A model for foveal and parafoveal sensitivity that could help our understanding about light adaptation

Abstract

The understanding of the mechanisms involved in light adaptation is a challenge not completely solved. Here, we present a model based on visual mechanisms described in the physiological and psychophysical literature able to fit psychophysical data in several stimuli conditions. We propose that the study of changes in the different mechanism and phenomena considered in our model could be a strategy to better understand light adaptation. We used a two-channel Maxwellian view optical system to measure detection luminance thresholds with the method of limits for steady and transient (SOA300) backgrounds field. With data for different retinal eccentricities (0°-15°), and adaptation luminances (0.06-110 cd/m2), preliminary results of this model were reported in a previous ICVS meeting. In this occasion we performed new measurements considering different combinations of background/test size, 10°/2°, 10°/0.45° and 1°/0.45°, and a final version of the model is presented. For steady backgrounds we describe how spatial summation changes with eccentricity and we associate this behaviour with increases in the receptive field sizes. We report changes in subtractive adaptation with both eccentricity and background field size and we connect these changes with horizontal cells and changes in their dendritic field size with eccentricity. Additionally, when we reduce the background field size, we propose that the increase in the thresholds is related with rod-cone interactions and photon noise. For transient backgrounds, we show how the gradual shift in the site of the gain control can explain the data. Finally, we believe this strategy is an interesting way to better understand the behaviour of the different visual mechanisms and phenomena involved in retinal light adaptation.