The retinal threshold gradient in the presence of a high-luminance target and in total darkness

The retinal threshold was measured along the horizontal meridian for the dark-adapted eye and for the same retinal region light-adapted by stray light from a small foveally fixated high-luminance target. This was done to quantify the irradiation phenomenon in the foveal-parafoveal boundary region. Two target luminances were investigated (5,531 and 13,000 fL). Seven Ss made threshold settings using a variable-intensity test spot whose retinal image was located at various angular separations from the foveal center as far as 10 deg in the temporal field. Threshold slopes were steeper for test spots imaged from 15min to 1 deg of arc from the foveal center than for spots imaged from ldegl5min to 10 deg of arc from the foveal center for the illuminated target conditions. The angular distance between the fovea and the intersection of the steep component with the shallow component of each threshold curve tended to shift toward the target’s edge with an increase in target luminance. These data are interpreted as an indication that a dynamic neural mechanism is involved in producing the irradiation phenomenon.