Figure 7.

How correlated neural noise could explain the superiority of hue discrimination. In each panel, the x- and y-axes represent the signals in the two ‘cardinal’ chromatic channels of the early visual system. The left-hand panels correspond to hue discrimination and those on the right correspond to saturation discrimination. (a) No noise is assumed: the responses generated by the target and the distractors plot as points in the two-dimensional space. (b) The case where independent Gaussian noise is present in the two cardinal channels. The circles represent one contour of the probability density function produced by a given chromaticity. (c) It is assumed that noise in the two channels is correlated. Now the probability density distributions are elliptical with positive slopes. When the stimuli differ in saturation (right-hand panel) the distributions for the discriminanda overlap, but when the stimuli differ in hue—when a change on one axis is accompanied by an opposite change on the other axis—the overlap of the probability density distributions is less.