Table 2.

Estimated receptor responses to the eight stimulus colours used for chicken short (S), medium (M) and long (L) wavelength-sensitive single cones and (D) double cones, relative to a barium sulphate reflectance standard under the same illumination (see also Osorio et al. 1999c). The response to the achromatic background was close to 0.3 for the four cone types. Values given are for the highest intensity colour, but intensities were varied at random over a uniform distribution with a contrast range of 0.3 (Osorio et al. 1999c). x and y chromaticity coordinates were calculated according to the formula given in the legend to figure 2. Aspects of colour that correspond to human ‘hue’ and ‘saturation’ are defined by polar coordinate frame centred on the origin. Saturation corresponds to the distance of the colour from the origin, and hue to the angle relative to a line running in the negative direction from the origin and parallel to the x-axis. The UV sensitive receptor was inactive because a coloured filter (Schott G475) blocked short-wavelength illumination (Osorio et al. 1999c). Examples of reflectance spectra that are close to the orange and blue used here and were produced by the similar Epson printer inks are illustrated by Miklósi et al. (2002; figure 1a,b).

	S	M	L	D	hue (°)	sat.	x	y
saturated colours
orange	0.19	0.28	0.37	0.31	180	0.15	0.1515	0.0000
blue	0.45	0.33	0.25	0.30	353	0.14	−0.1373	−0.0159
green	0.28	0.29	0.16	0.23	34	0.14	−0.1162	0.0783
red	0.165	0.17	0.285	0.22	204	0.16	0.1507	−0.0669
unsaturated colours
orange	0.27	0.33	0.40	0.36	183	0.092	0.092	−0.004
blue	0.46	0.37	0.32	0.35	351	0.087	−0.086	−0.014
green	0.30	0.31	0.22	0.27	36	0.084	−0.068	0.049
red	0.22	0.22	0.30	0.25	210	0.088	0.076	−0.044