Spatial phase or luminance profile discrimination?

The ability of human observers to discriminate differences in the relative phase of the components of high contrast compound gratings has been investigated. It is found that differences of less than 10⁷ in the phase angle of the higher harmonic can be detected reliably, if sufficient practice is given. However, examination of the mechanisms involved in making "phase" discriminations suggests that observers, in most studies of phase discrimination, may not code relative phase directly in making their judgements. Indeed, it appears that the most parsimonious explanation is that the observers detect differences in the contrast of local regions of the stimuli.