Threshold versus intensity functions in two-color perimetry

Abstract

Purpose : To determine the adaptational state and isolation of rod- and cone-mechanisms in two-color perimetry, a technique developed for assessing visual function in inherited retinal disease.

Methods : Seven normal subjects (aged 16-46yr), were examined using a MonCV Perimeter (MetroVision, Paris, France). Visual field tests were undertaken under scotopic conditions and then from -1.5 log cd.m^-2 (0.2 log td) to 2 log cd.m^-2 (3.7 log td; white background) in 0.5 log unit steps. Sensitivities were determined using a 4-2-2 dB staircase for 480nm and 640nm Goldmann size V targets at 17 locations within the central 60 degrees. Data were fitted with tvi functions of the form logT = logT₀ + log ((A+A₀)/A₀), where T is threshold, T₀ is absolute threshold, A is background intensity and A₀ is the "dark-light" constant.

Results : The minimum value for cone T₀ was at the fovea (-1.7 log td, 640nm stimulus) and for rod T₀ at ≥ ±9,±9 (-4.2 log td, 480 nm stimulus). Cone A₀ was lowest at the fovea (1.0 log td) whilst rod A₀ was lowest at ±9,±9 (-0.70 log td). No clear rod-cone break was observed for 640nm stimuli at any stimulus location. For 480nm stimuli at all test locations, there was evidence of transition from rod-detection to cone-detection at mesopic illumination levels. Detection mechanisms did not display Weber behaviour until the background luminance approached 1 log cd.m^-2/ 2.7 log td (see Figures 1 and 2).

Conclusions : 640nm targets are primarily detected by cones at all intensities; 480nm targets, however, are detected by rods until mesopic illumination levels are reached. These data are in keeping with spectral sensitivity estimates at fixed background intensities (Simunovic MP et al..TVST. 2016;5(3):10). Two-color perimetric tvi functions do not display Weber-like behaviour until photopic illumination levels are reached (1 log cd.m^-2/ 2.7 log td). These findings suggest that two-color perimetry will be vulnerable to perturbations in ocular media and pupil size under clinical conditions, where background intensities are fixed.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.

 

Averaged foveal threshold sensitivity for blue (480nm, blue circles) and red (640nm, red circles) Goldmann size V targets versus background intensity (log troland). Bars show standard errors. Data are fitted with tvi functions (see text). The diagonal dashed line plots a slope of 1 (Weber's Law).

 

Averaged peripheral (±3, ±3; ±9, ±9; ±15, ±15; ±21, ±21) sensitivity. Details as for caption 1.