Temporal phase response of the short-wave cone signal for color and luminance

Abstract

A chromatic discrimination paradigm was used to measure the temporal phase of the S (short-wave cone) signal relative to the L ȡ M (long-wave cone minus middle-wave cone) signal. Suprathreshold equiluminant red-green flicker that stimulates the L ȁ M mechanism was presented on a steady, intense yellow-green adapting field. Violet flicker that stimulates the S cones was added to the red-green flicker at different temporal phase angles, and the violet modulation depth was varied to achieve a chromatic discrimination threshold. A template was fitted to the data relating thresholds to phase: the location of the template symmetry axis showed that the S signal lagged L ȡ M by about 75ȁ90° at 10 Hz. This is about one half the phase lag obtained for luminance or motion discrimination. The phase discrepancy shows that there are separate luminance and chromatic mechanisms receiving S cone inputs. The hue of the flicker in the present study varied strongly with phase angle, with the positive and negative excursions of the S cone signal producing a reddish-blue and greenish-yellow, respectively, and these colors combined with the reddish and greenish hues produced by the L ȁ M signal. The observed phase shift, and measured color appearance of the combined flicker, account for the colors seen on a radially segmented disk of Munsell hues when rotated: the colors differ strikingly depending on the direction of rotation.