The scotopic visibility curve and cone intrusion

The scotopic visibility curves of two observers have been measured by determining their absolute thresholds for monochromatic lights and by direct comparison matching just above the threshold, in a 10 degree field with its centre 15 degrees above the fixation point. The results were all similar to the CIE scotopic curve. However, the threshold results in the long-wave part of the spectrum could be considerably modified by subliminal red and green lights. This is consistent with Drum's observations of sub-additivity at threshold. Above threshold, supra-additivity was obtained. A scotopic curve is evidently a non-linear combination of rod and cone responses which varies with the conditions of observation.