The effect of eccentricity and light level on the time course of light adaptation mechanisms

Abstract

Input visual signal is modified by neural adaptation mechanisms in order to be efficiently processed by posterior circuits. However these mechanisms do not work instantaneously and they are not similar in fovea and periphery. In this work we analyze the effect of transient and steady backgrounds on contrast detection threshold at different light levels and eccentricities. We used a two-channel Maxwellian view optical system to measure detection luminance thresholds with the method of limits for steady (LA) and transient (SOA300) background fields on three subjects with emmetropic vision. Data were collected for different retinal eccentricities (0°-15°), and adaptation luminances (0.06-110 cd/m2). Test/background size was 0.45°/1°. Comparison between LA and SOA300 TvI curves evidence that for most conditions, thresholds for SOA300 are higher than LA thresholds, with greater differences for photopic conditions. In mesopic light levels for 3°, 6° and 9° of eccentricity LA and SOA300 thresholds are similar, except at 6 cd/m2 for 6° where LA thresholds are higher than SOA300 data. A model consistent in known adaptation mechanisms (originally applied for steady conditions; Gloriani et al, Vision Research, 2016) showed that the photopic threshold increment for SOA300 is due to slowness of subtractive and molecular mechanisms, and variations in mesopic levels are due to an increment of noise involvement especially at higher eccentricities. This study evidence that the time course of mechanisms and factors affecting sensitivity is strongly dependent of light level and eccentricity.