Small, brief foveal stimuli: an additivity experiment

In an additivity experiment, increment thresholds were determined for 510 and 660 nm and for 470 and 570 nm. These thresholds were determined for each wavelength separately and for the wavelengths of each pair mixed. These measurements were made against a 1000-Td white background using fields subtending 0.16 degrees and 1.04 degrees visual angles and at 10- and 200-msec durations. For most conditions with the 510- and 660-nm pair, additivity of the reduction type was obtained. Generally, the larger field and longer duration produced larger additivity failures than the smaller field and shorter duration. For the 470- and 570-nm pair, two of the three observers showed additivity for the 0.16 degrees field in both the long- and the short-duration conditions. The larger field with longer durations yielded additivity failure for the three observers, but these failures were smaller than for the 510- and 660-nm pair.