Subjective colours produced by the achromatic pattern of a rotating Benham's disc depend upon the phase relationship and the contrast between the luminous signals of the arc and the background. By studying the effect of luminance and spectral adaptation with different illuminations specified in terms of LMS cone excitation we determine whether particular post-receptoral combination signals explain the differences in perceived colours. The results show that (I) the attributes of subjective colours may be determined from a combination oft and M cone excitations in an antagonistic and in a synergetic mode, and more precisely by the ratio of these two combinations; (2) the contribution of these two combinations is modulated by the duration of the white phase preceding the arc; (3) the effect of the S cone signal is small and essentially restricted to the arcs adjacent to the black surface of the disc. We discuss temporal aspects of non-specific spectral contrast units implied in achromatic detection (parvo- and magno-system). We argue that the mechanisms underlying Benham subjective colours, are situated in a neurophysiological site high enough in the visual information processing hierarchy to allow both types of non-spectral-specific signals to be modulated by the duration of the white phase following the dark half disc and preceding the ai-c, thus reducing the implication of retinal level. (C) 2000 John Wiley & Sons, Inc.