Internal noise and sampling efficiency in discrimination of second-order patterns

Abstract

The visual system is sensitive to both first-order luminance modulations and second-order modulations of carrier contrast. Studies have shown that sensitivity to second-order modulations is lower than the sensitivity to first-order stimuli. We sought to determine the factors which make second-order vision less sensitive than first-order vision. We evaluated internal noise and sampling efficiency of first- and second-order vision employing the phase-discrimination paradigm (Burgess and Ghandeharian, 1984) in conjunction with the equivalent input noise approach (Legge et al. 1987). The observers were presented with a 2-cpd pedestal or the pedestal plus a 2-cpd signal with a fixed phase shift relative to the pedestal. Detectability of the signal at various phase angles was measured in the absence and presence of static 2D Gaussian noise. Given sufficient a priori phase information, the observers performed phase-sensitive discrimination of both first- and second-order stimuli, although imperfectly. We found that the internal noise in phase discrimination of contrast modulations was five times higher than that in phase discrimination of luminance modulations. This finding may be due to the presence of an additional demodulating stage in the analysis of second-order information which adds extra noise. The sampling efficiency in discrimination of second-order signals was half that found for discrimination of first-order signals. This effect did not depend on the phase angle between the pedestal and the signal which suggests that second-order information processing is characterised by imperfect signal demodulation rather than to inefficient sampling strategy.