Age-related Change in Flicker Thresholds with Rod- and Cone-enhanced Stimuli

Purpose: Rod and cone photoreceptor-specific tests can be time-consuming. A new non-invasive test is described. The test is based on the measurement of flicker modulation thresholds with rod- and cone-enhanced visual stimuli, which requires only minimum adaptation time. Here, we investigated how the rod-and cone-mediated flicker thresholds vary with age.

Methods: Monocular thresholds with rod and cone-enhanced stimuli were measured in 140 healthy adults, (age range: 18-75 years), foveally (0°) and at four parafoveal locations, at an eccentricity of 5° in each of the four quadrants using five, adaptive, interleaved staircases. Temporal frequencies, stimulus sizes, background luminance and spectral composition, were adjusted appropriately to achieve approximately 1 log unit separation in sensitivity between the rod- and cone-enhanced stimuli. Spectrally calibrated, 'neutral density' filters were used to enable adequate control of display luminance for rod enhanced stimuli.

Results: The magnitude of central and parafoveal rod thresholds was significantly higher than the central and parafoveal cone thresholds, respectively (p < 0.001) in both the age groups. However, the rate of increase in central rod thresholds (y = 0.45x-12.79; linear regression equation) was not significantly steeper than the rate of increase in central (y = 0.29x-8.53) cone thresholds (p = 0.15). Centrally, cone thresholds showed a better correlation with rod central thresholds for the age > 45 years (Spearman correlation, ρ = 0.74, p < 0.001) compared to age ≤ 45 years (ρ = 0.41, p < 0.001).

Conclusions: Thresholds with rod- and cone-enhanced stimuli are largely invariant below 45 years of age and increase rapidly above this age. This age-wise normative database can be used as an effective functional-marker to assess photoreceptor sensitivities in retinal diseases.