Figure 1. The modified hue selection task.

(a) 2-deg colour patches (test stimuli) are presented at an eccentricity of 4 degrees. To obtain unique red (or green) the observer is asked to click on that patch that contains neither yellow nor blue. Similarly, to obtain unique yellow (or blue), an annulus consisting of yellowish (or bluish) patches is displayed and the observer is asked to click on that patch that is neither red nor green. In a previous experiment, we confirmed that the hue settings are not affected by the arrangement of the hue patches; randomised and ordered hue patches yield the same mean results. (b) The chromatic properties of the test stimuli are shown in a 3D solid in CIELUV space. The outermost white line indicates the gamut of our monitor. On a particular trial, hues of similar saturation and brightness were displayed in an annulus (see a). E.g. To obtain unique red, reddish hues lying on an arc in the CIELUV space, are displayed and the observer is asked to select that reddish hue that is neither yellow nor blue.