Test and field spectral sensitivities of colour mechanisms obtained on small white backgrounds: action of unitary opponent-colour processes?

It has been shown that for human foveal vision the test spectral sensitivity curve obtained in the presence of a large white background exhibits peaks at about 440, 530 and 610 nm and a small dip or notch at about 580 nm. Additionally, field spectral sensitivity curves for the medium- and long-wavelength sensitive colour mechanisms when derived in the presence of a small monochromatic background (auxiliary field) spatially coincident with the test field are sharper than the corresponding Stiles's II mechanisms and peak at about 530 and 605 nm. The short-wavelength sensitive colour mechanism shows no such effect, the peak remaining at about 440 nm. The test spectral sensitivity curve obtained on a large white background and the sharpened field spectral sensitivity curves obtained on a spatially coincident, monochromatic auxiliary field have each been interpreted in terms of an opponent-process theory of colour vision: the response of the non-opponent luminance system is considered to be reduced by selective achromatic adaptation (with the large white background) or by selective contour masking (with the spatially coincident auxiliary field). The present study combined the two techniques of achromatic and spatial adaptation: a small white auxiliary field spatially coincident with the test field was used in measurements of both test and field spectral sensitivities. Under these conditions, it was found that the test spectral sensitivity curve showed more clearly defined peaks and a deeper notch at about 580 nm than when obtained on the large white background and that the test curve was well fitted by the upper envelope of the short-wavelength and sharpened medium- and long-wavelength field spectral sensitivity curves, suggesting the possibility that three unitary opponent-colour mechanisms may underlie test and field spectral sensitivities. A test of this hypothesis is proposed concerning the effect of auxiliary-field chromaticity on the position of the notch at about 580 nm in the test spectral sensitivity curve.