The field adaptation of the human rod visual system

1. Incremental thresholds were measured in a retinal region 12 deg temporal from the fovea with a target of 200 ms in duration and 6 deg in diameter superimposed on background fields of various intensities and wavelengths. Measurements were made under rod-isolation conditions in five normal observers and in a typical, complete achromat observer who had no cone function. 2. The rise in threshold with background intensity changes with background wavelength in the normal trichromat observers. On 450, 520 and 560 nm backgrounds the average slope in logarithmic co-ordinates (0.78 +/- 0.04, S.D.) is similar to that found for the achromat--whose slope is independent of background wavelength (0.79 +/- 0.03)--but on a 640 nm background it more nearly approaches Weber's law (0.91 +/- 0.02). This indicates that the sensitivity of the rods to an incremental target is not determined by quantal absorptions in the rods alone but by quantal absorptions in both the rods and the cones. 3. Rod incremental thresholds were also measured in various colour-blind observers lacking one or more of the cone classes: a blue-cone monochromat, four deuteranopes and a protanope. For the blue-cone monochromat, like the achromat, the slope of the increment threshold curve is constant with background wavelength. For the deuteranopes and the protanope, like the normal, the slope increases with wavelength. The protanope, however, shows a smaller increase in slope, consistent with the lower sensitivity of his cones to long-wavelength light. 4. The dependence of the field adaptation of the rods on the cones was confirmed by field-mixture experiments, in which the incremental threshold was measured against bichromatic backgrounds, and in silent substitution experiments, in which backgrounds equated for their effects on either the cones or the rods but not both were instantaneously substituted for one another.