Contrast detection in luminance and chromatic noise.

We measured detection thresholds for a vertically oriented 1.2-cycle-per-degree sine-wave grating embedded in spatiotemporal broadband noise. Noise and signal were modulated in different directions in color space around an equal-energy white point. When signal and noise were modulated in the same direction, we observed a linear relationship between noise spectral density and signal energy at threshold. The slope of this function was the same whether the modulation was along a luminance axis or a red-green axis. If the signal was on one axis and the noise was on the other, no masking was observed. These results support the notion of two independent and equally efficient mechanisms tuned to these directions. We then measured threshold elevations for masks with both chromatic and luminance components. When signal and noise were modulated along the same line (for example, bright red and dark green), thresholds were elevated. When we inverted the phase of the chromatic component of the noise relative to the luminance component (bright green and dark red), the masking effect disappeared, even though the amount of noise in the putative luminance and chromatic mechanisms was exactly the same as before. This implies that detection performance is limited by mechanisms sensitive to both luminance and chromatic contrast signals. We characterized these mechanisms by their spectral tuning curves.