Age-related change in flicker thresholds with rod- and cone-enhanced stimuli

Abstract

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.

Visual function deficits in eyes with resolved endophthalmitis

“…The median values for the visual functions are provided in Table 2 along with a reference for normative age-matched values for each of the visual functions 10 , 13 – 15 . The median central retinal thickness in the cohort of patients was 260 μm (IQR: 91.75 μm).…”

“…Fifty-five per cent (11/20) of the patients had a visual acuity ≤ 0.3 logMAR, which is considered as a good visual outcome in endophthalmitis 1 . Among, those who had visual acuity of ≤ 0.3, none of the patients had a log contrast sensitivity higher than 1.66 (lower limit for age-matched normal values) 15 Only one patient (10%) had a lower threshold than age—matched cone flicker threshold values 10 . Only 18% (2/11) of the patients had CAD thresholds (RG and B-Y) and FMTs lower than the upper limits for the age-matched database 10 – 13 .…”

“…Among, those who had visual acuity of ≤ 0.3, none of the patients had a log contrast sensitivity higher than 1.66 (lower limit for age-matched normal values) 15 Only one patient (10%) had a lower threshold than age—matched cone flicker threshold values 10 . Only 18% (2/11) of the patients had CAD thresholds (RG and B-Y) and FMTs lower than the upper limits for the age-matched database 10 – 13 . These results indicate that the visual acuity is fairly preserved, however, the visual functions such as contrast sensitivity, chromatic discrimination and cone-mediated FMT remain impaired.…”

“…The current study has some limitations. The flicker thresholds were limited only to the central 5 o because, we had normative database for that eccentricity 10 and it is therefore easy to compare it against the database to make sure that flicker thresholds were not age-related changes. The visual function characterization of subtypes of infectious endophthalmitis (bacterial/fungal) was not done extensively, owing to the small sample size in each subgroup.…”

“…Flicker- Plus module in the AVOT setup uses a 5 Alternative-Forced-Choice (AFC) method to measure flicker threshold at five distinct locations namely one in the centre and each of the four quadrants parafoveally (at an eccentricity of 5° from the point of fixation) at a viewing distance of 1 m. The four quadrants were indicated as superotemporal (ST), superonasal (SN), inferonasal (IN) and inferotemporal (IT) based on the eye on which the test was carried out. The following spatiotemporal parameters were used: Temporal frequency: 15 Hz, a sensitive frequency to detect retinal deficits 9 ; the size of the centre and the parafoveal stimulus was 30 and 60 min of arc respectively 10 . The starting contrast for the flicker stimulus was set at 10% and it was kept constant for all the participants.…”