Data from: Superior discrimination for hue than for saturation and an explanation in terms of correlated neural noise

The precision of human colour discrimination depends on the region of
colour space in which measurements are made and on the direction in which
the compared colours—the discriminanda—differ. Working in a
MacLeod–Boynton chromaticity diagram scaled so that thresholds at the
white point were equal for the two axes, we made measurements at reference
points lying on lines that passed at 45° or −45° through the white point.
At a given reference chromaticity, we measured thresholds either for
saturation (i.e. for discriminanda lying radially along the line passing
through the white point) or for hue (i.e. for discriminanda lying on a
tangent of a circle passing through the reference point and centred on the
white point). The discriminanda always straddled the reference point in
chromaticity. The attraction of this arrangement is that the two
thresholds can be expressed in common units. All that differs between
saturation and hue measurements is the phase with which the short-wave
signal is combined with the long-/middle-wave signal. Except for
chromaticities very close to the white point, saturation thresholds were
systematically higher than hue thresholds. We offer a possible explanation
in terms of correlated neural noise.