How is color represented in the cortex? Most evidence suggests that the well-defined code found in the retina and lateral geniculate nucleus, in which two “cardinal axes” of the color circle structure are encoded by just two parallel sets of signals (alongside a third for luminance), gives way in the cortex to a relatively messy and inscrutable organization where no such privileged axes exist. Here we use metacontrast to investigate the cortical representation of color and brightness.

Using the BIGMAX stimulator we describe elsewhere, test discs of 1.1 deg diameter were flashed briefly (50 ms) and followed at an onset asynchrony of 120 ms by a contiguous masking ring. Ring and test consisted of perturbations of luminance and/or color from a neutral adapting background. Contrast thresholds for the test were measured using seen/not seen judgments.

A masking ring devoid of color strongly suppressed only colorless test discs of similar luminance polarity: colored tests were little affected by colorless masks. But this selectivity for color difference was not symmetrical: colorless luminance test discs were effectively masked by colored rings, so long as the ring included an appropriate luminance modulation. Similar results were obtained in working around the two cardinal directions in the isoluminant plane. These observations suggest that test discs escaped masking, and were seen in roughly their proper color and luminance contrast, if they generated at least one cardinal-axis signal that exceeded the corresponding signal elicited by the mask.

As this interpretation predicts, no comparably sharp and asymmetrical selectivity for direction of modulation was found when working around the diagonal directions in color space. On this evidence, the cardinal directions do have a special status in the neural representation of color at the site of metacontrast.