Why is second-order vision less efficient than first-order vision?

Research has shown that the sensitivity to second-order modulations of carrier contrast is lower than that to first-order luminance modulations stimuli. We sought to compare the efficiency of processing first- and second-order information. Employing a phase-discrimination paradigm we found that when humans were given sufficient a priori information of signal parameters they detected both luminance and contrast modulations of 0.6 and 2c/deg by a phase-sensitive algorithm. The overall detection efficiency for second-order patterns, however, was lower that that for first-order stimuli. To study the factors which limit the efficiency of first- and second-order vision, we measured detection performance for luminance and contrast modulations of 0.6 and 2c/deg embedded in Gaussian noise. The results showed that the detection of second-order patterns had lower sampling efficiency and higher additive internal noise as compared to the detection of first-order stimuli. Classification images for detecting contrast modulations of 2c/deg resembled the side-band component of the contrast modulations which suggests that human observers may detect contrast modulations of a sinusoidal carrier using first-order luminance channels. The lower sensitivity of the mechanism detecting second-order patterns might be due to higher levels of additive internal noise and lower sampling efficiency than those of the mechanism analysing first-order patterns.