CONICET | Buscador de Institutos y Recursos Humanos

The understanding of the mechanisms involved in the time course of light adaptation is a challenge not completely solved. The problem becomes very complex in the central 15 deg of retina, due to the significant change in cell density of all retinal layers. In this work, we present foveal and parafoveal threshold luminance measurements and a model that accounts for these data. The measurements were performed with a two-way Maxwellian view optical system on the temporal retina of the right eyes of four individuals with healthy vision. We considered photopic luminances (15-110 cd/m2) in retinal eccentricities from 0 to 15 degrees, for steady (LA) and transient (SOA300) adaptation. Circular and concentric adaptation and stimulus beams were considered with sizes 1.05º and 0.45º respectively. Transient adaptation field was 500 ms long whereas stimulus beam was always 40 ms long. The light source was a halogen incandescent lamp (T=3000 K). The method of limits was employed. The obtained thresholds increase with increasing adaptation field luminances, showing a greater slope for SOA300 measurements. A model has been built to explain these results. This model incorporates physiological and psychophysical mechanisms, some of them changing with eccentricity and field luminance, such as the different contribution of cones and rods and the contrast gain. The photopigment regeneration has been included for LA measurements whereas a gain control mechanism helps to explain LA and SOA300 results. Our results suggest that, in the first instants of the adaptation process, control gain might be due to cones in fovea but tends to be shifted gradually to ganglion cells as more peripheral regions are considered. In the adapted retina (LA) ganglion cells would be the responsible of the control gain in the whole retina. (Dunn et al., 2007, Nature, 449, 603?606).