What can colour thresholds tell us about the nature of the underlying detection mechanisms?

Two-colour increment thresholds, which have traditionally been analysed in terms of pi-mechanisms, were subjected here to a different form of analysis. Detection thresholds were measured for monochromatic test flashes and 10 bichromatic mixture combinations containing various proportions of two out of the five primaries: white, blue, green, yellow and red. The 1 degree, 0.5-s duration, foveal flashes appeared on a 1000-td blue background. All the thresholds were simultaneously fitted by assuming probability summation from a limited number of independent linear mechanisms. The assumption of two detection mechanisms provided a reasonable fit for all the bichromatic mixture thresholds. One mechanism received a positive contribution from both M and L cone types, whereas the other received an excitatory contribution from the L cone type, and an inhibitory contribution from the M cone type. The predictive power of the model was tested and compared to the pi-mechanism concept by fitting a spectral-sensitivity curve measured under identical conditions. It was concluded that parts of pi-mechanism curves probably reflect the responses of colour-opponent processes rather than those of single cone types.