nm, whereas the spectrum of the stimulation is low-pass, with a cutoff wavelength at 550 nm. Flicker frequencies ranged from 2 to 64 Hz. RESULTS In retinal arteries an increase was observed at all flicker frequencies, with a less-pronounced effect at 64 Hz. In retinal veins, all flicker frequencies except 2 and 64 Hz induced vasodilation. Generally, the flicker-induced diameter response was less pronounced in retinal veins than in arteries. CONCLUSIONS The flicker-induced diameter response of the larger retinal arteries and veins can be continuously measured with a modified RVA system that spectrally separates the flicker stimulation from the fundus illumination light. Vasodilation of retinal arteries was observed in response to short wavelength flicker with frequencies between 2 and 64 Hz, indicating that the parvo- and magnocellular neural pathways are activated with this stimulation.